Sound/Record Terms 1

Capacitor

(also called cap or condenser) A capacitor is an electronic component capable of storing electrical energy. It consists of two metal plates insulated from each other by a dielectric (an electrical insulator). The impedance of a capacitor in a circuit is inversely related to frequency. As the frequency increases the impedance of the capacitor decreases.

Amperage

(ampere, amps, current, I): The ampere is the unit of electric current. Current is the flow of electric charge carries past a point, or from one point to another. Analogous to forward motion (as result of pressure). Current is the instrument of work and is set in motion by the presence of voltage.

Pad

1. An electronic circuit designed to attenuate the output of a device by a given amount. For example, some microphones have so much output that they can overdrive the input stage of many mic preamps. To prevent this, mic designers will include a switchable "pad" on the output stage of the mic, attenuating, or reducing the mic's output by 10 or 20 dB. While many devices have built-in pads, it is also possible to purchase external pads, which plug in to a device's output and reduce its level. 2. A sustainy, "wash" or fill sound, usually used as harmonic background material in a musical arrangement. Arrangers often speak of using a "string pad" during a passage; this would be a section of strings playing long, sustained chords behind the melody. With the advent of samplers and synthesizers, other types of sounds have also become common as pads; just about any sound that can sustain can be used as a pad these days!

Phantom Power

A DC (direct current) voltage, usually 48 volts, applied to pins 2 and 3 referenced to pin 1 of an XLR microphone connector that can be used to power transducers with active electronics. Condenser microphones require a pre amp close to the very high impedance (See WFTD archive "impedance") diaphragm which requires power to operate. Back in the '50's and '60's this power was often provided by a separate power supply that came with the microphone. Later manufacturers began to provide a source for this power at the microphone input to mixers or pre amps. Since the power is carried on the same wires that carry the audio signal, and since most dynamic microphones and other passive devices are not affected by this DC voltage it was known as "phantom" power. The theory was that only devices that needed it would be wired in such a way that they would use it. Nowadays almost all condenser microphones and active direct boxes are able to use phantom power when it is present on a microphone cable. Consequently most mixing board manufacturers include this feature in their products.

Distortion

A change in the shape of a waveform. or more loosely, any change in signal form one point to another.

Attenuation

A decrease in the level of a signal is referred to as attenuation. In some cases this is unintentional, as in the attenuation caused by using wire for signal transmission. Attenuators (circuits which attenuate a signal) may also be used to lower the level of a signal in an audio system to prevent overload and distortion.

Unity Gain

A device or setting which does not change signal level (amplify or attenuate the signal) is said to be at "unity gain." Many processors are set up for unity gain; that is, they can be plugged into a system without changing its overall levels. In practice, unity gain is often a desired setting for maintaining gain staging, and for optimizing operating levels and signal to noise ratios.

Diode

A diode is a tube or semiconductor device intended to pass current in only one direction. An LED (Light Emitting Diode) is a variant that emits light as current passes through it.

Dynamic (Microphone)

A dynamic mic is one in which audio signal is generated by the motion of a conductor within a magnetic field. In most dynamic mics, a very thin, light, diaphragm moves in response to sound pressure. The diaphragm's motion causes a voice coil which is suspended in a magnetic field to move, generating a small electric current. Generally less expensive than condenser mics (although very high quality dynamics can be quite expensive), dynamics feature quite robust construction, can often handle very high SPLs (Sound Pressure Levels), and do not require an external power source to operate. Because of the mechanical nature of their operation, dynamic mics are commonly less sensitive to transients, and may not reproduce quite the high frequency "detail" other types of mics can produce. Dynamic mics are very common in live applications. In the studio, dynamics are often used to record electric guitar and drums.

Filter

A filter is an electronic device designed to reduce a signal's energy at a specific frequency. A true filter always acts as a subtractive device, not adding anything to the signal. In many filters, an amplifier is often incorporated into the circuit, allowing the frequency to be boosted or cut (active filter). Filters of different frequencies are often combined to create equalizers.

Efficiency

A measurement of how much of the input electrical energy to a speaker is converted into sound. The remaining energy is converted to heat. Most direct radiator speakers are 1 or 2 percent efficient; a horn-loaded speaker might approach 20 percent, some reach as high as 30 percent. High efficiency means that a lower powered amplifier can be used to produce the same level, but there is also a case to be made for less efficient speakers actually being more accurate due to better damping and less susceptibility to resonances.

Cardioid

A microphone polar (pickup) pattern. Characterized by strong sensitivity to audio from the front of the mic, good sensitivity on the sides (at 90 degrees, 6 dB less than the front), and good rejection of sound from the rear, the cardioid pattern can almost be visualized as a "heart-shaped" pattern (hence its name). The ability to reject sound from the rear makes cardioid patterns very useful in multi-miking situations, and where it is not desirable to capture a large amount of room ambience. Popular in both studio and live use (where rear rejection cuts down on feedback and ambient noise), cardioid mics are used for a very high percentage of microphone applications. Keep in mind that like all non-omnidirectional mics, cardioid mics will exhibit pronounced proximity effect (see WFTD archives, "Proximity Effect").

Figure 8

A microphone polar pattern in which the mic is (nearly) equally sensitive to sounds picked up from front and back, but not sensitive to sounds on the sides. This produces a pattern that looks like a figure 8 on paper, where the microphone is at the point of crossover on the 8. The pattern is also known as bi-directional.

Integrated Circuit (IC)

A monolithic microcircuit consisting of interconnected active and passive electronic elements interconnected on or within a single semiconductor substrate.

Matched Pair

A pair of microphones that have been measured to ensure that they are extremely close or identical in response. Using a matched pair of mics is usually considered ideal for stereo recording applications, as the two mics will respond in equal fashion, without one coloring the sound more than the other.

Passive

A passive audio device is one which does not use amplification circuits. By nature of design, passive devices do cause a loss in power (insertion loss). Because they do not contain amplifiers, and are "cut-only" or "subtractive" in operation, passive devices tend to not add noise or distortion to a signal (although noise may be added in compensating for insertion loss). Typical passive devices include direct boxes, splitters, equalizers and crossovers.

Ground Loop

A phenomenon that occurs when an audio (or video) system has multiple paths and path lengths to ground. When a ground loop occurs in an audio system it will manifest itself as a hum. The hum occurs at 60 Hz (Hertz) or some multiple of 60 Hz because that is the line frequency of AC (in the US). In video a ground loop can often shows up as rolling bars (sometimes quite faint) in the picture. The severity of the hum will vary depending upon numerous factors; sometimes it can be quite faint, while other times it can be so loud that it overloads an amplifier. Ground loops have been a problem since the very early days of connecting audio equipment together and there have been volumes of materials written on how to prevent and solve them. Any time one connects multiple pieces of audio gear together that are each plugged into an electrical outlet and connected to each other via signal cables, and possibly even electrically connected by their cases touching or being in the same rack, the risk of ground loops increases. There are many valid schools of thought on stopping them, some of which we will get in to in the coming months. At the most basic level, however, the only way to prevent or get rid of ground loops is to make sure that all of your equipment has only one path to ground. One way people sometimes accomplish this is to utilize AC plug ground lift adapters (a.k.a. widow-makers) on offending pieces of equipment as determined by troubleshooting the system, but this is considered unsafe and we highly recommend against it.

Out of Phase

A phrase used to characterize two or more signals whose phase relationship with each other is such that when one is at its positive peak the other is at (or near) its negative peak. This is also commonly referred to as being 180 degrees out of phase. Phase is a relative value that is measured in degrees (like angles). 90 degrees out of phase is more out of phase than 80 degrees, but less than 100 degrees. 180 degrees out of phase is completely backwards, which is characterized by one signal's highest peak correlating with another's most negative peak. Most signals are not entirely in phase with each other, and it's just as rare for them to be perfectly (180 degrees) out of phase. But people generally say "out of phase" to mean approximately 180 degrees out of phase. People also frequently say "out of phase" when the more technically correct term to use would be "polarity reversed." Phase implies a time delay, where one signal lags behind another one to some degree. Polarity refers to one signal being "backwards" from another. An example of this would be the "phase" switch on many mic preamps and mixing boards. Generally all this switch does is reverse pins two and three on the XLR connector entering the preamp, thereby reversing the "polarity" of the signal. There is no time delay of the signal. Nevertheless this is often referred to as "out of phase." A similar thing happens when you reverse the polarity of the speaker leads to one speaker in a two-speaker setup. That speaker is now operating with the opposite polarity of the other. No time delay was introduced, yet we often refer to this as "out of phase." This confusion occurs because when viewed on a display like an oscilloscope waveforms that are 180 degrees out of phase with each other will not look any different than two that are polarity reversed. Sonically the difference is generally pretty minute as well. So for all practical purposes the two terms can be used interchangeably. While it is technically true that any two signals not 100% in phase with each other could be referred to by the somewhat generic phrase, "out of phase," we generally don't use that terminology until the signals approach a 180 degree phase relationship with each other.

Insert

A point in the signal path of a circuit where it is possible to interrupt the signal and "insert" another signal. Most commonly deployed in mixing boards it usually shows up as a patch point on each channel and/or bus output. The purpose is to be able to interrupt a signal in the mixer, bring it outside the mixer for some sort of special processing, and then return the processed signal to the same point from which it left. Common applications include applying compression, gating, or EQ to a particular channel without affecting any other channels or using any extra buses. Sometimes an insert is called a "patch" or "injection point."

Hypercardioid

A polar pattern name typically used to describe microphone pick up characteristics. Hypercardioid patterns are similar to cardioid patterns in that the primary sensitivity is in the front of the microphone. They differ, however, in that the point of least sensitivity is at the 150 - 160 and 200 - 210 degree positions (as opposed to directly behind the microphone in a cardioid pattern). Hypercardioid microphones are thus considered even more directional than cardioid microphones because they have less sensitivity at their sides and only slightly more directly behind. Hypercardioid microphones are frequently used in situations where a lot of isolation is desired between sound sources.

Supercardioid

A polar pattern name used to describe the pickup pattern of some microphones. The supercardioid pattern is very similar to, and often confused with, the hypercardioid pattern. The supercardioid pattern is slightly less directional than the hypercardioid pattern, but the rear lobe of sensitivity is also much smaller in the supercardioid.

Floating Unbalanced Line

A quasi-balanced output stage consisting of an unbalanced output connected to the tip of a 1/4" TRS jack through an output resistor (typically in the 50-300ohms range). An equal valued resistor is used to tie the ring terminal to signal ground. The sleeve connection is left open or "floating." Thus, from the receiver's viewpoint, what is "seen" are two lines of equal impedance, used to transfer the signal. In this sense, the line is balanced", even though only one line is actually being driven. leaving the sleeve open, guarantees that only one end of the shield (the receiving end) will be grounded; basically a telescoping shield - a practice that often significantly improves ground loop problems in balanced systems. for trouble-free interconnections, balanced lines are generally the preferred choice.

White Noise

A random noise signal whose power per unit frequency interval is independent of frequency. In other words, white noise has equal energy at all frequencies. This is the type of noise found in the noise floor of most electronic equipment and is the result of the "thermal noise" of the components.

Pink Noise

A random noise signal whose power per unit frequency is inversely proportional to frequency over a specified range. In other words, pink noise contains the same amount of energy per octave.

Decibel

A ratio used to describe two power levels. Often one of the power levels is a known and accepted reference, making the decibel value look as if it is an absolute value when in fact it is always a ratio. Some common ways to express decibels in audio are...

Resistor

A resistor is a circuit component that provides a fixed value of resistance in ohms to oppose the flow of electric current.

Inter-modulation Distortion (IM)

A result of two (or more) frequencies being applied to a system with the distortion product occurring at some other frequency(s) - usually sum and difference frequencies.

Combi Jack

A single jack on a device that allows either XLR and 1/4-inch connection. This saves space on the jack panel of the piece of gear, since separate XLR and 1/4″ jacks aren't required. Most combi jacks are female, for input connections.

Subwoofer

A subwoofer is a specialized loudspeaker dedicated to reproducing extremely low frequencies. Because many "full-range" speakers can't handle frequencies below 45-50Hz, a subwoofer can be added that will output the low frequency information normally missing. In a typical 5.1 setup, the ".1" (or "point one") refers to the subwoofer. The first commercial subwoofer was developed in the 1970s, while the first documented use of a subwoofer in a pro recording environment was during the mixdown of Steely Dan's Pretzel Logic album. Early subs were huge (up to 34 inches), but today, thanks to advances in technology, subs with speakers of 8″ or 10″ are available, though the larger the driver, the lower the sub typically can go. Today almost all subwoofers are active or self-powered via an onboard amplifier.

Transformer

A transformer is an electrical device that transfers electrical energy from one or more primary circuits to one or more secondary circuits by electromagnetic induction. AC only.

Transistor

A transistor is a three terminal electronic device capable of amplification and switching. A supply voltage is applied to two terminals (emitter and collector) and the third terminal (base) uses a control voltage to vary the devices ability to pass current.

Graphic Equalizer

A type of EQ that is configured to provide a graphic display of the EQ settings. Years ago equalizers were all rotary knob based. When units began to arrive on the scenes that had 15, 30, or even 45 bands (frequencies) they could EQ at once it became difficult to see what was going on at a glance. Looking at a row of 30 knobs to get an overall idea of the EQ curve is pretty difficult. So equalizers that used sliders instead of knobs were developed and quickly won the favor of engineers due to their improved ergonomics. People liked how easy it was to see the overall EQ curve at a glance, but they also just liked using sliders more than knobs (something that we'd already figured out about mixers). The ONLY thing that makes an EQ "graphic" is this configuration of being able to see the curve at a glance. Contrary to popular belief there are graphic EQ's that have the same features as parametric EQ's, including Q controls and sweepable frequencies. Most graphic EQ's, however, only give you control of cutting or boosting a pre-selected set of frequencies at a pre-selected Q.

Parametric Equalizer

A type of equalizer having several "parameters" for control of various filters that can be applied to audio signals. Parametric equalizers are most widely used in situations where very fine control over the audio signal is desired. In order for an equalizer to be parametric it must at least have control over gain, Q, and frequency. In most cases each of these controls are on rotary potentiometers, but there are a few graphic style parametric equalizers on the market. Some equalizers have selectable frequencies that can be adjusted, but no Q control. These are known as quasi-parametric or sweepable equalizers.

Star Ground

A type of grounding scheme used in some studios to prevent ground loops. It requires isolating each piece of gear from AC ground (using a ground lift adapter) and running a separate ground wire from the chassis of each piece (including the racks themselves) back to the main studio ground (we call this "Technical Earth"). This "Tech Earth" gets connected back to a main AC ground, and/or a large copper rod driven 18 feet into the ground. Thus every piece of gear still has AC fault protection, but no earth grounds are tied together. Technically limited ground loops can still exist in the studio signal wiring, but the path length differences are minimized to an extent that it isn't likely to be a problem. Star Grounding is a time consuming and complex wiring scheme, but is generally very effective at preventing ground loops and works great in conjunction with other measures such as telescoping shields. Occasionally you'll still find some piece of gear that requires audio transformers to eliminate ground loops. Generally you'll find that with today's equipment, you really don't have to go as far as all this. Telescoping shields, balanced lines, and careful consideration to signal cable wiring with today's equipment is often good enough.

Electret

A type of microphone design, similar to condenser. Basically, there is a permanently charged plate in the mic element. As the diaphragm moves in response to sound pressure, it creates a changing capacitance with the plate. The big advantage to using electret (also called back-electret, or occasionally prepolarized condenser) technology is that it does not require an external polarizing voltage (battery or phantom power). In some cases, the microphone includes an impedance changing preamp that requires battery or phantom power, but the electret element itself does not require voltage. Electret mics can lose their charge in high humidity and high temperature environments, so some care should be used in storing and using them. If the electret loses charge, the mic's sensitivity will suffer, resulting in an reduced signal to noise ratio.

Ribbon Mic

A type of velocity microphone. A velocity microphone responds to the velocity of air molecules passing it rather than the Sound Pressure Level, which is what most other microphones respond to. In many cases this functional difference isn't important, but it can certainly be an issue on a windy day. Very old ribbon mics could be destroyed from the air velocity created just by carrying them across a room. A ribbon mic works by loosely suspending a small element (usually a corrugated strip of metal) in a strong magnetic field. This "ribbon" is moved by the action of air molecules and when it moves it cuts across the magnetic lines of flux causing a signal to be generated. Naturally ribbon mics have a figure 8 pick up pattern. You can think of it like a window blind; it is easily moved by wind blowing at it, but usually doesn't move when wind blows across it from left to right. Ribbon mics were the first commercially successful directional microphones.

C-weighted

A type of weighting curve designed into filters for equipment measuring sound output levels. the C-curve is basically "flat", with -3dB corners of 31.5Hz and 8kHz, respectively. It is designed to loosely correspond to how humans perceive sound at higher volume levels as indicated by curves such as the fletcher munson curvers.

dBu or dBv (little v)

A voltage measurement which has been taken with no concern for circuit impedance but is being reference to 0.775 volts as though the circuit were 600ohms (which may or may not be the case). Because circuit impedance is unknown the actual power is not defined and the measurement is, strictly speaking, not a true dB measurement. In pre transistor circuitry (tubes) most audio lines were terminated into a 600ohm impedance. In such a circuit, a zero-decibel reference level at one milli-watt is reached when there is a 0.775 volt drop across 600ohm. although 600ohm lines are seldom used anymore, the notation dBu is still used to specify a zero reference voltage of .755 volt. Remember, it is not a power or wattage ratio but represents a voltage ratio.

AFL

AFL (After Fade Listen) is used in mixing boards to override the normal monitoring path in order to monitor a specific signal at a predefined point in the mixer. Unlike PFL (see WFTD archive "Pre-Fade Listen"), the AFL signal by definition is taken after the fader of a channel or group buss such that the level of the fader will affect the level heard in the AFL monitor circuit. AFL is sometimes also taken after the pan pot which also allows the engineer to monitor the signal with the pan position as it is in the mix. AFL is a handy way to monitor a small group of related instruments by themselves with all of their eq, level, and pan information reproduced as it is in the overall mix. An AFL circuit that includes pan information is often called "solo" (see WFTD archive "solo") or "solo in place" depending upon who builds the mixer.

CMRR

Abbreviation for Common Mode Rejection Ratio. In balanced lines there is always a positive signal on one cable and a negative or opposite polarity signal on the other. The idea is that anything common to both wires will eventually get canceled at the receiving end (as noted in WFTD Balanced). The degree of cancellation at the receiving end, however, is dependent upon a number of factors relating to the integrity of the circuit involved. CMRR is a spec that defines to what degree common signals are canceled at the input of a balanced system.

FOH

Abbreviation for Front of House. This distinction comes from theater work where Front of House is any part of the theatre in front of the proscenium arch. In concert and PA applications it is generically used to describe anything in the audience area. Specifically the main, or house mixing position is referred to as the FOH position, which is meant to differentiate the main house mixer from the monitor mixer normally located to the side of the stage.

LED

Abbreviation for Light Emitting Diode. It is not an acronym. You do not pronounce it as "lead." It is pronounced EL - EE - DEE. An LED is an electronic component that glows when current passes through it. LED's are found in all sorts of electronic equipment these days from watches to laser disc players (in fact the laser that reads the disc is usually an LED). The lights that glow on top of your keyboard are almost surely LED's. The numerical readout and meters on your DAT machine are almost surely LED's. LED's do not always produce visible light. Infrared LED's are used in wireless remote control devices as well as things like wireless headphone systems.

TRS

Abbreviation for Tip Ring Sleeve. This is the descriptively accurate term used to describe 1/4″ (or 1/8″) balanced connectors. A TRS plug can be found at the end of most headphone cords if you want to know what one looks like. They look like a standard 1/4″ plug with an extra section in them. The three sections of the shaft are called the Tip, Ring, and Sleeve (a "standard" 1/4″ connector just has a tip and sleeve). TRS connectors are used wherever it is desired to have two conductors plus a ground (shield) in one plug. Common uses are as a way to connect balanced equipment (where the TRS plug has a positive, negative, and ground connection), or stereo unbalanced equipment (left and right are on the Tip and Ring, with a common ground) like headphones, or as an insert for your mixer or other processor (Tip or Ring is the send with the other being used as the return and again ground is common).

Inductor

Also called coil. Designed to provide a controlled amount of inductance. It is usually constructed by rapping a coil of wire around a ferrous material. Inductance is the ability of a device to store electrical energy in the form of a magnetic field. The inductance (directly related to impedance) of an inductor is directly related to frequency. At higher frequencies the effective impedance of an inductor will rise. At lower frequencies it will lower, until very low frequencies are reached where (because of the way they are constructed) they can sometimes become non linear.

Transient Voltage Suppressor (TVS)

Also known as spike protection. A semi-conductor device can be irreparably damaged in picoseconds with trillionths of a second of exposure to over-voltage. Some of these devices are designed to clamp the voltage and then return to normal operation while others destroy themselves in the process of protection.

Aux Return

An aux input used for bringing a signal back into a mixer. Aux returns are often used to bring the output of external effects, such as echo, delay, and reverb into a console.

Amplifier

An electronic amplifier is a device for increasing the power of a signal. It does this by taking power from a power supply and shaping the output to match the (relatively low power) input signal. This process invariably introduces some noise and distortion into the signal, and the process cannot be 100% efficient - amplifiers will always produce some waste heat. An ideal amplifier can be described as "a straight wire with gain", as the output is an exact replica of the input, just larger. Different designs of amplifier are used for different types of applications and signals. We can broadly divide amplifiers into three categories - small signal amplifiers, low frequency power amplifiers and RF (radio frequency) power amplifiers. Each of these calls for a slightly different design approach, mainly because of the physical limitations of the components used to implement the amplifier, and the efficiencies that can be realized.

Proximity Effect

An increase in bass or low frequency response when a sound source is close to a cardioid or bi-directional (Figure 8) microphone. Proximity effect is an exaggeration of low frequencies caused by the use of ports to create directional polar pickup patterns, so omni-directional mics are not affected. Depending on the mic design, proximity effect may easily result in a boost of up to 16 dB, usually below 100 Hz. Proximity effect increases as you get closer to the microphone, so the bass boost will vary depending on your distance from the mic. Vocalists tend to like proximity effect since it fattens up their voice, but a constantly varying bass boost can wreak havoc on headroom and carefully set levels! Obviously, if a vocalist is "eating the mic" to get proximity effect, the Inverse Square Law (WFTD 6/12) tells us that the levels the mic sees are increasing dramatically as well - distortion can easily result, from either mic diaphragm breakup or electronic overload. (You may occasionally see proximity effect referred to as "bass tip-up.")

Equalizer (EQ, EQing)

Based on the root word, equal, an equalizer is an audio device whose function is to equal out the tonal characteristics of a sound. At least that was the idea back in the days when they were first conceived as a tool used to get flat response in telephone lines and to make up for the deficiencies in audio equipment and acoustic spaces. Nowadays it could more aptly be named an "unequalizer" since they are more often used creatively to alter the relative balance of frequencies to produce desired tonal characteristics in sounds. An equalizer has the ability to boost and/or cut the energy (amplitude) in specified frequency ranges by employing one or more filter circuits. There are many different types of EQ's in use today in many widely varying applications, but they fundamentally all do the same thing.

Wavelength

Denoted by the Greek lower-case lambda symbol (l), the distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelength of any frequency may be found by dividing the speed of sound (approximately 1100 feet (or 34 meters) per second at sea level) by the frequency. Thus the wavelength of a 60hz sine wave would be approximately 18.3 feet. Knowing wavelengths of sounds is very important when designing or working with acoustic spaces such as studios, control rooms, and speaker enclosures.

Polar Pattern

Depending on their design and construction, microphones respond to sound coming from different directions with varying degrees of sensitivity. A plot or graph of this response is called a polar pattern (sometimes polar response curve). Looking at a mic's polar pattern will tell you how directional it is, how well it will reject sound from certain directions, etc. It is important to note that polar patterns are frequency dependent. Typically, low frequency response will be almost omnidirectional; the polar pattern will be come more directional as frequency rises.

Direct Current (DC)

Direct current is a current that always flows in the same direction; that is, the polarity never reverses. Direct current is very easily employed in circuits for doing work. It has the significant disadvantage of incurring significant losses when sent over long distances. DC is widely used in electronic circuits of all types to operate equipment but is never used to transmit power over any distance. 12V @ 0Hz

Trim

Found on most mixers, trim controls provide the initial level setting for each channel's input gain. In most cases, trim adjusts gain of the microphone preamp, but it may also apply to line level signals. Optimizing this gain stage will make a tremendous difference in the mixers signal to noise ratio and in gain staging later in the signal chain.

Impedance

Impedance is the resistance to alternating current (AC) flow, the behavior of AC circuits is somewhat more complicated than direct current (DC) circuits. In an AC system, the impedance depends not only on the resistance, but also the reactance. Reactance in turn varies with frequency.

Pre-Fade Listen (PFL)

In a console, pre-fade listen is a one of several possible means of overriding the normal monitor signal routing for various purposes. PFL generally sends a signal to monitor outputs regardless of the setting of that channel's fader, and simultaneously mutes the other channels. In other words, PFL allows you to solo a channel even if the fader is pulled all the way down. Note that on most consoles, this affects monitors only, and does not interfere with main, tape, or aux outs. In broadcast situations, PFL is often referred to as "cueing".

Voice Coil

In a dynamic loudspeaker the voice coil is a winding of wire around a cylinder that is attached to the main part of a speaker. The voice coil is inserted into the "gap" created between the magnet and pole piece of a speaker magnet assembly and, when current travels through the wire, generates a magnetic field that causes the speaker to move against the permanent magnet. The alternating current in the voice coil causes alternating magnetic fields to form which interact with the permanent magnet to cause the speaker to move back and forth producing sound.

Bus

In audio (not transportation) terms, a Bus is a point in a circuit where many signals are brought together. For example: Most electronic items have a Ground Bus where all of a device's individual ground paths are tied together. In mixers, we have Mix Busses, where multiple channels' signals are brought (or blended) together; Aux Busses, where feeds from channels are brought together to be routed to an external processor or monitor send, etc. In general, the more busses a mixer has, the more flexible the routing capabilities of that mixer will be.

Sensitivity

In audio terms, sensitivity is the minimum amount of input signal required to drive a device to its rated output level. Normally, this specification is associated with amplifiers and microphones, but FM tuners, phono cartridges, and most other types of gear have a sensitivity rating as well. In general, higher sensitivity is better (less input signal required for full output), but there are definitely situations where a device can be TOO sensitive (picture a very sensitive microphone in front of a wound-up Marshall guitar amplifier!) resulting in unwanted distortion.

Subgroup

In audio, a subgroup is a group of sources (channels, tracks, etc.) that are combined into one bus on a mixer. The term is very closely related to, but not the same as, submix. Subgroups are created for a variety of reasons, most of which have to do with convenience. For example, it is very handy (especially when mixing live) to have sets of common sources grouped together into a subgroup so they can all be controlled with one fader. It is common to create drum sub, a keyboard sub, a backing vocal sub, etc.

Balanced

In audio, the opposite of Unbalanced. For us balanced refers to a type of AC electrical signal having two "legs" independent of ground. One is generally considered positive (+) and the other negative (-) in voltage and current flow with respect to ground. Unlike unbalanced audio lines there is no "signal" carried in the shield or ground connection unless there is a fault. The main benefit is that any noise that gets induced into the line will be common to both the positive and negative sides and thus canceled when it arrives at its destination, assuming the destination is balanced. This phenomenon is called "Common Mode Rejection" and basically just means that any signals common to both the positive and negative legs of balanced lines get canceled. This happens because when the receiving device looks at the signal the common noise actually shows up as out of phase with itself, and gets cancelled. Think of it as if the negative (-) signal gets inverted to positive (+) before use, which puts the desired audio signal in phase with the already positive other leg and at the same time causes the undesired common noise to become out of phase with itself. Clear as mud? Balanced lines are generally much better for long cable runs due to their ability to reject induced noises. XLR and TRS type cables are designed to transmit balanced audio from one balanced device to another. A standard 1/4-inch guitar cable is an example of an unbalanced cable. Another (newer) application of balancing that is becoming popular in audio systems is the idea of balanced power systems. Fundamentally the concept is the same. There is a positive and negative (with respect to ground) leg of electricity at every electrical outlet. The idea is that the power supply of any devices connected can then reject any noise induced on the AC line and thus will produce cleaner audio. We'll talk more about balanced AC systems in the future.

Amplitude

In physics and electronics amplitude is literally the maximum absolute value of a periodically varying quantity. In layman's terms it is the strength of a signal or sound without regard to its content. Amplitude measurements of audio signals generally refer to the signal voltage, which is only one component of what determines power (watts), or the ability to do work. Thus it is important to understand that amplitude alone does not singly determine power (or loudness in audio), but does affect it. In the physical world the amplitude of a sound is measured in dB of SPL (Sound Pressure Level), which again does not define the true sound power or intensity (many people are confused about this), only the sound level at a point in time. We'll cover this in more detail later when we define those words.

Diaphragm

In the audio world, diaphragm refers to the component in a microphone that vibrates sympathetically with air disturbances such as sound waves. It is typically a circular shaped very thin piece of mylar or other delicate low mass material that will range from .2 to 2 inches in diameter. When the diaphragm in a microphone vibrates it generates an electrical signal often by either moving an attached coil of wire in and out of a magnetic gap (in the case of moving coil microphones) or by changing the distance between it and another electrically charged plate (as in condenser microphones). These electrical impulses are then present at the output of the mic and ready for amplification as an audio signal.

Omnidirectional

Literally, from all directions. In audio, microphones are said to be omnidirectional if they can detect sound equally from all directions. Speakers are omnidirectional if they radiate sound in all directions equally; this tends to be the case with subwoofers and low frequency drivers. Low frequencies, in general, tend to be omnidirectional, versus high frequencies which tend to "beam" or be very directional.

Line Level

Literally, the average voltage of an electronic audio signal. While technically any voltage over 25 millivolts RMS is considered line level, in the modern audio world we narrow the scope a bit to the two line level references in use today: Balanced "pro" gear runs at around +4 dBm (1.23 volts), while unbalanced "semi-pro" gear operates at approximately .316 volts (-10 dBV). "Pro" and "semi-pro" may be almost meaningless terms anymore, but the two operating levels must still be dealt with. The important thing is to match the levels of the gear you are using so that -10 equipment isn't directly feeding +4 equipment, and vice versa. If you use gear of both levels, there are various level matching devices on the market to properly interface the items.

Nominal Level

Nominal implies that something is according to plan, with only insignificant differences. A nominal level implies a "normal" or, perhaps, typical level in equipment. The nominal operating level of a piece of equipment is thought of as the typical signal level with which it operates. Though this is somewhat vague, the phrase often gets generically used in audio to specify a signal level. For example, on equipment with +4 dBu inputs and outputs the nominal operating level is said to be +4 dBu. This level, which is also its zero reference level, is what it is designed to deal with in terms of typical audio program material. There is sufficient headroom above this level to accommodate peaks or loud sections of audio without distortion. When we refer to nominal levels in audio equipment we are generally referring to zero reference levels. The two phrases are often used interchangeably even though "zero reference" is much more precise.

Direct Box

Often abbreviated DI (for direct insertion), a device commonly used to convert high impedance (see WFTD archive Impedance) unbalanced signals from a stage instrument (like a bass or keyboard) into a low impedance balanced signals. This puts the signal at the proper voltage level for the mixer and prevents the instrument from becoming loaded down with too low impedance, which could cause tonal shifts and distortion. It also allows the signal to be transmitted over long lengths of cable. They are always used in live sound to get a signal out of an instrument on stage out to the mixing board, which can be as much as 500 feet away. A simple direct box consists only of a small transformer, but more sophisticated designs employ electronic gain stages that more closely resemble the input section of a modern instrument amp. They may also have some combination of ground lift switches, equalization switches, level matching switches, isolated line outputs, and more.

Ohm's Law

Ohms law is a simple raltion between the current, voltage, and resistance in a circuit. Simply put, a resistance of 1ohm will conduct (allow to flow) 1amp of current when a voltage of 1V is placed across it. This relationship is illustrated by the following formula: I=Current in amps, V=Voltage in volts, R=Resistance in ohms. I=V/R V=IR R=V/I

Instrument Level

One of four audio signal levels present in studios and live sound systems - the others being mic, line (two flavors), and speaker level. Instrument level is the level (or voltage) put out by an instrument such as an electric guitar, an acoustic guitar with a pickup, or an electric bass guitar. There is no standard for instrument level; it is assumed to fall between mic level (on the low end) and line level (on the high end), and it can range from a few millivolts for passive or piezo pickups to several volts on instruments with active pickups and built-in preamps.

Active

Opposite of Passive. In the world of music technology passive devices are those things that only subtract elements from a signal. An active device has its own power and can, if necessary, add to or amplify a signal in some way. Active devices have the advantage of being less "lossy" in how they take care of signals and can produce better results under the proper circumstances. The disadvantage to most active devices is that they usually add some amount of noise and distortion to a signal (op amps always add noise and distortion), and may color the sound in ways some passive devices may not. Active devices also tend to be more expensive due to the cost of components and because a power supply is required for them to operate.

Telescoping Shield

Refers to a shield configuration in shielded balanced cables. When the cable shield is connected at one end of a cable, but left "floating" or unconnected at the other it is said to be telescoping. A cable's shield does not necessarily have to be connected to ground at both ends in order to provide effective shielding so they are often telescoped in studio wiring to reduce the likelihood of ground loop problems. Which end of a cable's shield gets left floating (the source end or the destination end) is a subject that is debated in the tweaky circles of studio designers, but most people lift it at the source.

Off-Axis

Refers to an audio source that is not directly in front of a transducer, especially a microphone. This results in off-axis coloration; a distortion or change in the frequency response of the reproduced audio signal. Often this coloration is put to good use. For example, many engineers intentionally set up mics on guitar amps so that they are slightly off access to control the amount of high frequencies captured. A microphone will generally produce the "truest" results if it is used on-axis (oriented directly in front of the sound source).

Channel Strip

Refers to one channel of a mixing board. Due to the layout of most mixers - channels in columns across the face with the functions of each channel arrayed from top to bottom - a channel is sometimes referred to as a strip. Over the years some mixers have been so highly regarded for their sound quality that a market was developed by marrying a channel strip to a power supply and I/O connections for stand alone use. One could plug a mic directly into the channel strip and record directly to a recorder without having to take the big mixer to the location. Even when such a mixer is available the channel strip approach is often used because it is thought that the simpler signal path of the external strip produces a more pure audio signal to record. This is similar to the popular approach of using high quality stand-alone mic preamps as the only device between mics and recorders, with the major difference being that a channel strip has many more capabilities than a simple mic amp. It may have EQ, compression, gates, and more, depending upon the unit. The idea is that it's everything you're likely to need in the signal path to make a great recording. Channel strips are so popular these days that they are generally considered to be a separate product category from preamps and other processing.

Large Diaphragm

Refers to the size of the diaphragm used in a microphone. Any microphone with a diaphragm larger than (and potentially including) 3/4″ is considered to be a Large Diaphragm microphone. In general, Large Diaphragm microphones tend to have a "big" sound that engineers find especially pleasing where a little more character might be advantageous, such as is the case with most vocals. Large diaphragms are generally more sensitive than small diaphragm or medium diaphragm mics because of the increased surface area. A common myth is that large diaphragm mics capture more low frequencies than small diaphragm mics. Sometimes their coloration may make it sound like this is the case, but a properly designed small diaphragm mic is more likely to be accurate throughout a wide range of frequencies, whereas the coloration of a large diaphragm mic can tend to enhance certain desirable characteristics in a sound, which sometimes amounts to more apparent bass or low end.

Small Diaphragm

Refers to the size of the diaphragm used in a microphone. While there are no final standards regarding a diaphragm size that defines Small Diaphragm, most professionals and manufacturers agree that any diaphragm smaller than 5/8″ would be considered a Small Diaphragm. Generally speaking, Small Diaphragm microphones tend to do a good job of capturing high frequency content and transients. They will tend to have a bit more "air" to their sound and often have less coloration than medium diaphragm or large diaphragm microphones. Most of this is due to the reduced mass of the smaller diaphragm, which allows it to more closely follow any air disturbances it is subjected to.

absorption coefficient

Represented by the Greek letter Alpha, a measure of the relative amount of energy that will be absorbed when a sound hits a surface. Absorption coefficients are always a value ranging from 0 to 1 that when multiplied by the surface area in question yield a percentage of sound that will be absorbed by that surface. This percentage is in units known as Sabins, after the Harvard professor and acoustician, Wallace Sabine. An absorption coefficient of 1 means that all acoustic energy striking the surface will be absorbed and none reflected. A coefficient of 0 means that all the energy is reflected. The latter condition is virtually impossible and the former condition is rare. The absorption coefficient varies by frequency because most materials have different absorption characteristics at different frequencies. Acousticians use absorption coefficients to help determine the RT-60 or reverberation time of rooms, and as such many common building materials have been measured and results published for their absorption coefficients.

dBW

Same as dBm except referenced to 1 watt the dBW is related to the dBm as follows: dBW = dBm + 30

Aux Send

Slang for Auxiliary Send, a circuit pathway (or bus) in a mixing console that supplies an independent mix, which can be routed to an external (auxiliary) device such as an effects processor or monitor system. Most modern consoles have several aux sends on each channel so several devices can process the input to any channel or groups of channels.

Impedance Balancing

Sometimes also called pseudo-balancing, quasi-balancing, resistor balancing, ground compensated balancing, or any of a dozen other similar names. There are some differences between these, but basically it's a method of creating an output that will function similar to a balanced output without having to employ all of the electronics normally required in a fully (differentially) balanced system. The benefit is reduced cost and sometimes increased overall flexibility without losing much performance in many situations. Here's what it is and how it works: In a truly balanced output there are two conductors and a ground wire. The two conductors each have the output signal on them, but at opposite polarities. Refer to our WFTD, Balanced, for more background on this. Having this signal on both wires is beneficial for several reasons, but one of the biggest benefits is just in having the two wires, even if signal is only on one of them. A balanced input is able to look at the hot and cold wire's signal and compare the difference between them. Any differences get amplified; any signal that's the same between the two gets cancelled. That means noise and hum picked up in the wires along the way will get cancelled, and any signal - even if it's only on one wire - will get amplified. It's a great system, but comes at a cost in terms of the components required. If you've understood everything so far you can now see that you can drive an unbalanced output into a balanced input and still get much of the benefit of the CMRR (Common Mode Rejection Ratio) of that input. In such a configuration it is common to tie the 'cold' wire going to the balanced input directly to the ground at the source end of the wire. Any noise picked up along the wire can get cancelled at the other end. However, because the cold wire is tied to ground the noise picked up is more or less killed right there, or 'shunted' to ground as they say. Consequently most of it doesn't show up at the balanced input, which never gives that input device the opportunity to use it to cancel the same noise that was pickup up on the 'hot' signal wire. Further it opens up the possibility for noises on the ground line to get into your audio through the negative side of that balanced input. In a ground compensated system that cold wire is not tied directly to ground. Instead a resistor is placed in between the cold and ground so that noises picked up in the wire are NOT shunted off to ground. Now they appear at the other end just as they would if the line were a differentially balanced line. They get cancelled and the noise mostly goes away. The effectiveness of this scheme is largely dependent upon the CMRR ratio of the input device and the accuracy of the resistance in simulating a source impedance that's the same as the hot wire. This is not a truly balanced system, but in terms of noise cancellation behaves similarly. If you are working with a device that employs impedance balancing you should connect it to other balanced devices just as if it's a normal balanced output with hot, cold, and ground leads. If you are connecting its output to an unbalanced device you can use a standard unbalanced cable (which actually makes it easier than connecting a balanced output to an unbalanced input [what to do with that third conductor]). Some equipment can use its 'cold' output terminal as what is known as a 'ground sense' line. Thus even when driving an unbalanced input (where you'd have the cold terminal tied to ground) this sense line is able to add any noise or hums from the ground back into the hot signal where they would get canceled at the other end. They get canceled there because the same signal would be appearing at the ground of that device (since it came from the ground wire in the first place). In unbalanced inputs, ground serves as a reference against which the signal on the hot lead is taken. If the hot and ground have the same waveform on them then nothing appears at the device to amplify, and again the noise ends up getting canceled. This isn't as effective as a balanced input, but the performance is certainly better than a typical unbalanced line.

Dynamic Range

The Dynamic Range of a program, system, or component is the range of volume levels from softest to loudest. It is a loosely defined measure by itself and is therefore beset accompanied by another descriptive term to define its meaning. It is often equated with signal to noise ratio but these are not always the same thing. The signal to noise ratio of a device is easily described as the level difference between the noise floor of the device and its maximum output level. The meaning of dynamic range is also subtly changed depending on whether the term "maximum" is employed. The "maximum" dynamic range is closely related to the signal to noise ratio. In a device, this is probably not its useful dynamic range since one is neither concerned with a signal that is at the noise floor or at clipping. The useful dynamic range implies an undistorted and noise free signal and is therefore considerably less than the maximum dynamic range.

Sound Pressure Level (SPL)

The acoustic volume or loudness of sound, measured in decibels. SPL is a function of a signal's amplitude. Aside from the usual (and justified) warnings about hearing damage from high SPLs, it is worth noting that because of the way our ears function, sounds will appear to have a different timbre (or tone) to us at different SPL levels. This is important to keep in mind, especially when mixing in a studio environment. Be sure to check your mixes at a variety of volume levels to ensure that the mix is accurate. The old rule of thumb is that if a mix sounds good at a low SPL, it will sound great at higher levels...

Condenser Microphone

The condenser microphone is a very simple mechanical system, with almost no moving parts compared to other microphone designs. It is also one of the oldest microphone types, dating back to the early 1900's. It is simply a thin stretched conductive diaphragm (metal or gold sputtered mylar) held close to a metal disk called a backplate. This arrangement basically produces a capacitor, and is given its electric charge by an external voltage source. This source is often phantom power, but in many cases condenser mics have dedicated power supply units. When sound pressure acts on the diaphragm it vibrates slightly in response to the waveform. This causes the capacitance to vary in a like manner, which causes a variance in its output voltage. This voltage variation is the signal output of the microphone. There are many different types of condenser microphones, but they are all based on these basic principles.

Input Impedance

The impedance present at the input of a device. The input impedance affects the signal entering the device coming from a connected external device. Passive ribbon microphones, for example, generally perform best when connected to a preamp with a high input impedance.

Mic Level

The level (or voltage) of signal generated by a microphone. Typically around 2 millivolts. Compare this with the two normal line levels (1.23 volts and .316 volts), and it becomes apparent just how much amplification is going on in a microphone preamp, and why it is essential that preamps be of as high quality as possible!

Capsule

The portion of a microphone that converts acoustic energy to electrical energy. The capsule usually includes shock mounts, acoustic isolators, protective covers and electronic circuitry in addition to the basic transducer. Also called an element. It's basically the 'heart' of any microphone.

Voltage Regulation

There are many ways to provide voltage regulation. There is usually one in every piece of electronic equipment. The choice of which type to use is a compromise between cost, size, heat, amount of protection, and to what type of load the voltage is being delivered. Each type has it's own set of characteristics making it more or less appropriate for various situations.

Overcurrent Protection

This is normally accomplished with fuses and circuit breakers which open (breakers) or destroy themselves (fuses) in the event of an over-current situation. This is known as thermal current protection. The other type is the electromagnetic protector. This is often used in high power circuits. The disadvantage of fuses (and to some extent breakers) is that they are relatively slow.

2:1 Rule of Ambience

To capture an equal amount of room ambience, a cardioid microphone must be placed twice as far from a source as an omnidirectional pattern microphone. Keep this in mind the next time you are trying to capture some of a room's natural sound when recording!

dB SPL

Used to describe acoustic sound pressure levels. Acoustic sound pressure levels measured in dB are commonly referenced to a level of .0002 microbars. This is the sound level threshold of human hearing. The SPL in decibels is the number of decibels above this threshold.

dBV

Used when showing decibel data measure in volts as above. Although the decibel is always a ratio of two power levels, it may be conveniently calculated from a ratio of two measured voltages, provided the resistance is unchanged. The notation dBV is used to specify that the zero-reference voltage is 1 volt.

Inverse Square Law

Useful when setting up a microphone or speaker, the inverse square law states that, in a free field the intensity of sound drops by 6 dB for each doubling of distance from the source. Now, none of us ever work in a truly free field (no reflective surfaces), but for most applications these numbers are accepted as workable. In real world terms, this means that for each time you double the distance between your sound source and a listener or microphone, the power of the audio drops by 75% - a fairly significant amount! How much is this in terms of volume? Well, it depends on the source you consult, we've seen both 6 dB and 10 dB convincingly listed as doubling or halving the volume (let's just say it's subjective and leave it at that...) - regardless, 6 dB is a very noticeable drop in level! Consider this the next time you place a microphone or speaker: Rather than just cranking up or attenuating the mic preamp or amplifier level for gain control, look at the distance to your source...

Voltage

Voltage is the existence of a potential (charge) difference between two objects or points in a circuit. Analogous to pressure. Voltage by itself does no work - it merely represents the ability to do work.

Alternating Current (AC)

Whenever electrons in a conductor flow in both directions, alternating current exists. AC is in most cases not very useful for doing work and must be rectified to DC in order to be useful. AC does have the advantage of being able to be sent long distances without significant losses. AC is mostly used for power distribution and is rarely used to actually operate equipment. (It is usually converted to DC first). 120V @ 60Hz

equivalent input noise (EIN)

a rating of the overall noise performance of an amplifier and typically a microphone preamplifier. in mic pre amps this measurement is normally made with a 15ohm resistor load on the pre amp to simulate the load of a 150ohm microphone. The theoretical limit in this scenario is -1131.8 to -130.9 dBm because this is the thermal noise power of the resistor.

GFI (ground fault interrupt)

compares the current on the Hot lead to the current on the neutral lead. If there is any difference then that means there is a problem and the extra current is being shunted to ground. This can be a very dangerous condition and should.

Drop Out Relay

in the event of a power outage or under-voltage (brown out) a relay trips which will interrupt the voltage supply and turn the unit off until manually reset.

Overvoltage protection

self explanatory. too much voltage on the line can cause electronic components to fail.

Tube

similar in function to a diode or transistor. widely used prior to the advent of semiconductor technology. Tube circuits usually must function in situations of much higher voltages and lower impedance than solid state circuits.

A-weighted

sound measurement most accurate to human hearing curve. large band pass like curve from 20Hz - 20kHz

dBA

the measure of a test signal which has passed through an "A" weighted filter.

dBm

used when showing decibel data in milliwatts. in circuit theory, the zero-reference level is usually one milliwatt. however, since other reference levels are sometimes applied, the notation dBm is used to clearly indicate that one milliwatt is the zero-reference level. Also, unless otherwise stated the impedance of the measured circuit is assumed to be 600ohm.