Audio Signals & Systems Test #2

What problem can occur as the result of an impedance mismatch between devices?

A problem that can occur as the result of an impedance mismatch between devices is that the frequency response is being altered.

What type of signal represents the most simple pattern of vibration?

A sinusoidal (or sine) wave, also called the simple harmonic motion, is the most simple form of vibration.

When looking at a complex wave (such as in the waveform view of a track in Pro Tools), how can one tell between a waveform with a lot of high-frequency content and one without?

A waveform with a high-frequency content is more jagged while a waveform without is smoother.

What is the primary function of a step-down transformer?

The primary function of a step-down transformer is to reduce the voltage that is going into a device.

What is the voltage and frequency of a typical AC power circuit (i.e., US wall outlet).

The voltage and frequency of a typical AC power circuit is 120V and 60 Hz.

Why is the word "analog" used in the context of audio recording?

"Analog" is used in the context of audio recording when talking about audio that is recorded in either a magnetic, mechanical, or optical form.

If electricity is modeled using the analogy of water, how is "resistance" conceptualized?

"Resistance" is conceptualized because it is similar to a valve. It can increase or decrease both its flow and pressure.

Given the list of typical ensemble instruments shown in Figure 6.30, what frequency range encompasses (roughly) the "tone" portion for all of the instruments?

Anywhere between close to 100Hz and 700Hz.

When two speakers with the same impedance (e.g., 8Ω) are wired in series to the same amplifier, how much of the total voltage from the amplifier goes across each speaker?

Half of the voltage is what goes through each speaker.

The text discusses harmonics but not overtones. What is the difference between these two terms? (You might have to do some web research to determine the answer

Harmonics are any of the essential multiplications of the fundamental frequency while overtones are any frequencies greater that the fundamental frequency.

Generally speaking, do higher harmonics in a complex waveform contain more or less energy (i.e., have more or less relative amplitude) than lower harmonics?

Higher harmonics in a complex waveform contain less relative amplitude than lower harmonics.

When taking the base-10 logarithm of a number, what value gives a negative answer? Why?When taking the base-10 logarithm of a number, what value gives a negative answer? Why?

If the value between 0 and 1, it will give you a negative number. Any value greater than 1 will give you a positive number. This is because the numerator is less than the denominator.

Name some benefits of bridging impedances as compared to matching impedances.

Most of the voltage of bridging impedances transfer to the input. A device can also connect to multiple inputs with no voltage loss, which causes distortion.

If a guitar amp is being recorded by a close mic and a room mic, why does moving the room mic change the overall frequency response of the recorded guitar amp?

Moving the mic changes the overall frequency response of the recorded guitar amp because the different delay times between the close and room mics shift the comb filter to different parts of the spectrum.

Looking at Table 9.1, what is the difference (in dB) between the quietest sound that can be heard and the loudest sound that can be heard comfortably?

The difference between the quietest sound that can be heard and the loudest sound that can be heard comfortably is 120 dB because that separates the threshold of hearing from the threshold of pain.

If a guitar and a piano both played the same exact note (e.g., A440), what is responsible for the difference in perceived timbre between these two instruments?

The difference in amplitude of the harmonics is what's responsible for the difference in perceived timbre between those two instruments.

What effect does constructive interference have on amplitude?

The effect constructive interference has on amplitude is that it makes the amplitude double.

What is the speed of sound in air, roughly? Give both feet-per-second (ft/s) and miles-per-hour (mph). (You may have to Google this or use a calculator.)

The speed of sound in air is about 1,130 ft/s or 770 mph.

Based on the answer to the previous question, how many decibels represent a perceptual doubling of loudness?

10dB is how many decibels represent a perceptual doubling of loudness.

Which speaker will put a heavier load on an amplifier: a 4 Ω speaker or an 8 Ω speaker?

A 4 Ω speaker will put a heavier load on an amplifier. This is because the load increases while the resistance decreases.

In what situations would the Inverse Square Law not entirely hold true?

It wouldn't hold true where reflections and reverberation amplify the signal.

What is the basic principle behind additive synthesis?

The basic principle behind additive synthesis is that periodic sounds can be defined by the frequency and amplitude of the fundamental.

Given the difference between the lowest and highest audible frequency, what is the range of wavelengths for sound moving through air, generally speaking?

The range of wavelengths for sound moving through air, or general speaking, are. 57 ft or 0.7 in.

What happens when you take the base-10 logarithm of a negative number? Why?

There isn't a real number that can be raised by a positive number and get a negative number as an outcome.

How loud is 0 dB?

0db is as loud as whatever it's being compared to.

How is a comb filter created, and what effect does it have on an audio signal?

A comb filter can be created through either a short delay under 50 ms or identical signals. The effect a comb filter has on an audio signal is that it can cause certain frequencies to increase amplitude while causing others to decrease.

Which creates a greater change in decibels, doubling the voltage or doubling the wattage?

A doubling of the voltage creates a greater change in decibels because power increases by the squaring of voltage.

What is the heaviest load that could be put on an amplifier? What is the lightest load that could be put on an amplifier?

A resistance-less wire would be the heaviest load that could be put on an amplifier. Not connecting anything to the amplifier output would be the lightest load that could be put on an amplifier.

What is the difference between a transverse wave and a longitudinal wave, and which one describes sound in air? Which one describes sound as depicted in Pro Tools?

A transverse wave is a wave that moves perpendicular while a longitudinal wave is a wave that pushes and pulls in the same direction the sound travels. You could also say that a transverse wave moves up and down while a longitudinal wave moves back and forth. A transverse wave describes sound as depicted in Pro Tools. If you think of a guitar string, it moves up and down when you pluck it. This is how the sound is produced.

Does a typical battery produce direct current (DC) or alternating current (AC)? Why?

A typical battery produces a DC current because one pole is positive and the other is negative, which allows a one-way-flow of current.

Since most modern amplifiers are constant-voltage devices, how might we best conceptualize what happens when we change the load (i.e., impedance) on the amplifier with regard to power (or current) output? How does this differ from our basic analogy for electricity?

Amplifiers can be conceptualized as dams with a constant pressure on the walls of it. This then releases water (or a current) when plugging in a speaker. This differs from our basic analogy for electricity because it's changing the resistance.

What is the difference between "amplitude" and "loudness" in terms of a periodic signal?

Amplitude is a more technical description of how much sound change occurs, while loudness is a more subjective measurement to each individual.

Given two different sound sources that are producing identical sine waves, what would cause the sine waves to be "out of phase" when they reach the listener?

Any distance difference between the listener would cause sine waves to be "out of phase."

As resistance increases, does the electrical "load" increase or decrease? Why?

As resistance increases, the electrical "load" decreases, but with an increase in resistance. This is because a lower resistance causes more electrons to flow, which makes a greater current.

What frequencies are baffles generally ineffective at stopping, and why? How should we handle these instruments in the studio?

Baffles are generally ineffective at stopping at lower frequencies. This is because they need a natural outlet or else it creates an overall "boomy" or uneven sound.

Why do comb filters have what Case (the author) describes as a "not necessarily musical" sound on the affected signal?

Comb filters have boosts that cut linear multiples in frequency domains, but our ears hear frequency relationships as a logarithmic.

How is crest factor measured, and what is it meant to assess?

Crest factor is measured by the ratio between the peak and rms level. It is meant to assess percussive or transient sounds.

According to the Inverse Square Law, every doubling of distance corresponds to how much of a decrease in level, as measured in decibels? Also, what formula models this decrease?

Every doubling of distance creates a 6 dB decrease in level. The formula that models this decrease is dB=20 log (V2/V1)

By what factor does the power of an amplifier have to increase to make it sound "twice as loud"? How many Watts, for example, is twice as loud as a 100W amplifier?

I would take a 10dB increase in power to create a sound twice as loud. Double wattage = 3dB Double voltage = 6dB

Given two periodic waves of the same frequency, how many degrees offset are the waves if they are "in phase"? How many degrees offset are the waves if they are "out of phase"?

If the waves are "in phase," it would be 0 degrees. If the waves are "out of phase," it would be anything up to 360 degrees.

How fast does sound travel when it is in electrical form, moving through a wire?

In an electric form, sound essentially travels at the speed of light or 186,000 miles/sec.

Why is it meaningless to say that "the average level of a typical rock concert is 110 dB," assuming this statement is taken literally?

It's meaningless because decibels only have meaning in comparison to something else. This quote makes it not clear what the concert is 110 dB more than.

What do the letters "JND" stand for, and what do they represent, both in general terms as well as specifically?

JND stands for Just Noticeable Difference. It represents the smallest difference in loudness (about 1dB amplitude) that can be perceived.

Between 0 and 50ms, does lengthening the delay time—like from 1ms to 2ms—cause more audio frequencies to be cancelled out or fewer audio frequencies to be cancelled out?

Lengthening the delay time causes more lower end audio frequencies to be cancelled out.

How can one create a square wave from a combination of sine waves?

One can create a square wave from a combination of sine waves by using only odd harmonics with each one at an amplitude of 1/n, with n being the harmonic.

Why might one periodic sound wave sound louder than a second periodic sound wave that has the identical peak voltage? What would be an example of this?

One periodic sound wave might sound louder than another periodic sound wave that has the identical peak voltage because the amplitudes of the waves might be different. An example of this would be a sine wave.

What is the difference between the peak amplitude of a signal and the peak-to-peak amplitude?

Peak amplitude is measuring the highest point above zero reached by the wave in a given cycle while peak-to-peak amplitude is measuring the difference between the highest and lowest points in a given cycle.

Do phase offsets get larger or smaller as frequency decreases, given a particular delay time? E.g., with a 5ms delay, will low frequencies have more phase shift than high frequencies?

Phase offsets get smaller as frequency decreases, given a fixed delay time. Lower frequencies won't have more phase shift than higher frequencies.

What does "phase" refer to in terms of a periodic waveform, and how is phase measured?

Phase refers to its current position with respect to the completion of a full cycle. Phase is measured in degrees.

What is the difference between "power" and "intensity" of a sound?

Power is measured per unit of time and intensity is measured per area or second.

Why does RMS stand for, and how is an RMS calculated?

RMS stands for root-mean-square. An RMS is calculated by taking a large number of samples of the sound wave at regular intervals, squaring the samples to remove negative values, adding those up, dividing the sum by the total number of samples, then square rooting the result.

What is "reactance," and how does it relate to impedance?

Reactance is the frequency-based measurement of a type of opposition to AC electricity because of capacitance or inductance. Impedance is related to reactance because reactance has a phase angle of plus or minus 90 degrees. When that value is added to the resistance of the circuit, you get impedance.

Why does simply recording an electric guitar amp with a single microphone typically involve comb filter effects?

Recording an electric guitar amp with a single microphone typically involves comb filter effects because the sound of the amp going to the microphone combined with a slightly delayed version of the sound that bounces from the floor.

What are some important differences between resistance (R) and impedance (Z)?

Resistance is the opposition to current flow while impedance is the opposition to the alternating current flow. Also, resistance is used for direct currents (DC) while impedance is used for alternating currents (AC).

Why is the peak amplitude of a signal not a good indicator of loudness?

Some signals don't spend as much time at their peaks than signals that are more sustained.

Given a particular frequency, what factors affect the wavelength of the sound wave?

Some. Factors that affect the wavelength of the sound wave are the speed of sound and temperature or humidity changes.

What is the essential assertion of the Inverse Square Law?

Sound dissipates from the source as its intensity decreases in proportion to the inverse of the square of the distance away from the sound source.

What is sound?

Sound is disturbances in the air, which then produces vibrations. Those disturbances then expand away from the sound source in a ripple type effect.

What is the highest frequency in a square wave of 125 Hz?

Square waves have a infinite frequency bandwidth.

In a real-world sound, what is the primary difference between the beginning (attack) portion of the sound and the steady-state (sustain) portion of the sound?

The amplitudes are usually changing pretty rapidly during the attack portion of the sound compared to the sustained portion of the sound.

What is the basic principle behind a Fourier transform? (Wikipedia might help here, too.)

The basic principle behind a Fourier transform is that it is a function of frequency that's magnitude represents how much frequency is present in the original function, as well as whose argument is the. phase offset of the basic sinusoid in that frequency.

Conceptually, most modern amplifiers are considered to be "constant voltage" devices, in that they will theoretically produce a constant voltage across various loads. So as the resistance of the load decreases, what happens to the power it dissipates? (See Table 8.1.)

The dissipated power will decrease as the load decreases.

Alex Case (the author) says that delay times under 50ms are not heard as an echo but rather as something else. What distance between two microphones would correspond to a 50ms delay? What frequency has a period of 50ms?

The distance between two microphones that would correspond to a 50ms delay would be 56.3ft. The frequency that had a period of 50ms is 20Hz.

What effect does destructive interference have on amplitude?

The effect destructive interference has on amplitude is that it makes amplitude become zero.

What electrical concept is analogous to "pressure," and what term reflects "zero pressure"?

The electrical concept that is analogous to "pressure" is voltage. The term that reflects "zero pressure" is electrical ground.

What electrical concept measures the flow of electrons (akin to the flow of water in a pipe)?

The electrical concept that measures the flow of electrons is current.

What is the essential gist of the "3-to-1" rule in microphone placement?

The essential gist of the "3-to-1" rule is that there is no significant leakage from the secondary mic and the little that does leak through can be realistically ignored if the distance of the secondary mic from the source of sound is 3 times greater than the distance of the main mic from the source of the sound.

If a complex wave is composed of the frequencies 135 Hz, 270 Hz, and 540 Hz, what is the overall frequency of the complex wave?

The fundamental is 135 Hz and the harmonics and overtones are 270 Hz and 540 Hz because they're doubles of the fundamental.

Why does the intensity (or "loudness") of a sound decrease as the as distance from the sound source increases?

The intensity of a sound decreases as the distance from the sound source increases because the sound spreads out more as the sound goes further from the source. (power is distributed a bigger and bigger area)

What is the level difference in dB between 0 dBu and 0 dBV. (See Table 9.3 for reference.)

The level difference between 0 dBu and 0 dBV is 2.2 dB.

If one speaker is connected to an amplifier, and then a second speaker is connected in series with the first speaker, does the load increase or decrease?

The load (or power) would decrease because less of the current would flow since there is more impedance that has been added to the circuit.

If one speaker is connected to an amplifier, and then a second speaker is connected in parallel with the first speaker, does the load increase or decrease?

The load would increase because more of the current would flow since there would be less impedance that existed in the circuit.

In the decibel equation for power (Equation 13.3 on page 311), why is the logarithm of the power ratio multiplied by 10?

The logarithm of power ratio is multiplied by 10 because you have to multiply by 10 to get dB.

What is the mathematical average voltage for any sine wave with a consistent peak amplitude?

The mathematical average voltage for any sine wave with a consistent peak amplitude is zero volts.

What is the period of a sound wave? Do all sound waves have a period?

The period of a sound wave is a sine wave's waveform repeated exactly over time which then results in predictable and measurable characteristics. Yes, all sound waves have a period.

If one speaker is connected to an amplifier, and then a second speaker is connected in parallel with the first speaker, does the total power dissipated increase or decrease? (Assume that the amplifier is a constant voltage source, i.e., that its voltage output is constant.)

The power output would increase as the load increases.

What is the primary purpose of a rectifier in the power supply stage of a device?

The primary purpose of a rectifier in the power supply stage of a device is to convert AC current into DC current.

What is the professional standard operating level (i.e., 0 VU) as measured in dBu? What is the semi-professional standard operating level (i.e., 0 VU) as measured in dBV? What are each of these values, expressed in Volts?

The professional standard operating level is +4 dBu (1.23 V). The semi-professional standard operating level is-10 dBV.

When sound is traveling through air, what do we call regions that have higher pressure or density than average? What do we call regions that have lower pressure or density?

The regions with higher pressure or density are called compressions and the regions with lower pressure or density are called rarefactions.

When two speakers with the same impedance (e.g., 8Ω) are wired in parallel to the same amplifier, how much of the total voltage from the amplifier goes across each speaker?

The same total voltage would go through both of the speakers.

In terms of delay times, what situations would cause "constructive interference."

The situations that would cause "constructive interference" would be when the delay time equals or is an integer multiple of the period.

In terms of delay times, what situations would cause "destructive interference."

The situations that would cause "destructive interference" would be when the delay time makes the frequency be halfway through a full cycle.

If one speaker is connected to an amplifier, and then a second speaker is connected in series with the first speaker, does the total power dissipated increase or decrease? (Assume that the amplifier is a constant voltage source, i.e., that its voltage output is constant.)

The total power would decrease as the load decreases.

Why are decibels used to measure changes in loudness, rather than—for example—just using amplitude or power differences between two sources?

This is because our loudness perception exists on a logarithmic scale. So, the decibels convert linear changes in amplitude into a quantity that is logarithmic.

What does it mean that the relationship between period and frequency is reciprocal?

This means that both period and frequency are equal and in agreement with eachother.

Why should transient signals, such as a kick or snare drum, be recorded with a lower average signal level (i.e., on a VU meter) than more sustained signals, such as a bass or vocal?

Transient signals should be recorded with a lower average signal level than sustained signals because the peak amplitudes a lot higher than the average amplitude. So, it's better and safe to record at lower than average levels so you don't run into the issue of the channel distorting.

What unit (e.g., volts, ohms) measures power, and what does this unit represent?

Watts measures power (1 Watt equals 1 Joule per second).

How is wavelength measured?

Wavelength is measured from crest to crest.

What does wavelength measure?

Wavelength measures the distance that sound will travel during a single cycle.

Why are microphones (and loudspeakers also) called "transducers"?

We call microphones and loudspeakers "transducers" because they convert one form of energy (acoustical pressure) into a different form of energy (electric voltage).

How is the amplitude of sound measured in air?

We can measure the amplitude of sound with displacement amplitude. It describes what happens with air molecules when they are disturbed by a sound wave. A sound wave causing a proportionally greater displacement also represents a way to measure a wave's amplitude.

If we model a single frequency moving through the air via a sine wave, where on the sine wave are the maximum and zero points of air pressure change? What about the maximum and zero points of particle velocity on this same sine wave?

When particle velocity is zero is when air pressure is at a maximum. When particle velocity is at a maximum, air pressure is at zero.

What is the primary difference between a balanced and an unbalanced connection?

With balanced forces, equal forces are exerted in opposite directions, which provides a net force of zero. With unbalanced forces, a net force acts on an object which causes it to accelerate and change the motion of the object.

What is the primary difference between "matching" impedance and "bridging" impedance?

With matching impedance, the input and output impedances are the same. With bridging impedance, the input impedance is great than the output impedance.

Given a 100W amplifier at full volume, how could you increase the intensity of the sound?

You could increase the intensity of the 100W amplifier by getting closer to the amplifier. This is because the power would be spread across a smaller area.