Sound Reinforcement Final Exam

What are the different types of distortion in a sound system?

1. Overdriven sound 2. Harmonic distortions 3. Intermodulation distortion

Three different uses of a sound system

1. Public announcements 2. Direct Public address 3. Performance

A 10 band, 15 band, and 31 band graphic EQ can affect how much of an octave per slider?

10: 1 octave 15: 1/2 octave 31: 1/3 octave

Class A/B amplifier

2 pairs of transistors pass small amounts of power at idle (eliminates crossover distortion). 60-65% efficient

Class D amplifier

2 transistors- 1 is off while the other is in use. "switch mode" amp, 80-90% efficient. Common in line arrays

Three Phase Power

5 wire connection. 3x wires (120 or 210v each) 1x neutral 1x ground Running in different phases for each of 3 cycles. Uses cam locks.

Illegal frequency bands

600MHz and 700 MHz

Compansion

A signal is compressed when it is sent across a network and expanded when it is recieved

Point source

A single speaker or multiple speakers placed far apart

Digital signal processor

A specialized chip that processes digital information and transmits signals very quickly.

AFL and PFL

AFL: After-fade listen- check L/R position and mix levels PFL: Pre-fade listen- set pre-amp and EQ

Advantages/disadvantages of a permanent system

Advantages: specific to the acoustics of the venue, can be hidden by architecture, has optimal room-position Disadvantages: engineers have to learn it when they come to the venue, expanding it's capabilities would require further installations

Advantages/disadvantages of a portable system

Advantages: specific to the band, doesn't have to be re-learned at each venue, can be adjusted for needs Disadvantages: more wear and tear, more travel costs

Best practices for wireless antenna placement:

At least two meters above the ground, (1/2) to 1 wavelength apart

Difference between a mixer bussing and matrix sections:

Bussing sections combine channels and route to outputs, while the mix matrix is devoted to additional routing such as front/side fills, delays, dispersed system, etc.

Consumer line level vs. Professional line level

Consumer: -10 dBu Professional: +4 dBV

How does a room's acoustics play into the way we set up our system?

Different room acoustics require different acoustic treatments and speaker placement in order to optimize phase-time relationships

DCA and VCA

Digitally-controlled amplifier and voltage-controlled amplifier. They allow control of the levels of multiple instruments with one fader. Does NOT combine signals

How do you calculate delay times for speakers?

Distance x 1.1 milliseconds

Parallel

Dual wires connect (+) to (+) and (-) to (-) Impedance(ohms) = 1/((1/S1) + (1/S2))

Basic components of a drive rack

EQ, limiter, crossover network

Ribbon driver

Electro dynamic driver. Contains aluminum ribbon, permanent bar magnets, perforated pole piece, cutaway section

EMI

Electromagnetic interference. Interference caused by motors, power lines, and fluorescent lights. Cables can be shielded to protect signals from EMI. Additionally, EMI shielding prevents signal emanation, so it can prevent someone from capturing network traffic.

White noise

Equal energy per frequency

Pink noise

Equal energy per octave

Different types of cable shielding

Foil, wire, and braided. Braided is the most protective, but the least flexible.

Hard patch vs. soft patch

Hard patch: manually plugging in circuits Soft patch: connections are made digitally

Local I/O

I/O modules that are connected to the controller

Phase time

In phase, out of time: still increases amplitude but contains a delay Out of phase, out of time: sounds cancel each other out

How does cupping a mic affect it's sound?

It covers up the ports, changing the mic's polar pattern to omnidirectional

Basic signal flow of a drive rack

L/R > Stereo EQ > Compressor/limiter > crossover > HF/MF/LF EQ, Compressor, and delay

Frequencies of VHF and UHF wireless systems:

Low-band VHF: 49-108 MHz High-band VHF: 169-216 MHz Low-band UHF: 450-806 MHZ High-band UHF: 900-952 MHz

What equipment/information does a stage plot show?

Microphones, instruments, monitors, DI boxes, amps, channel inputs, phantom power, extra needs.

Class A amplifier

Most common. 1 DC transistor, low distortion levels, transistor is always 1/2 on, waste 4-5 watts in heat for each watt of audio output

How can I reduce proximity effect?

Move the mic further from the sound source, add a pop filter, or change the mic polar pattern to omnidirectional

Why is antenna diversity important?

Multiple antennas are necessary so that they can still pick up a wireless mic signal even if one antenna stops working or drops the signal.

What microphones are not susceptible to wind noise?

Omnidirectional

Monitor setups

One monitor in front of a mic Two monitors (W, t, t, W) in front of a mic Two monitors at angles towards the mic (can have mono mixes, 2 separate mixes, or a stereo mix) One in front of mic, two on either side at an angle

RFI

Radio frequency interference. Interference from RF sources such as AM or FM transmitters. RFI can be filtered to prevent data interference, and cables can be shielded to protect signals from RFI.

Types of enclosures:

Sealed: acoustic suspension, tight transient response, low distortion, less efficient in power Ported Enclosure: bass reflex, has port for air to escape/enter, less efficient transient response, more power efficient Passive Radiator: internal air, drone cone, steeper LF rolloff

What does the horn do to the sound of a speaker's driver?

Serves to improve the coupling efficiency between the speaker driver and the air

What does a system diagram(line diagram) show?

Signal flow from the mic/line input to the monitor/speaker output.

What information needs to be put onto an input list?

Source, microphone/DI, channel input, phantom power, and band

How does squelch affect a wireless signal? What does it do?

Squelch is muting the audio output of a receiver in the absence of the desired radio signal. This prevents the receiver from picking up noise when a signal is lost

What happens when I place my subs close to the wall?

The bass is boosted

Absorption coefficient

The degree to which a material absorbs sound waves. Goes from 0-1 with 1 being 100% absorptive

Haas Effect (precedence effect)

The ear/brain will use the time of first arrival to locate the sound source direction

Line level

The level of audio between pieces of audio equipment. +4 dBV, -10 dBu

Power alley

The line between subwoofers where each woofer is in phase and noticeably louder. Boosts the low frequencies

Transient response

The measure of how quickly a mic's diaphragm will react when it is hit by an acoustic wavefront.

What is a 2 way, 3 way, or 4 way crossover network?

The number of points where frequencies are "crossed over"

Power output

The power an amp can produce into a stated load impedance Continuous: via sine wave at various frequencies (constant) Program: RMS, 1 min Peak: < 1 second

What does a speaker's polar plot show you and why is it important?

The vertical and horizontal dispersion of a speaker, important for speaker placement

Why would we place our subs together and in mono at the center of the stage rather than in stereo?

To minimize phase cancellations and out-of-time sound waves coming from the subs.

Class B amplifier

Two AC transistors (1 for + side, 1 for - side), off when no signal is present. 70-80% efficient. Causes crossover distortion(therefore not used in audio)

Optical cable

Uses laser light to transmit signal

Bi-amp mode

Using two channels of amplification to power a single speaker

Power formula

Watts = Amps x Voltage P = IE

Condensation effect

When condensation on condenser mics causes unwanted noise

Occlusion

When the musician's ears are tired after several hours of monitoring at high levels

Non-destructive solo

You can solo an instrument to monitor it without it being soloed in the mains

Grounding rods/cables

a safety mechanism used to disperse extra electrical energy into the ground

Dampening:

aka "overshoot" lowers "ringing time" or smearing caused by the speaker's momentum

Bridged amplifier

both amplifier channels are combined and the amp becomes mono. 1 loaded output. Hook-up: C1 and C2 at same time (+/-)

Switch

connects a group of local evices

Ground wire

connects circuits in the home to the earth, giving charges alternate path in case of a short circuit

Distributed System

connects to multiple spaces/rooms.

Horn loaded compression driver

contains: phase plug, magnet, diaphragm, and voice coil

Active crossover

crossover frequencies are set by user. Located before power amp

Piezo driver

crystal coupled to diaphragm, efficient/lightweight, used in alarms

Impedance load formula

current = voltage/resistance W = v/ohms

Touring PA

extreme, needs many trucks, set up, time, knowledge. Line array system.

dB(Z)

flat frequency response

dB(C)

for higher volumes

dB(A)

for lower volumes: emphasizes highs and lows

dB(B)

in-the-middle, less used

Dynamic driver

longer horn for low frequencies, increases efficiency, loses fidelity

Frequency response

measure of the range of frequencies a microphone can hear and reproduce

Passive crossover

non-adjustable frequencies, located after power amp, inside of speakers

Side fill monitors

provide additional coverage across the stage, used only when needed. Helps musicians hear themselves at different parts of the stage.

Transistor

raises the strength of a weak signal. All amplifiers require transistors to work

Band PA

reinforces small to large venues, easily scalable, 10-2,000 people

Router

routes signals across networks

Series

single wire to connect (+) terminal to (-) terminal. Impedance(ohms) = S1 + S2

Personal PA

smallest PA system, 1-12 people, 10-20 ft

RT60

the amount of time it takes for reverb to decay 60 dB

Crossover slope

the rate which the audio level increases/decreases per octave as the frequency increases/decreases

Coaxial speaker

the woofer and tweeter share center point. This helps with phase alignment, particularly in the crossover range

Why is a baffle/enclosure needed for a speaker to function properly?

to prevent the sound waves generated at the front of the speaker to interfere with the sound waves from the back

Common stage terms

upstage (near back), centerstage, downstage(near front), stage right(audience left), stage left(audience right), wings, proscenium arch, apron, thrust, audience/house

Line source

uses physics to force sound at a greater distance by lining up multiple speakers in close proximity

Ground loop

when a voltage difference is seen across the ground cables when using two separate ground points. Cut the shield connection inside the TRS connector.

Inverse square law

you lose 6dB of audio for every doubling of distance from a speaker