MUI 608 Final Study Guide

compression

a vibrating mass squeezes air molecules into a given area as it moves outward. This causes the area being acted upon to have a greater than normal atmospheric pressure.

rarefaction

as the compressed molecules move away from the source, an area of lower than normal pressure is created.

seven characteristics of a waveform

amplitude, frequency, velocity, wavelength, phase, harmonic content, envelope.

velocity of sound at 59 degrees Fahrenheit, at sea level

1130 ft/sec.

velocity of sound change with temperature

increases at a rate of 1.1 ft/sec for each Fahrenheit degree increase.

formula for wavelength

λ = V/f.

relationship between wavelength and velocity

velocity is directly proportional to the wavelength.

wavelength of a 1000 Hz wave

.343 meters.

two identical waves 180 degrees out of phase

perfect inverse relationship.

phase shift

describes one waveform's lead or lag time with respect to another; results from a time delay between two (or more) waveforms.

transducer

any device that converts one form of energy into another.

difference between woofers and tweeters

woofers reproduce low frequencies, usually 8-18 inches in diameter; tweeters reproduce highest frequencies, usually 5-12 inches in diameter.

full range loudspeaker

integrated systems with woofer and tweeter, possibly midrange speaker.

why decibels use a logarithmic scale

to deal with the broad sensitivity of our ears in a convenient way; reduce large numbers into smaller, more manageable numbers.

reference level 0 dB SPL

.0002 dyne/cm^2 (threshold of hearing).

doubling the distance between a mic and a sound source

6 dB reduction in sound pressure level.

reference level 0 dBu

.775 volts.

reference level 0 dBV

1 volt.

reference level 0 dBm

1 milliWatt.

increase of 3 dBm

2x power.

increase of 6 dBSPL

2x voltage.

RMS (root mean square)

electrical signals are often described using this measurement rather than their "peak" measurement.

Fletcher Munson Equal Loudness Curve

indicates the ears sensitivity to different frequencies at various levels.

dynamic range of a sound system

the difference, in dB, between the loudest and quietest portion of a program.

head room

the difference between the nominal (average) and peak level in a program.

signal to noise ratio

difference between the nominal level and the noise floor of the system.

feedback

occurs when sound from the loudspeakers is picked up by the mic and a continuous loop is reintroduced into the sound system.

formula for Max Acoustic Gain

20log(d0) - 20log(dS) + 20log(d1) - 20log(d2) - 6.

max acoustic gain for free field

free of large boundaries, reflective surfaces or obstructing objects.

wind velocity effect

a crosswind will add a velocity vector to a sound wave, shifting the direction of propagation; will make the sound appear to originate from a different location; example is wind instruments or wind noise in microphone recordings.

wind gradient effect

when air layers are moving at different speeds from one another; example is sound propagation in outdoor concerts.

humidity effect on sound waves

as sound travels, the air absorbs some of the sound wave's energy; dry air absorbs more acoustical energy than does moist air.

absorption coefficient

measurement of the energy lost when a sound wave strikes a specific material; can range from 1 to 0.

reduction in dB SPL with absorption coefficient of .45

10log(.55) = -2.6 dB.

standing waves

occur at parallel surfaces or 90º corners where the boundary surfaces are reflective; sound waves bounce off the surfaces, this interference will result in a stationary pattern in the air, creating areas of lower air pressure and higher pressure; created by frequencies that fit within the room's boundaries.

other names for standing waves

room resonances, natural frequencies, or modes.

treatment for standing waves

non-parallel surfaces, absorption and diffusion help minimize this

RT60

decay time/reverb time; amount of time it takes the reverberating acoustical waves to decrease by 60 dB.

critical distance

the distance from the center of the sound source to the point at which direct sound is equal in amplitude to the reverberant sound.

transformer

transforms electrical energy from one circuit to another.

transformers in audio components

step up/step down signal voltage, increase or decrease the impedance of a circuit, convert a circuit from unbalanced to balanced, block DC current in a circuit while allowing AC current to flow, electrically isolate one audio device from another.

power bandwidth vs frequency response

power bandwidth can be used to assess what the amplifier does at a higher power, showing the number of Hz between the points where output power drops by 3 dB.

frequency response

describes how accurately (in terms of frequency) a device recreates at its output the signal it receives at its input

white noise

equal energy per Hz

pink noise

white noise that has been modified with a "pinking filter"; removes the 3 dB per octave increase in energy found in white noise- equal energy per octave

hum

result of the leakage of AC power into the audio circuit

buzz

similar to hum but contains harmonically related noise energy across the audio spectrum, including high frequency energy

popcorn noise

sporadic crackling that can be caused by internal breakdown within transistors or by dirty patch points or other connections

EIN (equivalent input noise)

measure of how quiet a mic preamp is, measured at the output of the preamp

THD (total harmonic distortion)

composite measure of all the harmonics; can be caused by the clipping of a wave form at an amplifier or by resonances in loudspeakers

intermodulation distortion

when at least two input signal frequencies interact to form new, non-harmonically related output frequencies

impedance

total opposition to the flow of alternating current (audio signal) in an AC electrical circuit and is measured in ohms and represented by Z

bridging connection

one where the input termination impedance is a minimum of 10 times higher than the source impedance of the output driving it

matched connection

one where the input's terminating impedance is roughly the same as the output's source impedance

mic level

range extends from no signal up to about -20 dBu, it includes the outputs of microphones, guitar pick-ups, phone cartridges and tape heads, prior to any amplification

line level

-20 dBu to +30 dBu, it includes electronic keyboard outputs, preamp and console outputs and the inputs and outputs of most signal processors

crosstalk

signal leakage between channels of a mixing console, a stereo signal processor or an amplifier. It can also occur between the input and output of a single channel

carbon mic

carbon granules are held between two metal plates; sound waves move the plates and compress the granules

crystal mic

use the piezoelectric (pressure-electricity) effect; when piezo crystals receive pressure, they generate a voltage analogous to that pressure

electromagnetic induction

when conductive metal moves in a magnetic field it creates voltage

dynamic/moving coil mics

durable, can handle loud sounds, not a flat frequency response, slow transient response, great for live sound applications due to strong rejection of background noise and overall durability

ribbon mics

generally have a low output, most are bidirectional, valuable and delicate, can break at relatively low SPLs, can be great on horns, vocals, electric guitar, drum overheads, and stringed instruments

condenser mics

very accurate, very good transient response, need power source, can be harsh or brittle, can distort at relatively low SPLs, sometimes too sensitive

phantom power

used to give the two metal plates a charge, 48 volts DC is sent from the console to the microphone via the mic's cable

electret condenser

a charge is manufactured into the capsule permanently, the capsule does not require additional power, although they require power for amplification, lavalier mics are one type

PZM (pressure-zone) mic

electret condenser mic that is attached to a metal plate, designed to attach to a surface, often used for lectures or conferences

polar pattern

describes how well the microphone receives sound from different directions

cardioid polar pattern

polar pattern that reacts to sound pressure arriving only from the front, great for live sound reinforcement, reducing feedback and improving isolation

omnidirectional polar pattern

react equally to pressure from all directions, not common for live sound stage use

bidirectional polar pattern

react to sound pressure arriving from the front and back

hypercardioid polar pattern

tighter response on axis than cardioid, some response at 180 degrees

supercardioid polar pattern

more focused version of hypercardioid, higher rejection at rear

flat frequency response

if a device has an equal response to all frequencies

pad

reduces microphone output by specified amount, typically 10 dB or 20 dB

shock mount

suspends mic in an elastic harness, eliminating mechanical connection between stand and mic

pop filter

acoustically transparent barrier placed between a singer's mouth and microphone in order to reduce low-frequency pops

proximity effect

boost in the low-frequency response of a directional microphone that occurs when the sound source is relatively close to the microphone

common mode noise rejection

used with balanced connections to cancel out any noise that may have entered the cable during the signal's travel

open mic

a mic whose signal is being sent through the console's main mix to the speakers (basically it's on)

doubling open mics

the system gain must be reduced by 3 dB to avoid feedback

spaced pair

same type of microphone- two omni mics spaced at a distance of 3 to 30 ft apart

XY pair

usually 2 cardioid mics of same type placed facing each other between 90-135 degrees

mid side mic technique

one mid mic (cardioid) pointing at source, panned center; one side mic (figure 8) oriented sideways at the source- copy the side signal and reverse phase (pan L R)

ORTF

two cardioid mics placed at 110º and 17 cm apart, distance of human ears

monitor mixer

usually located on either side of the stage; custom mix so each performer can perform better; allows artists to hear some of the stage mix and allow communication between performers

front of house mixer

controls the sound that the audience will hear; goal is to create a balanced mix that sounds good in the venue

importance of a good monitor mix

allows the artist to hear themsleves and each other; quality also affects the sound “in the house” because of the bleed- need to avoid unwanted frequencies going into the crowd

monitor wedges EQ

the monitor sound will inevitably bleed into the audience, so you need to take out unwanted frequencies that could cause feedback/distortion in the audience

ways to reduce feedback on stage monitor systems

• use high pass filters when appropriate

• use directional mics

• flip the polarity of the signal

“ringing out” a monitor system

method of tuning the EQ of each monitor channel to suppress resonances in the speaker

steps involved in ringing out a monitor system

• step 1- setup the stage as it will be used in the performance, with mics in place

• step 2- go through each monitor, one at a time, gradually turning up the level while a mic is “talked” (spoken into)

• step 3- turn up the level until feedback begins

• step 4- use your ears (or a spectrum analyzer if available) to determine which frequency is feeding back

• step 5- find that frequency on your graphic eq and cut by 3 dB

• step 6- have the person continue speaking at the mic, bring the monitor level up slowly, if the same frequency feedback, cut it a few more dB, if it is a different frequency, cut it 3 dB

• step 7- do this until you reach a point where several frequencies are feeding back, or where you’ve adjusted as many frequencies as you can

FOH mixer placement

directly in front of one speaker stack; measure the distance between the two speaker stacks

where FOH mixer shouldn’t be placed

don’t place the console further than twice the interstack distance; mixing the console too far from the stack will cause time delays; do not place too close to stage, or the mix will be too quiet and the mixer might hear the monitor bleed

power amplifier

electronic device that takes a small electrical signal and boosts it to a higher level so it can drive output devices like speakers and headphones- ensures sound is loud and clear

equalization

allows the engineer to selectively change the balance of frequencies in a signal; use various filter types to modify sound

equalization in live sound

• corrects specific problems in a room/monitor

• overcomes deficiencies in the frequency response of an instrument

• blends multiple instruments and sounds into a better overall mix

bandwidth

describes how wide or narrow the range of frequencies is around a center frequency that is being boosted or cut

Q

quality factor of an EQ, determines how wide or narrow the range of frequencies affected by the EQ adjustment is

Q and bandwidth relationship

inverse- wide bandwidth = low Q, narrow bandwidth = high Q

peaking filter

peak-shaped bell curve can be either boosted or cut around a center frequency