Audio Notes

Iqbal Asif carries World Music readers through some more tech terminology to enhance their appreciation of sound and music

 Absorption. The absorption of sound is the process, by which sound energy is diminished when passing through a medium or when striking a surface, i.e., sound is attenuated by absorption The physical mechanism is usually the conversion of sound into heat energy. A material’s ability to absorb sound is quantified by its absorption coefficient, whose value ranges between 0 (total reflection) and 1 (total absorption), varies with sound frequency and the angle of incidence.

AC-3 (audio coding 3) Dolby’s digital audio data compression algorithm adopted for HDTV transmission and used in DVDs, laserdiscs and CDs for 5.1 multichannel home theater use.

Acoustic feedback The phenomenon where the sound from a loudspeaker is picked up by the microphone feeding it, and re-amplified until the system runs away and rings or feeds back on itself producing scream or squeal found in sound systems.

Acoustic treatments There are only three classic (physical) tools available to treat a room: absorbers, reflectors and diffusers. Absorbers attenuated sound; reflectors redirect sound, and diffusers uniformly distribute sound. these tools change the temporal, spectra and spatial qualities of the sound

Amplitude... The maximum absolute value of a periodically varying quantity. or The maximum absolute value reached by a voltage or current waveform.

Audio relating to humanly audible sound, i.e., audio is all the sounds that humans hear (approximately 20 Hz – 20 kHz). or relating to the broadcasting or reception of sound.

Background music – music without lyrics and not performed by the original artist, used as an alternative to silence.

Balanced line A balanced circuit as “a circuit in which two branches are electrically alike and symmetrical with respect to a common reference point, usually ground.”, that two lines are driven equally and oppositely with respect to ground. Balances lines are the preferred method (for hum free) interconnecting of sound systems

Unbalanced line is one that transmits the audio signal between one wire and ground.

CD (compact disc) Trademark term for the Sony-Philips digital audio optical disc storage system. The system stores 80 minutes (maximum) of digital audio and sub code information, or other non-audio data, on a 12-centimeter diameter optical disc. The disc is made of plastic, with a top metallized layer, and is read by reflected laser light.

Chorusing Recording. An effect where the audio signal is given multiple delays so as to sound like several instruments playing at once. The delay times are short, typically 20-45 milliseconds, and each delayed signal may be pitch-shifted. The effect is similar to hearing a “chorus,” where everyone is singing the same thing but at slightly different times and pitches –

DAT (digital audio tape recorder) 1. A digital audio recorder utilizing a magnetic tape cassette system with rotary heads similar to that of a video recorder.

Decibel Abbr. dB Equal to one-tenth of a bel. A measuring system first used in telephony. The preferred method and term for representing the ratio of different audio levels. It is a mathematical shorthand that uses logarithms to reduce the size of the number [dB equals 20 log x/y, where x and y are the different signal levels]. Being a ratio, decibels have no units. Everything is relative. Since it is relative, then it must be relative to some 0 dB reference point

0 dBu Preferred informal abbreviation for the official dB (0.775 V);

Distortion Audio distortion: Distortion is the name given to anything that alters a pure input signal in any way other than changing its size. The most common forms of distortion are unwanted components or artifacts added to the original signal, including random and hum-related noise. Anything unwanted added to the input signal changes its shape (skews, flattens, spikes, alters symmetry or asymmetry, even if these changes are microscopic).

Dolby Digital® Dolby’s name for its format for the digital soundtrack system for motion picture playback. Utilizes their AC-3 system of digital compression. The signal is optically printed between the sprocket holes. Now being introduced to Home Theater on laserdisc and DVD. Dolby Digital may use any number of primary audio delivery and reproduction channels, from 1 to 5, and may include a separate bass-only effects channel.

DTS (DTS Cinema) A competing digital soundtrack system for motion picture playback developed by Digital Theater Systems Inc. Its novelties are: 1) not requiring a special projector to read digital code off the filmstrip like its competitors; 2) using only very moderate compression (3:1 verses Dolby’s 11:1); and 3) offering 20-bit audio. The discrete digital full bandwidth six (6) channel sound is contained on a CD that is played synchronously with the film. The synching time code is printed between the standard optical soundtrack and the picture.

DVD “digital versatile disc”and before that it meant “digital video disc” A 12-centimeter (4.72″) compact disc (same size as audio CDs and CD-ROMs) that holds 10 times the information. The DVD standard specifies a laminated single-sided, single-layer disc holding 4.7 gigabytes, and 133 minutes of MPEG-2 compressed video and audio.

Dynamic microphone A microphone design where a wire coil (the voice coil) is attached to a small diaphragm such that sound pressure causes the coil to move in a magnetic field, thus creating an electrical voltage proportional to the sound pressure. Works in almost the exact opposite of a dynamic loudspeaker where an electrical voltage is applied to the voice coil attached to a large cone (diaphragm) causing it to move in a magnetic field, thus creating a change in the immediate sound pressure.

Dynamic range The ratio of the loudest (undistorted) signal to that of the quietest (discernible) signal in a unit or system as expressed in decibels (dB). Dynamic range is another way of stating the maximum S/N ratio. With reference to signal processing equipment, the maximum output signal is restricted by the size of the power supplies, i.e., it cannot swing more voltage than is available. While the noise floor of the unit determines the minimum output signal, i.e., it cannot put out a discernible signal smaller than the noise. Professional-grade analog signal processing equipment can output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu. This gives a maximum dynamic range of 120 dB – pretty impressive numbers, which coincide nicely with the 120 dB dynamic range of normal human hearing