This column aims to give musos, creative types, DJs Techno and Rave producers and such like minded people, enough info so that when the sound engineer or PA operators ask..." What levels ya output, do ya need a DI, what sort of connectors ya got, got all your adapters, waddaya want in foldback etc etc?" they won't be completely stuffed by all this techno-babble!!

There are many things to consider, like the various signal levels, inputs & outputs, connectors...etc, etc. and the list goes on! It won't replace lots of hands on experience, but at least by learning some of the jargon you'll know the basics and not sound like a complete beginner!!

This week we'll continue with the A - Z of Live Audio looking at the 'Ds.'

DAMPING FACTOR : Damping factor is a measure of how well an amplifier can actually control the movement of a speaker cone or diaphragm by preventing it from moving farther than it is supposed to. Damping factor is a ratio arrived at by dividing the speaker impedance by the amplifier's internal output impedance. (Be aware that ANYTHING connected in the speaker line - including the speaker cable itself or a crossover - looks to the speaker like an increase in the output impedance of the amplifier, thus lowering the effective damping factor.) A low damping factor (high amplifier output impedance) allows a woofer to continue to move after the signal stops, resulting in an indistinct and mushy low frequency response. A high damping factor (200 or above) provides excellent control over low frequency woofers and produces a tight, clean bass.

DAMPING : In the context of reverberation, damping refers to the rate at which the reverberant energy is absorbed by the various surfaces in the environment.

DAISY CHAIN : Term used to describe in-series electrical connection between devices or modules.

dB (DECIBEL): A unit for describing the ratio of two voltages, currents, or powers, based on a logarithmic scale. (Originally the "bel" in honour of Alexander Graham Bell was the logarithmic term called the "transmission unit," used to express the transmission losses of long telephone lines. The "bel", being too large for practical use, was later changed to "decibel".) The decibel has no actual numerical value, but is used only to express a ratio and must always be referenced to some base of measurement. The basis of the decibel system is logarithmic mathematics. (The logarithm: The exponent of that power to which a fixed number - the base - must be raised in order to produce a given number - called the antilogarithm. The decibel uses logarithms to the base 10 called LOG. This is not to be confused with the so-called natural logarithm to the base "e," called LN, used in many electronic formulas.) When no reference is given, dB refers to Sound Pressure Levels.

dBA : Sound Pressure Level (SPL) measured with an "A" weighting filter.

dBm : A unit of measurement of audio signal level in an electrical circuit, expressed in decibels referenced to 1 milliwatt. The "m" in dBm stands for "milliwatt." In a circuit with an impedance of 600 ohms, this reference (0dBm) corresponds to a signal voltage of 0.775 VRMS (because 0.775 V across 600 ohms equals 1mw).

dBu :A unit of measurement of audio signal level in an electrical circuit, expressed in decibels referenced to 0.775 VRMS into any impedance. Commonly used to describe signal levels within a modern audio system.

dBu : Primarily a British term for gain referenced to 0 dBu = .7746 volts RMS.

dBv : Variation on dB referenced to 0dB = 0.775 volts. (A unit of measurement equal to the dBu but no longer in use. It was too easy to confuse a dBv with a dBV, to which it is not equivalent.)

dBV : A unit of measurement of audio signal level in an electrical circuit, expressed in decibels referenced to 1 volt RMS; 0 dBV = 1 volt across any impedance. Commonly used to describe signal levels in consumer equipment. To convert dBV to dBu, add 2.2dB.

dBV : A decibel scale

dBW : A term for power gain referenced to 0 dBW = 1 Watt.

O dB : In the measurement of SPL or Sound Pressure Level, 0 dB is referenced to the threshold of hearing or auditory perception of a tone of 1000 cycles (hertz) per second (1 kHz). When measuring differences in SPL, the amount of change in sound pressure level perceivable is directly proportional to the amount of stimulus, i.e the more sound present, the greater the change must be, to be perceived.
In gain functions, 0 dB is unity gain

3 dB : The amount of SPL gained by doubling the power to a speaker. The amount gained by doubling the number of speakers.

+/- 3 dB : Plus or minus 3 dB is a measurement of frequency response that exhibits no more than +3 dB and no less than -3 dB below a given reference. It is actually a 6 dB window.

-6 dB : The amount of loss in SPL as you double the distance away from a sound source.

dB/Octave : A means of measuring the slope of a filter. The more dBs per octave, the sharper the filter slope.

DC : Direct Current.

DCO : Digitally Controlled Oscillator.

DDL : Digital Delay Line.

DECAY : The progressive reduction in amplitude of a sound or electrical signal over time. In the context of an ADSR envelope shaper, the Decay phase starts as soon as the Attack phase has reached its maximum level. In the Decay phase, the signal level drops until it reaches the Sustain level set by the user. The signal then remains at this level until the key is released, at which point the Release phase is entered.

DECAY ; The gradual reduction in sound energy once the sound source is turned off.

DE-ESSER : Device for reducing the effect of sibilance - 'Sss' - in vocal signals.

DELAY : In sound work, delay usually refers to an electronic circuit or effects unit whose purpose it is to delay the audio signal for some short period of time. Delay can refer to one short repeat, a series of repeats or the complex interactions of delay used in chorusing or reverb. Signal time delay is central to many audio effects units. When delayed signals are mixed back with the original sound -delay feedback - a great number of audio effects can be generated, e.g. - phasing and flanging, doubling, Haas-effect positioning, slapback, echo, regenerative echo, chorusing and hall-like reverberation.

DETENT : A point of slight physical resistance (a click-stop) in the travel of a knob or slide control, often used to indicate unity gain or '0' point, e.g. centre of a control such as a pan or EQ cut/boost knob.

DIAPHRAGM : A thin flexible sheet that can be moved by sound waves as in a microphone, or can produce sound waves when moved as in a loudspeaker or compression driver.

DIAPHRAGMATIC ABSORPTION : Absorption of sound energy due to the flexing of wall panels at low frequencies. Wall panels when mounted on a solid backing, but separated from it by an air space (such as 2 x 4 studs) will respond to impinging sound waves by vibrating. This results in absorption of sound energy due to frictional losses caused by the flexing of fibres in the wall panel.

DI BOX : Direct Injection, a way of bypassing the use of microphones, eliminating all microphone and stage noise, leaving just the sound of the instrument itself. A very "clean and clinical sound". A DI takes an instruments output - instrument or line level - and reduces it to a mic Level. It also converts the High Impedance (Hi Z) instrument signal into a Balanced Low Impedance (Lo Z) signal that the mixing desk can handle. A device for matching the signal level impedance of a source to a tape machine or mixer input.

DIFFERENTIAL AMPLIFIER ; An amplifier whose output is proportional to the difference between the voltages applied to its two inputs. Used to balance or offer common mode rejection of interference signals.

DIFFRACTION : The bending or redistribution of acoustic sound waves in a room caused by some obstacle, such as a column or divider. (Huygens Principle). The longer the wavelength in comparison to the obstacle, the more the wave will diffract around it. Only low frequency wave forms can be diffracted.

DIFFUSION :The scattering of sound waves by a solid object.

DIGITAL : Refers to the processing of audio signals as having discrete values as opposed to a continuous analog audio signal. In digital audio the continuous analog signal is converted to an encoded discrete value or digital 'word' in the form of codes comprising 1s & 0s.

DELAY : Digital processor for generating delay and echo effects.

DIGITAL REVERB : Digital processor for simulating reverberation.

DIP : A reduction (attenuation or cut) in gain at a certain frequency also called a notch.

DIPPING : The opposite of peaking, doh!. A dip is an EQ curve that looks like a valley, or a dip. Dipping with an equalizer reduces a band of frequencies.

DIN CONNECTOR : Consumer multipin signal connection format, also used for MIDI cabling. Various pin configurations are available.

DIRECT COUPLING : A means of connecting two electrical circuits so that both AC and DC signals may be passed between them.

DIRECTIVITY : Area of coverage of a speaker or microphone.

DISPERSION : The spread or distribution or coverage of sound generated from a horn or loudspeaker. For any given frequency, the area of dispersion is defined as that area between the -6 dB down points of that frequency plotted against amplitude. It is measured in degrees related to an imaginary line descending from the centre of the speaker cone. As you move away from the imaginary line, up or down, right or left, - 'off axis' - the loudness level of the sound decreases. When the sound level decreases rapidly on either side of the imaginary line, the speaker is said to be highly directional.

DISTORTION : Any undesired change in the wave form of an electrical signal passing through a circuit or transducer, so that what comes out is no longer a true replica of what went in. When reproducing sound, distortion is unwanted. (Musos sometimes desire the application of electronically induced distortion for musical effect in the production of their 'sound.' Not to be confused with unwanted system distortion.)

DOUBLING : A delay effect, where the original signal is mixed with a medium (20 to 50 msec) delay. When used carefully, this effect can simulate double-tracking i.e. recording a voice or instrument twice.

DRIVER : The electric motor structure portion of a horn loaded loudspeaker system that converts electrical energy into acoustical energy and feeds that acoustical energy into the entry of a horn throat or the narrow end of the horn. Most often used when referring to a high frequency compression driver, called a driver for short. The definition also includes the loudspeaker in a horn loaded woofer or mid bass horn.

DRY : A signal that has had no effects added.

DUCKING : A system for controlling the level of one audio signal with another. For example, background music can be made to 'duck' whenever there's a
voice-over.

DYNAMIC MICROPHONE : A type of microphone that works on the electric generator principle, where a diaphragm moves a coil of wire within a magnetic field.

DYNAMIC RANGE (Acoustic) : In a musical instrument, the dynamic range is the difference in decibels between the loudest and softest level of notes that can be played on that instrument.

DYNAMIC RANGE : In electronic equipment, dynamic range is the difference in decibels between the highest, peak clipping or overload level, and lowest, minimum acceptable level before the signal disappears into the noise floor. Compatible with that signal system or transducer.

DYNAMICS : Way of describing the relative levels within a piece of music.

Next time we'll look at ...the 'E's'.

In the meantime, if you've got any audio questions give me a yell, send me an email or post a message on roadtalkz. I'm not the font of all knowledge, but if I don't know the answer to your questions, I know enough guru's who will!!

See-ya-later.

Willy T.

Email : willyt-downunderdesk@roadogz.com

This is extracted from the book: "LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems." ISBN 0 646 36408 1. (c) Giraffe Communications 1998. Order via e-mail or at all good Music & Book shops.

(c) 2002. Giraffe Media.