**ampere** , amp Abbr. I, also A.
1. A unit of electric current in the International standard

meter-kilogram-second (mks) system. It is the steady current that
when flowing in straight parallel

wires of infinite length and negligible cross section, separated
by a distance of one meter in free

space, produces a force between the wires of 2E-7 newtons per
meter of length. 2. A unit in the

International System specified as one International coulomb per
second and equal to 0.999835

ampere. [After André Marie Ampère.]

**amplifier classes** Audio power amplifiers are classified
according to the relationship between the

output voltage swing and the input voltage swing, thus it is primarily
the design of the output stage

that defines each class. Classification is based on the amount
of time the output devices operate

during one complete cycle of signal swing. This is also defined
in terms of output bias current [the

amount of current flowing in the output devices with no applied
signal]. For discussion purposes

(with the exception of class A), assume a simple output stage
consisting of two complementary

devices (one positive polarity and one negative polarity) -- tubes
(valves) or any type of transistor

(bipolar, MOSFET, JFET, IGFET, IGBT, etc.).

Class Aoperation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have "bottom side" transistors but these are operated as fixed current sources, not amplifying devices. Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning you draw about 5 times as much power from the source as you deliver to the load!) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion. [Much mystique and confusion surrounds the term class A. Many mistakenly think it means circuitry comprised of discrete components (as opposed to integrated circuits). Such is not the case. A great many integrated circuits incorporate class A designs, while just as many discrete component circuits do not use class A designs.]

Class Boperation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is d ue to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.

Class ABoperation is the intermediate case. Here both devices are allowed to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demand s. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.

Class AB plus Bdesign involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.

Class Cuse is restricted to the broadcast industry for radio frequency (RF) transmission. Its operation is characterized by turning on one device at a time for less than one half cycle. In essence, each output device is pulsed-on for some percentage of the half cycle, instead of operating continuously for the entire half cycle. This makes for an extremely efficient design capable of enormous output power. It is the magic of RF tuned circuits (flywheel effect) that overcomes the distortion create d by class C pulsed operation.

Class Doperation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle (Sampling Theorem). Theoretically since the output devices are either completely on or completely off they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the load, so the power dissipated by the dev ice is zero (found by multiplying the voltage across the device [zero] times the current flowing through the device [big], so 0 x big = 0); and when the device is off, the voltage is large, but the current is zero so you get the same answer. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%

**amplitude** 1. Greatness of size; magnitude. 2. Physics.
The maximum absolute value of a

periodically varying quantity. 3. Mathematics. a. The maximum
absolute value of a periodic curve

measured along its vertical axis. b. The angle made with the positive
horizontal axis by the vector

representation of a complex number. 4. Electronics. The maximum
absolute value reached by a

voltage or current waveform.

**analog** A real world physical quantity or data characterized
by being continuously variable (rather

than making discrete jumps), and can be as precise as the available
measuring technique.

**bandpass filter **A filter that
has a finite passband, neither of the cutoff frequencies being
zero or

infinite. The bandpass frequencies are normally associated with
frequencies that define the half power

points, i.e. the -3 dB points.

**band-limiting filters **A low-pass and a high-pass filter
in series, acting together to restrict (limit) the

overall bandwidth of a system.

**bandwidth** Abbr. BW The numerical difference between
the upper and lower -3 dB points of a

band of audio frequencies. Used to figure the Q, or quality factor,
for a filter.

**bel** Abbr. b, B Ten decibels. [After Alexander Graham
Bell.] The Bel was the amount a signal

dropped in level over a one-mile distance of telephone wire. See:
decibel

**Bell, Alexander Graham** (1847-1922) Scottish-born American
inventor of the telephone. The first

demonstration of electrical transmission of speech by his apparatus
took place in 1876. Bell also

invented the audiometer, an early hearing aid, and improved the
phonograph.

**capacitor** A capacitor can store
small amounts of electrical charge. It consists of two electrodes

with a non-conducting layer in between them. If you think of the
capacitor as a small reservoir of electricity that will be released
later, you will have a rough idea of how it works.

**compression **1. An increase in density and pressure in
a medium, such as air, caused by the

passage of a sound wave. 2. The region in which this occurs.

**compression wave** A wave propagated by means of the compression
of a fluid, such as a sound

wave in air.

**condenser microphone **A microphone design where a condenser
(the original name for capacitor)

is created by stretching a thin diaphragm in front of a metal
disc (the backplate). By positioning the

two surfaces very close together an electrical capacitor is created
whose capacitance varies as a

function of sound pressure. Any change in sound pressure causes
the diaphragm to move, which

changes the distance between the two surfaces. If the capacitor
is first given an electrical charge

(polarized) then this movement changes the capacitance, and if
the charge is fixed, then the

backplate voltage varies proportionally to the sound pressure.
In order to create the fixed charge,

condenser microphones require external voltage (polarizing voltage)
to operate. This is normally

supplied in the form of phantom power from the microphone preamp
or the mixing console.

**current **Symbol i, I Electricity. a. A flow of electric
charge. b. The amount of electric charge

flowing past a specified circuit point per unit time, or the rate
of flow of electrons. [As electrons flow

in one direction, the spaces left behind, called holes, appear
to flow in the opposite direction. Thus,

current can be visualized as electron flow (negative current flow),
or in the opposite direction, hole

flow (positive current flow, sometimes called conventional current
flow).]

**cutoff frequency **Filters. The frequency at which the
signal falls off by 3 dB (the half power point)

from its maximum value. Also referred to as the -3 dB points,
or the corner frequencies.

**decibel** Abbr. dB Equal to one-tenth
of a bel. [After Alexander Graham Bell.] The preferred

method and term for representing the ratio of different audio
levels. It is a mathematical shorthand

that uses logarithms (a shortcut using the powers of 10 to represent
the actual number) to reduce

the size of the number. For example, instead of saying the dynamic
range is 32,000 to 1, we say it is

90 dB [the answer in 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. To distinguish between reference points
a suffix letter is added as

follows:

- 0 dBu A voltage reference point equal to 0.775 Vrms. [This
reference originally was labelled

dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to

dBu (for unterminated).] - +4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.
- 0 dBV A voltage reference point equal to 1.0 Vrms.
- -10 dBV Standard voltage reference level for consumer and
some pro audio use (e.g.

TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units

operating at -10 dBV levels.) - 0 dBm A power reference point equal to 1 milliwatt. To convert
into an equivalent voltage

level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an

equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for

instance, yields an equivalent voltage of 0.224 V -- something quite different. Since modern

audio engineering is concerned with voltage levels, as opposed to power levels of yore, the

convention of using a reference level of 0 dBm is obsolete. The reference levels of 0 dBu, or

-10 dBV are the preferred units. - 0 dBr An arbitrary reference level ("r" = "re")
that must be specified. For example, a

signal-to-noise graph may be calibrated in dBr, where 0 dBr is specified to be equal to 1.23

Vrms (+4 dBu); commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to +4

dBu." - 0 dBFS A reference level equal to "Full Scale."
Used in specifying A/D and D/A audio data

converters. Full scale refers to the maximum voltage level possible before "digital clipping," or

digital overload of the data converter. See: Overs. The Full Scale value is fixed by the internal

data converter design, and varies from model to model.

**diatonic** 1. Music. Of or using only the eight tones
of a standard major or minor scale without

chromatic deviations.

**digital filter** Any filter accomplished in the digital
domain.

**digital signal** Any signal which is quantized (i.e.,
limited to a distinct set of values) into digital words

at discrete points in time. The accuracy of a digital value is
dependent on the number of bits used to

represent it.

**digitization** Any conversion of analog information into
a digital form.

**DSP (digital signal processing)** A technology for signal
processing that combines algorithms and

fast number-crunching digital hardware, and is capable of high-performance
and flexibility.

**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. In fact, under the right circumstances, both elements
will operate as the other, i.e., a

dynamic loudspeaker will act as a microphone and a dynamic microphone
will act as a loudspeaker

-- although not too loud.

**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 minimum output signal is determined by the noise floor of
the unit, 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 (from just audible
to uncomfortably loud).

**dyne** A unit of force, equal to the force required to
impart an acceleration of one centimeter per

second per second to a mass of one gram. Old usage for sound pressure.

**electret microphone** A microphone
design similar to that of condenser mics except ultiziing a

permanent electrical charge, thus eliminating the need for an
external polarizing voltage. This is done

by using a material call an electret [acronym for electricity
+ magnet] that holds a permanent charge

(similar to a permanent magnet, i.e., a solid dielectric that
exhibits persistent dielectric polarization).

Because electret elements exhibit extremely high output impedance,
they often employ an integral

built-in impedance converter (usually a single JFET) that requires
external power to operate. This

low voltage power is often supplied single-ended over an unbalanced
connection, or it may operate

from standard phantom power.

**filter** Any of various electric,
electronic, acoustic, or optical devices used to reject signals,

vibrations, or radiations of certain frequencies while passing
others. Think sieve: pass what you want,

reject all else. For audio use the most common electronic filter
is a bandpass filter, characterized by

three parameters: center frequency, amplitude (or magnitude),
and bandwidth. Bandpass filters

form the heart of audio graphic equalizers and parametric equalizers.

**FIR (finite impulse-response) filter** A commonly used
type of digital filter. Digitized samples of

the audio signal serve as inputs, and each filtered output is
computed from a weighted sum of a finite

number of previous inputs. An FIR filter can be designed to have
completely linear phase (i.e.,

constant time delay, regardless of frequency). FIR filters designed
for frequencies much lower that

the sample rate and/or with sharp transitions are computationally
intensive, with large time delays.

Popularly used for adaptive filters.

**frequency **1. The property or condition of occurring
at frequent intervals. 2. Mathematics.

Physics. The number of times a specified phenomenon occurs within
a specified interval, as: a. The

number of repetitions of a complete sequence of values of a periodic
function per unit variation of an

independent variable. b. The number of complete cycles of a periodic
process occurring per unit

time. c. The number of repetitions per unit time of a complete
waveform, as of an electric current.

**hertz **Abbr. Hz. A unit of frequency
equal to one cycle per second. [After Heinrich Rudolf

Hertz.]

**high-pass filter** A filter having a passband extending
from some finite cutoff frequency (not zero) up

to infinite frequency.

**IIR (infinite impulse-response) filter**
A commonly used type of digital filter. This recursive

structure accepts as inputs digitized samples of the audio signal,
and then each output point is

computed on the basis of a weighted sum of past output (feedback)
terms, as well as past input

values. An IIR filter is more efficient than its FIR counterpart,
but poses more challenging design

issues. Its strength is in not requiring as much DSP power as
FIR, while its weakness is not having

linear group delay and possible instabilities.

**impedance** The numerical measure of the complex reaction
to current flow that a general electrical

network has when excited with an ac voltage.

**inverse square law Sound Pressure Level**. Sound propagates
in all directions to form a spherical

field, thus sound energy is inversely proportional to the square
of the distance, i.e., doubling the

distance quarters the sound energy (the inverse square law), so
SPL is attenuated 6dB for each

doubling.

**kHz** (kilohertz) One thousand (1,000)
cycles per second.

**loudness** The SPL of a standard
sound which appears to be as loud as the unknown. Loudness

level is measured in phons and equals the equivalent SPL in dB
of the standard. [For example, a

sound judged as loud as a 40 dB-SPL 1 kHz tone has a loudness
level of 40 phons. Also, it takes

10 phons (an increase of 10 dB-SPL) to be judged twice as loud.]

**low-pass filter** A filter having a passband extending
from DC (zero Hz) to some finite cutoff

frequency (not infinite). A filter with a characteristic that
allows all frequencies below a specified

rolloff frequency to pass and attenuate all frequencies above.

**mixer **At its simplest level, an
audio device used to add (combine or sum) multiple inputs into
one or

two outputs, complete with level controls on all inputs. From
here signal processing is added to each

of the inputs and outputs until behemoth monsters with as many
as 64 inputs are created -- at a cost

of around 10-20 kilobucks per input for fully digitized and automated
boards. At these price points a

mixer becomes a recording console.

**notch filter** A special type of
cut-only equalizer used to attenuate (only, no boosting provisions

exist) a narrow band of frequencies. Three controls: frequency,
bandwidth and depth, determine

the notch. Simplified units provide only a frequency control,
with bandwidth and depth fixed

internally. Used most often in acoustic feedback control to eliminate
a small band of frequencies

where the system wants to howl (feedback).

**octave **1. Audio. The interval
between any two frequencies having a ratio of 2 to 1. 2. Music
a. The interval of eight diatonic degrees between two tones, one
of which has twice as many vibrations per second as the other.
b. A tone that is eight full tones above or below another given
tone. c. An organ stop that produces tones an octave above those
usually produced by the keys played.

**ohm **Abbr. R, (Greek upper-case omega). A unit of electrical
resistance equal to that of a

conductor in which a current of one ampere is produced by a potential
of one volt across its

terminals. [After Georg Simon Ohm.]

**one-third octave **1. Term referring to frequencies spaced
every one-third of an octave apart.

One-third of an octave represents a frequency 1.26-times above
a reference, or 0.794-times below

the same reference. The math goes like this: 1/3-octave = 2E1/3
= 1.260; and the reciprocal,

1/1.260 = 0.794. Therefore, for example, a frequency 1/3-octave
above a 1 kHz reference equals

1.26 kHz (which is rounded-off to the ANSI-ISO preferred frequency
of "1.25 kHz" for equalizers

and analyzers), while a frequency 1/3-octave below 1 kHz equals
794 Hz (labeled "800 Hz").

Mathematically it is significant to note that, to a very close
degree, 2E1/3 equals 10E1/10 (1.2599

vs. 1.2589). This bit of natural niceness allows the same frequency
divisions to be used to divide

and mark an octave into one-thirds and a decade into one-tenths.
2. Term used to express the

bandwidth of equalizers and other filters that are 1/3-octave
wide at their -3 dB (half-power) points.

3. Approximates the smallest region (bandwidth) humans reliably
detect change. See: critical bands.

Compare with: third-octave

**op amp (operational amplifier)** An analog integrated
circuit device characterized as having two

opposite polarity inputs and one output, used as the basic building
block in analog signal processing.

**pascal **Abbr. Pa A unit of pressure
equal to one newton per square meter.

**passband** The range of frequencies passed by an audio
low-pass, high-pass or bandpass filter.

Normally measured at the -3 dB point: the frequency point where
the amplitude response is

attenuated 3 dB (decibels) relative to the level of the main passband.
For a bandpass filter two

points are referenced: the upper and lower -3dB points. The -3dB
point represents the frequency

where the output power has been reduced by one-half. [Technical
details: -3dB represents a

multiplier of 0.707. If the voltage is reduced by 0.707, the current
is also reduced by 0.707 (ohms

law), and since power equals voltage-times-current, 0.707 times
0.707 equals 0.5, or half-power.]

**period **Abbr. T, t 1. The period of a periodic function
is the smallest time interval over which the

function repeats itself. [For example, the period of a sine wave
is the amount of time, T, it takes for

the waveform to pass through 360 degrees. Also, it is the reciprocal
of the frequency itself: i.e., T =

1/f.]

**phantom power** The term given to the standardized scheme
of providing power supply voltage to

certain microphones using the same two lines as the balanced audio
path. The internationl standard is

IEC 268-15, derived from the original German standard DIN 45 596.
It specifies three DC voltage

levels of 48 volts, 24 volts and 12 volts, delivered through 6.8
k ohms, 1.2 k ohms, and 680 ohms

matched resistors respectively, capable of delivering 10-15 ma.
The design calls for both signal

conductors to have the same DC potential. This allows the use
of microphone connections either for

microphones without built-in preamps, such as dynamic types, or
for microphones with built-in

preamps such as condenser and electret types.

**phon** A unit of apparent loudness, equal in number to
the intensity in decibels of a 1,000 Hz tone

judged to be as loud as the sound being measured.

**polarity **A signal's electromechanical potential with
respect to a reference potential. For example, if

a loudspeaker cone moves forward when a positive voltage is applied
between its red and black

terminals, then it is said to have a positive polarity. A microphone
has positive polarity if a positive

pressure on its diaphragm results in a positive output voltage.
[Usage Note: polarity vs. phase shift:

polarity refers to a signal's reference NOT to its phase shift.
Being 180° out-of-phase and having

inverse polarity are DIFFERENT things. We wrongly say something
is out-of-phase when we

mean it is inverted. One takes time; the other does not.]

**psychoacoustics **The scientific study of the perception
of sound.psychoacoustics The scientific study of the perception
of sound.

**Q (upper-case) Quality factor**.
Filters. The selectivity factor, defined to be the ratio of the
center

frequency f divided by the bandwidth BW.

**rarefaction **1. A decrease in density
and pressure in a medium, such as air, caused by the passage

of a sound wave. 2. The region in which this occurs.

**root mean square** Abbr. rms, RMS Mathematics. The square
root of the average of the squares

of a group of numbers. A useful and more meaningful way of averaging
a group of numbers.

**slew rate** 1. The term used to
define the maximum rate of change of an amplifier's output voltage

with respect to its input voltage. In essence, slew rate is a
measure of an amplifier's ability to follow

its input signal. It is measured by applying a large amplitude
step function (a signal starting at 0 volts

and "instantaneously" jumping to some large level [without
overshoot or ringing], creating a step-like

look on an oscilloscope) to the amplifier under test and measuring
the slope of the output waveform.

For a "perfect" step input (i.e., one with a rise time
at least 100 times faster than the amplifier under

test), the output will not be vertical; it will exhibit a pronounced
slope. The slope is caused by the

amplifier having a finite amount of current available to charge
and discharge its internal compensation

capacitor.

**S/N or SNR (signal-to-noise ratio)** An audio measurement
of the residual noise of a unit, stated

as the ratio of signal level (or power) to noise level (or power),
normally expressed in decibels. The

"signal" reference level must be stated. Typically this
is either the expected nominal operating level,

say, +4 dBu for professional audio, or the maximum output level,
usually around +20 dBu. The noise

is measured using a true RMS type voltmeter over a specified bandwidth,
and sometimes using

weighting filters. All these thing must be stated for a S/N spec
to have meaning. Simply saying a unit

has a SNR of 90 dB means nothing, without giving the reference
level, measurement bandwidth, and

any weighting filers. A system's maximum S/N is called the dynamic
range.

**sone** A subjective unit of loudness, as perceived by
a person with normal hearing, equal to the

loudness of a pure tone having a frequency of 1,000 hertz at 40
decibels sound pressure level.

**sound pressure** The value of the rapid variation in air
pressure due to a sound wave, measured in

pascals, microbars, or dynes - all used interchangeable, but pascals
is now the preferred term.

Instantaneous sound pressure is the peak value of the air pressure,
often used in noise control

measurements. Effective sound pressure is the RMS value of the
instantaneous sound pressure

taken at a point over a period of time.

**sound pressure level or SPL** The RMS sound pressure expressed
in dB re 20 microPa (the

lowest threshold of hearing for 1 kHz. [As points of reference,
0 dB-SPL equals the threshold of

hearing, while 140 dB-SPL equals irreparable hearing damage.]
See: inverse square law

**third-octave **Term referring to
frequencies spaced every three octaves apart. For example, the

third-octave above 1 kHz is 8 kHz. Commonly misused to mean one-third
octave. While it can be

argued that "third" can also mean one of three equal
parts, and as such might be used to correctly

describe one part of an octave spit into three equal parts, it
is potentially too confusing. The

preferred term is one-third octave.

**volt** Abbr. E, also V. The International
System unit of electric potential and electromotive force,

equal to the difference of electric potential between two points
on a conducting wire carrying a

constant current of one ampere when the power dissipated between
the points is one watt. [After

Count Alessandro Volta.]

**wavelength **Symbol (Greek lower-case
lambda) The distance between one peak or crest of a sine wave
and the next corresponding peak or crest. The wavelength of any
frequency may be found by dividing the speed of sound by the frequency.