Select the first letter of the word from the list above to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the '#' link. To the table of contents.

If you can't find what you want here, check out the Rane glossary

**AB test
**A direct comparison between two pieces of equipment, or two design alternatives.
Everything in the system is unchanged except the item under test, which is switched
between the two alternatives. Also see ABX test.

**Absorption coefficient**

The sound absorption coefficient defines the fraction of sound energy absorbed
by, for example, one reflection from a wall. Energy is proportional to the square
of sound pressure, so for an absorption coefficient of 0.2, 20% percent of the
energy is absorbed, reducing the sound pressure by 10.6%.

**ABX test**

ABX company has created hardware which allows an individual to test if they
can distinguish any difference between two pieces of equipment, or other variables.
The hardware randomly switches between the two alternatives, and neither the
person running the test, nor the listener (they can both be the same person)
know which alternative is active. This is a classic "double blind" test design
to eliminate bias. Arny Kruger has posted free
downloadable software to run tests yourself between two .wav files.

**Acoustics**

The effect of a room on music. Rooms are typically described as "dead" when
they have a lot of sound absorbing stuff like drapes and heavy furniture in
the room, which reduce the number of audible reflections (echoes). A room with
bare concrete walls has tons of audible reflections and is extremely "live".

**Acoustic Suspension **

A type of loudspeaker enclosure design, made popular by the Acoustic Research
AR3 among others. The cone of a driver is suspended by a "surround" (see the
driver cutaway drawing) which acts like a spring,
to restore the cone to its central position. The air in a sealed enclosure also
acts like a spring, and is used as such in this design. The result is a lower
resonant frequency, and lower bass, than would otherwise be obtainable. These
designs also typically use an enclosure filling material that makes the enclosure
act larger than it really is. See the section on Thiele-Small
analysis for greater detail.

**Active **

Containing tubes or transistors that require an external source of power, and
typically provide amplification

**Aliasing **

An effect that occurs when an analog signal is digitally sampled. See the graphical
example of 44,100 Hz sampling [by coincidence, 44.1 kb]. Samples (black
circles) of a 10,000 Hz tone are identical to samples of a 34,100 tone. In general,
samples of a tone of frequency f are identical (except for a phase shift) to
samples of a tone of frequency Fs - f, where Fs is the sampling rate. Thus if
f equals, say Fs/2 + f0, the digital recording will produce a false tone of
Fs/2 - f0. Anti-aliasing filters are used to eliminate tones above Fs/2 to avoid
this effect. See the Signal
Processing Section for more detail.

**Amp **

Short for amplifier, and also the abbreviation of Ampere, the unit for electrical
current. Electrical current is sent through the speaker input terminals to make
the speaker produce sound. A good 120 watt amplifier can momentarily send a
peak current of 80 amps through a speaker, which will really rattle your windows;
maximum sustained average current at 120 watts, which is also very loud, is
around 4 amps.

**Amplifier **

Same as power amplifier. Converts a low-power signal from a pre-amplifier into
a high-power signal (tens to hundreds of watts) needed to drive a loudspeaker.

**Anechoic chamber**

A room where the walls, ceiling, and floor have been covered with acoustic absorbing
material to (nearly) totally eliminate reflections. Useful for engineering development
of loudspeakers since it isolates the performance of the speaker. But the response
in a real room will be __very__ different.

**Bass-reflex**

See vented enclosure.

**Biamplified**

Using two stereo power amplifiers. For a typical system with two tweeters, midranges
and woofers, one amplifier drives two woofers, and the second amplifier drives
the midranges and tweeters. This is discussed further in the section
on sound system design

**c **

The speed of sound, 344.4 meters per second at 1 atmosphere pressure, 20^{0}
C; 347.3 m/s @ 25^{0} C; very little change with pressure.

**Capacitor **

A component in crossover networks. Available at electronic supply stores. Capacitance
is denominated in Farads. A cousin of a resistor, but presents a high resistance
to low frequencies, and low resistance to high frequencies. Perversely, more
capacitance means less resistance.

**Cascade **

To connect the output of one network to the input of a second network.

**Circuit **

A conglomeration of electronic components. More or less interchangeable with
"network", but networks are usually small, tidy and passive, and circuits tend
to be big, messy and active.

**Class A **

A type of amplifier design that generally produces less distortion than class
AB, or B amplifiers, but with lower efficiency lower power output.

**CLIO**

A PC based audio measurement system made by Audiomatica.
Includes a calibrated microphone and software. This system can measure impedance,
component values of inductors and capacitors. Most important, the time-domain
and frequency response, both amplitude and phase, of drivers and systems can
be measured. It also measures room reverberation time, SPL sound levels, and
sundry other stuff. This was an absolutely essential tool in developing my system,
and I highly recommend it to anyone serious about building sound systems

**Coloration **

An alteration of the character of music due to imperfect frequency response.
Can make music sound shrill, dull, boomy, etc.

**Comb filtering **

A filter that reduces the frequency response at a fundamental tone and all of
its odd harmonics. If a signal is combined with its duplicate, except that the
duplicate is delayed by a path-length difference d (a time-delay of d divided
by the speed of sound) the two signals will cancel at all frequencies where
d is an odd multiple of half of the wavelength. For example, if d=12 inches,
cancellation will occur at wavelengths of 24, 8, 4.8, etc. inches, corresponding
to frequencies of 565, 1695, 2825, etc. Hz. If these frequencies are plotted
as vertical lines on a linear frequency scale, it will look something like the
teeth of a comb. There is a doubling of the sound pressure at the frequencies
between these teeth. The overall result is a roller-coaster frequency response.

**Cone **

The cone-shaped part of a loudspeaker driver that moves the air. Made of paper,
polypropylene, or recently out of high-tech materials such as Kevlar. See a
cut-away drawing of a driver (8.2 kb).

**Convolution **

A mathematical process. To convolve a 128-point HRTF
with a sound wave, both functions must be sampled at the same rate, e.g. 44,100
samples per second. The 128 HRTF samples then cover a time period of 2.9 milliseconds.
The HRTF samples are first flipped in time. They are then aligned with the first
128 sound wave samples. Paired samples are multiplied together, and the 128
products are added together to create one new HRTF-processed time sample. The
HRTF is then shifted one notch to align with sound samples 2-129, and another
multiply and add is performed. This shift, multiply, and add is repeated until
you run out of sound samples. Since a 5-second sound segment has 220,500 samples,
we are talking about 28 million multiplications!

**Critical Band **

The ear behaves as if it contains a bank of filters, each filter passing frequencies
within the critical band. Above 1 kHz the critical bandwidth is close to 1/6
octave. Below 100 Hz it is approximately 30 Hz. Two tones separated by more
than a critical bandwidth are received by the cochlea more-or-less independently.
Within a critical band a louder tone can make a softer tone inaudible, which
is called "masking"

**Crossover **

An electronic network for dividing sound into high and low frequencies for reproduction
by woofers, tweeters, etc. A crossover for a two-way system would consist of
a high-pass filter that sends the high frequencies to the tweeter, and a low-pass
filter that sends the low frequencies to the woofer. For a three-way system
a band-pass filter is added to send the middle frequencies to the midrange speaker.
The response beyond the crossover frequency is reduced by an amount that depends
on the order of the crossover filters. A 1^{st} order filter reduces
the response 6 dB per octave, a 2^{nd} order 12 dB per octave, a 3^{rd}
order 18 dB per octave, and a 4^{th} order 24 dB per octave. Crossover
design is discussed in the section on sound
system design.

**Crossover frequency**

The nominal dividing line between frequencies sent to two different speaker
drivers.

**Damping factor**

A characteristic of amplifiers; 8 Ohms (the nominal impedance of common loudspeakers)
divided by the output impedance. A large damping factor is allegedly good because
it gives an amp tighter control over the loudspeaker. This is not true, and
a damping factor of 1000 is really not better than a factor of 200, as shown
in the section on Thiele-Small
analysis.

**dB**

A logarithmic scale. A 3 dB increase means twice as much power; a 6 dB increase
means 4 times as much. A 60 dB increase is one-million times as much, and a
120 dB increase is one-million times one-million as much power.

**Degrees of freedom**

The number of independent variables that must be specified to determine the
state of a system. For example, to specify the position of a molecule, the x,
y, and z coordinates are required, so it has 3 degrees of freedom. A collection
of N molecules has 3N degrees of freedom. However in some cases, such as a diatomic
molecule, certain degrees of freedom are restricted, reducing the number. See
the short discussion elsewhere

**Diffraction
**When a sound wave from a loudspeaker reaches an edge of the speaker
cabinet, it creates a new source of sound that appears to emanate from the edge.
This is called edge diffraction. Diffraction will occur any time a wave encounters
an obstacle, or abrupt change in a surface. An animated
cartoon (107 kb) shows a blue wave emanating from a dome tweeter that diffracts
first from the rim of the tweeter as a green wave, and then from the cabinet
edge as a red wave. Rounding the corners of a speaker cabinet will reduce, but
not eliminate, diffraction. See the section on loudspeaker
construction, for more discussion on this topic.

**Diffuse-field Equalization**

A manner of equalizing data, such as head-related
transfer functions, such that the power spectrum average over all directions
is uniform. This eliminates measurement effects which are a function of frequency,
but not a function of the direction of the incoming sound.

**Dome**

Same function as a cone, but shaped as it is named. Used
for many tweeters and some midranges. Made of fabric, aluminum, titanium, or
other high-tech material.

**Doppler distortion**

Harmonic and intermodulation distortion caused by the motion of a loudspeaker
cone. See the analysis for a piston
in a tube.

**Doppler shift**

A change in pitch caused by a relative motion between the sound source and an
observer. The pitch is higher when the source and observer are getting closer,
and lower when they are moving apart. A
mathematical analysis is given in the physics section.

**Driver**

"Driver" always refers to a single naked speaker (e.g. a woofer or a tweeter)
rather than an ensemble of drivers, enclosure, and crossover network, which
is a loudspeaker. The primary parts of a driver are (1) the cone, which moves
the air, and (2) the voice coil and magnet, which moves the cone. See a cut-away
drawing of a driver (8.2 kb). Sometimes "loudspeaker" is used to mean "driver."

**e **

A magical mathematical number equal to 2.71828......... It is an irrational
number, so unlike rationals, the decimals run on forever, never forming a repeating
pattern. One form of Euler's equation, e^{iπ}+1=0,
relates 5 special numbers. This equation has been converted to music by Tom
Dukich

**Electrostatic Speaker**

A loudspeaker that uses a large, flat (sometimes curved), thin membrane instead
of a cone. The membrane is situated between two screens that are charged with
a high electrical voltage to move the membrane. Ideally the entire membrane
moves back and forth as a unit. The large area and low moving mass are advantages.
The limited membrane travel seriously limits the low frequency response. Also
presents a difficult load impedance to an amplifier.

**Filter **

An electronic circuit designed to selectively pass certain sound frequencies.
Also see crossover.

**Feedback **

Positive feedback in an amplification loop causes the output signal to grow
uncontrollably, and results in the annoying squeal when a microphone gets too
close to a loudspeaker. Negative feedback is a technique intentionally employed
in typical amplifier designs to reduce distortion. Hence, positive feedback
is a negative quality , and negative feedback is a positive quality (some folks
dispute the latter). If engineers didn't make things hard to understand we wouldn't
get paid so much.

**Fletcher-Munson Curves **

Curves originally obtained by two Bell Lab scientists relating subjective sound
levels to objective levels of pressure/power. See discussion
in the section on music and ears. Or view the
curves themselves [102 kb].

**Fourier transform **

A graph of sound pressure vs. time is an illustration of a time-domain function.
Sound can also be described as a frequency-domain function, as the superposition
of a group of sinusoidal waves spanning the audio frequency spectrum. Mathematically
both functions contain exactly the same information and each function perfectly
defines the sound. A Fourier transform, and inverse transform, convert functions
back and forth between the time and frequency domains. This is a error-free
transformation, but the width of the frequency range and frequency resolution
(fine-grain detail) is limited by the time resolution and duration. The longer
the duration, the finer the detail, and the finer the time detail, the wider
the frequency range. A fast Fourier transform is a technique for performing
these mathematical operations very quickly with a computer. See the Signal
Processing Section for more detail.

**Frequency **

The rate at which sound pressure varies. Denoted by f and denominated in Hertz=cycles
per second; abbreviated Hz, (or kHz, meaning thousands of Hz).
A 1000 Hz (1 kHz) tone pushes your eardrum back and forth 1000 times each second.
The lowest bass note on an organ keyboard is a rumbling 16.4 Hz; the highest
is a squeak of 18,794 Hz, which is inaudible to most males over the age of 40.
These sample tones are audible with good loudspeakers or headphones, but are
not audible with many computer speakers: a 100 Hz
tone (12 kb wav file);and a 10kHz tone (44
kb wav file). Frequency can also be defined mathematically as the derivative
of phase with respect to time, in which case it is usually denoted by ω
and denominated in radians per second. ω=2πf.

**Frequency band**

A specific range of frequencies. For example a typical subwoofer band is 20
Hz to 100 Hz.

**Frequency response**

The response of a system to a fixed input voltage, at different input frequencies.
Same as frequency domain response.

**Group delay**

Mathematically group delay is the derivative of the phase, in radians, with
respect to frequency. Filters cause such a delay. Group delay can also mean
an average of this delay over some frequency band.

**Harmonic **

A tone whose frequency is an integer times the frequency of the fundamental
(lowest) tone. Every note played on a musical instrument consists of a fundamental
tone plus many harmonics.

**Harmonic distortion**

Spurious harmonics produced by the sound system. Usually not quite as objectionable
as intermodulation distortion because harmonics occur naturally in music. See
the discussion in the section on music and ears.

**Head-related transfer function**

The sound wave that reaches the eardrum is quite different than the sound wave
traveling through the air towards the ear. The incident sound is modified by
reflections from the head, pinna, and within the ear canal.
These effects are mathematically represented by the head-related transfer function
(HRTF). An impulsive sound will reach the eardrum spread out over time, and
this function of time is the HRTF. An example is shown
here [44.6 kb].

The frequency-domain representation of the same HRTF is here [47.5]. It is different for sounds arriving from different angles. The magnitude dependence on angle is shown here [35.7 kb]. This is the HRTF for the right ear, located at 0-degrees elevation and 90-degrees azimuth. An angle of 0-degrees elevation and -90-degrees azimuth represents a source on the opposite (left ear) side of the head. 90-degrees elevation is directly overhead. The HRTF is a little different for every person, but one can expect the general characteristics to be similar. All examples shown here are drawn from the diffuse-field equalized data, posted on the web by Bill Gardner and Keith Martin (this link seems to occasionally disappear).

**High-end Audio**

Equipment that is always expensive and pretentious, and sometimes excellent. Also see Hi-Fi Fetishism.

**i (or j) **

The basic "imaginary" number equal to the square root of minus one. Multiplying
a quantity by i is equivalent to adding a 90^{0} phase shift. Quantities
involving i are called "complex" numbers, and if you don't already know about
this stuff it is a bit complex.

**Impedance**

The technically correct term for the resistance produced by inductors, capacitors,
and networks. For the technically minded, the real part of an impedance is equal
to resistance, and the imaginary part is called reactance.

**Impulse response**

The response of a system to a "spike" input that abruptly rises from zero and
then abruptly decays back to zero. The response of the system to any input can
be predicted from the impulse response. The impulse response measured in a room
also shows a series of echoes of the direct response, which are reflections
from walls, etc. See the Signal
Processing Section for more detail.

**Incoherent addition**

Sound from two loudspeakers can combine coherently or incoherently. This is
generally different at different spots in the room, and the following refers
to one specific spot. Coherent addition means all of the pressure peaks and
lows of the sound wave from one speaker arrive at exactly the same time as the
peaks and lows of the other. The pressures then add. Twice the pressure means
4 times the acoustic power, so the sound level increases 6 dB. Incoherent addition
means the peaks and lows from one arrive at random times compared to the other.
In this case the power adds and the sound level increases 3 dB. For frequencies
below 100 Hz, where the sound tends to be monaural, and the distance between
pressure peaks and lows is more than 5 feet, addition tends to be coherent.
At higher frequencies addition tends to be incoherent.

**Inductor **

A component in crossover networks. Available at electronic
supply stores, or can be made by winding wire in a doughnut shaped coil. Inductance
is denominated in Henries. A cousin of a resistor, but presents a high resistance
to high frequencies, and low resistance to low frequencies. More inductance
means more resistance.

**Infinite baffle**

A speaker enclosure that is completely sealed except for openings occupied by
the driver cones. The enclosure performance is similar to mounting the drivers
on a panel of infinite extent, which explains the name. Also called a sealed
enclosure, which is actually more accurate.

**Inter-modulation distortion**

Spurious tones created by the sound system. The frequency of a spurious tone
equals the frequency of one tone in the music added to or subtracted from the
frequency of a second tone in the music. Sounds awful. Also see discussion in
the section on music and ears.

**j (or i) **

The basic "imaginary" number equal to the square root of minus one. Multiplying
a quantity by j is equivalent to adding a 90^{0} phase shift. Quantities
involving j are called "complex" numbers, and if you don't already know about
this stuff, it is a bit complex.

**k **

Abbreviation for 1000. 1kHz = 1000 Hz. Alternatively, frequently used as the
algebraic symbol for the propagation
constant.

**Kinetic energy**

Energy due to motion. Proportional to an objects mass and the square of its
velocity. As a car accelerates, energy from burning gasoline is converted into
kinetic energy. To slow a car down, the kinetic energy must be converted into
heat by the brakes.

**Linear **

If the input amplitude is increased by a factor of, say 1.2, then the output
amplitude increases by a factor of 1.2, no more and no less. This is the ideal
for all parts of a sound system, since no distortion is introduced.

**Load **

Whatever is connected to the output of an amplifier or electronic circuit.

**Litz wire **

Wire woven in a pattern to reduce the "skin effect."
For ordinary stranded wire each strand tends to stay the same distance from
the wire center. The magnetic field produced by the current in the wire effects
the strands differently depending on this distance, causing the skin effect.
In Litz wire the strands weave in and out, such that all strands experience
almost the same magnetic field, and carry nearly equal current.

**Lobing**

A vertical array of two or more drivers will beam energy in a series of vertical
lobes, over a portion of the frequency band. Probably more than you want to
know about lobing can be found by following
this link.

**Loudspeaker **

Converts electrical energy to sound energy. This can mean an entire system such
as a woofer, midrange, tweeter, crossover network, and enclosure. It can also
mean an individual speaker driver such as a woofer.

**Matlab
**A software package for engineering analysis that does just about anything
an engineer could desire. Lots of graphics capability, full kit of analysis
tools. Produced by Mathworks (Cleve
Molar, a founder of Mathworks, was a classmate of mine at Caltech - too bad
I didn't have his entrepreneurial spirit).

**Midrange**

A speaker driver 2-5" in diameter for reproducing the middle range of sound
frequencies.

**Minimum phase filter**

Not an easy thing to explain. A given amplitude response can in theory be produced
by an infinite set of filters, each of which creates a different phase response.
A phase response that is a non-linear function of frequency smears the response
in time. The filter that produces the amplitude response with the least time-smearing
is the minimum phase filter of the set.

**Mode **

Characterized by having a specific pattern (e.g. of sound pressure). Some sound
features, like wavelength, can have any value. With modes there is no halfway;
there are modes 1,2,3, etc., but no mode 1-1/2. Also see the discussion of resonant
modes in a room.

**Network **

A bunch of electrical components hooked together.

**Non-linear**

Anything other than linear. A non-linear relationship
between the input and output of an amplifier, loudspeaker, or whatever creates
harmonic and intermodulation distortion.

**Octave **

A one octave increase in frequency means doubling the frequency. A note one
octave higher than another sort of sounds the same, but at a higher pitch. Play
a one octave step from 440 (A4 standard for orchestra tuning) to 880 Hz.
(180 kb wav file; requires patience). 1/3 octave smoothing means averaging the
frequency response over 1/3 octave intervals. This smoothes out (i.e. obscures)
abrupt changes in frequency response.

**Oscilloscope
**A piece of electronic lab equipment with a screen similar to a television
screen, which shows an instantaneous picture of the voltage of an electrical
signal. When connected to the output of an audio amplifier the picture is an
analog of the sound pressure coming out of the loudspeaker

**Passive **

Not requiring an external source of power; containing resistors, capacitors,
inductors, etc., but no tubes or transistors; provides no amplification.

**Pentode **

See tube.

**Phase **

A measure of how well two waves are aligned. "In phase" means the crests align
perfectly and add together; "180^{0} out of phase" means crests align
with valleys and the waves cancel each other. It is easier to show
graphically (18.8 kb) than to describe. A system has a flat (i.e. good)
phase response if the relative phase of the input and output signals is the
same for all frequencies (actually a linear variation is equally good). This
is particularly important for good transient response.

**Phon**

The unit of subjective sound level. Defined such that the sound level in phones
equals the level in SPL for a 1000 Hz tone. Generally used in tests where listeners
are asked when a tone at a different frequency sounds as loud as a 1000 Hz tone,
to define the subjective frequency response of the ear. See discussion
in the text on music and ears.

**Pi (π) **

Another magical mathematical number, 3.14159....... It is an irrational number,
so unlike rationals, the decimals run on forever, never forming a repeating
pattern. Entire books have been devoted to Pi, which has roots far deeper than
its relationship with circles.

**Pink noise**

Pink noise is random noise where the power is spread uniformly over a specific
spectrum of frequencies, such as 20-20,000 Hz for audio. It sounds like a hiss,
much like tuning between FM stations. For a theoretically infinite spectrum
it is called white noise.

**Pinna**

The exterior part of the ear. The ridges are important in determining the direction
sound comes from - see photo [19.4 kb]

**Placebo effect
**If you believe something will make an improvement, it will make an improvement
(usually). A scientific fact so well documented that trials of new drugs are
always blind-tested vs. a placebo (something with no "real" effect). The fact
that a significant percentage of people taking the placebo are always cured
is a rather remarkable proof of the power of the brain. It follows that one
is often happier as a stupid believer than as a wiseacre cynic.

**Port **

See vented enclosure.

**Power Spectrum**

A power spectrum plot shows the relative levels of power at different frequencies.
The example power spectrum plot discussed
in the section on music and the ear shows that the power around 4 kHz is about
20 dB below (1% of) the power at the lowest frequencies, and power drops another
20 dB for frequencies above 12 kHz.

**Pre-amp **

Preamplifier. Converts an electrical signal from a CD player or other music
source into a stronger signal to drive a power amplifier. Also controls the
source selection, volume, and treble and bass response.

**Propagation constant**

The mathematical term which defines wave propagation. It is equal to 2π
divided by the wavelength.

**Push-pull**

A type of amplifier design. A tube or transistor that outputs positive voltage
swings is matched with a second tube or transistor that outputs negative voltage
swings. This results in a symmetrical response for positive and negative voltages.

**Q**

The "quality factor" of a network or system. The lower the losses, the higher
the Q. In electrical circuits losses occur due to resistance. In mechanical
systems losses occur due to friction. An example of a high Q mechanical system
is a bell, which rings for a long time after it is struck.

**Resistor **

A basic component in electronic circuits that resists the flow of electrical
current. Resistance is denominated in Ohms. Higher resistance results in less
current. All electrical components (except superconductors) have some resistance,
wanted or not. Most home speakers are nominally rated at 8 Ohms of resistance
(4 Ohms for cars), but in reality the resistance is closer to 6 Ohms in most
of the band, has a peak of 20-100 Ohms at resonance, and also rises at high
frequencies due to voice coil inductance.

**Resonance **

A reinforcement of sound due to echoes adding in phase with the original source.
In general, a motion or electrical or other response at a frequency where a
system responds much more strongly than at other frequencies. The higher the
system Q, the narrower the frequency band where
the response is strong.

**Reverberation **

The echoes in a room that one hears after the original sound stops. The classic
example is organ music in a cathedral. The usual measure of reverberation time,
denoted RT_{60}, is equal to the time it takes the sound to decay 60
dB after the sound source stops, in seconds.

**Scalar**

A physical quantity that involves magnitude, but not direction. Examples are
speed, temperature, and pressure. Quantities that also involve direction, such
as velocity, are called vectors.

**Scientific notation**

A convenient way of writing very large or very small numbers. 2x10^{27}
means 2 followed by 27 zeros. (The 27 should be a superscript; some web browsers
don't get it right). Basically the decimal point is moved 27 places to the right,
filling in any spaces with zeros. So 1.6x10 ^{27} means 16 followed
by 26 zeros. 2x10^{-27} means moving the decimal point 27 places to
the left, or a decimal point followed by 26 zeros and then the number 2.

**Sensitivity
(of a loudspeaker) **

Sensitivity is normally defined as the SPL level produced
at a distance of 1 meter from the speaker, with 1 Watt of input power. However
it is also common to specify an input of 2.83 volts instead of 1 watt of input
power. This would be equivalent if the input impedance were 8 Ohms, but it is
usually really closer to 6 Ohms.

**Single-ended **

A type of tube amplifier design where the output stage is biased such that a
DC current exists when the input voltage is zero. A positive (negative) input
voltage causes the output current to increase (decrease) relative to the DC
bias level. The output transformer does not respond to the DC bias current,
so the final output is only the AC part due to the input voltage. This type
of design does not respond symmetrically to positive and negative input voltages.

**Skin effect **

At very high frequencies electrical current tends to flow in a thin layer on
the surface of a conductor, a layer of thickness roughly equal to the "skin
depth" δ. The skin depth for copper at 20 kHz is about 0.5 millimeters (0.018
inches). The 16 AWG gauge wire I use for speaker connections consists of 19
strands of .28 millimeter (.011 inch) diameter wire. The overall diameter of
the bundle is 1.5 millimeters (.060 inches). Assuming that the strand bundle
acts like a solid wire of the same diameter (a good assumption), the skin effect
for this wire is shown by a plot of the current
[40.1 kb] magnitude as a function of wire radius. The blue curves are the exact
Bessel function solution for a cylindrical conductor. The red curves are the
exponential solution exp(-x/δ) for an infinitely thick rectangular slab.
The Bessel solution is very close to the exponential solution for wires greater
than 10 δ in diameter. However when the thickness is of the same order as
δ, the Bessel function solution exhibits much less of an effect. The effective
resistance of the wire is increased by 3.6% and 13.2% at 10kHz and 20 kHz respectively.
Since the total resistance of a 10 meter length is about 0.1 Ohm, this increase
of about .01 Ohms is insignificant compared to the typical voice coil resistance
of 6 Ohms. The wire inductance is slightly reduced by the skin effect, but is
tiny compared to the voice coil inductance in any case. The skin effect can
be reduced by using Litz wire.

**Slew rate **

A characteristic of amplifiers. The maximum rate of change of output voltage
in response to a square-wave input, usually measured in volts per microsecond.
The higher the better.

**Solid-state **

See transistor.

**Speaker **

Same as loudspeaker.

**Spectrum **

A range of frequencies; e.g. the standard audio spectrum extends from 20 Hz
to 20 kHz.

**SPL **

Sound pressure level. A logarithmic dB scale is used, akin to the Richter scale
for earthquakes. A 3-dB increase means doubling the power. Zero dB SPL is the
threshold of hearing; a quiet room has a background SPL of about 40-dB; a loud
rock concert can go to 120-dB; the threshold of pain is 135 dB. Peak sound pressure
is 3 dB higher than root-mean-square (RMS) average pressure. SPL normally refers
to RMS pressure. Also see sensitivity.

**Subwoofer **

Like a woofer but even bigger in diameter
and lower in frequency.

**Thiele parameters **

A set of parameters that define the behavior of a driver
in an enclosure. Used for designing enclosures and analyzing the sound response.
See the section on Thiele-Small analysis,
and also the detailed definitions of
the parameters.

**Time domain response **

The response of a system measured over an interval of time. One can mathematically
transform the time domain response to obtain the frequency domain response (and
vice-versa), using a Fourier transform.

**Transistor **

The basic component of most amplifiers. A glob of magic stuff (semiconducting
solid-state material) that amplifies electrical current. From smaller than a
pin head up to an Oreo cookie in size (transistor case included). Reviled by
some high-end types as having a harsh inhuman
sound.

**Triode **

See tube.

**Tube **

Short for vacuum tube; a small glass bottle with all the air removed, containing
metal sheets and screens (three for a triode, five for a pentode) to control
electron flow. The basic component of amplifiers built prior to the 60's. Many
high-end audio types swear they sound better than transistors. High power output
tubes are usually pentodes; some folks think triodes produce a cleaner sound.
(My old Dynakit Stereo 70 tube amp put out enough heat for the whole house).

**Tweeter**

A speaker driver on the order of 1" diameter for reproducing the highest sound
frequencies.

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**Vented
enclosure**

Same as Bass-reflex. A speaker enclosure with a enclosure "port", which is a
hole or a cardboard tube connecting the inside of the box with the outside world.
The port generates sound along with the speaker driver, and generally extends
the bass response. See further discussion in the
section on Thiele-Small analysis.

**Vector**

A physical quantity that involves direction as well as magnitude. Velocity is
a vector; it's magnitude is speed, and the vector points in the direction of
travel.

**Voice coil **

The coil of wire in a speaker driver that conducts the electrical current from
the amplifier. This current creates a magnetic field that interacts with the
permanent magnet of the speaker to move the speaker cone. See a cut-away
drawing of a driver (8.2 kb).

**Volt**

Using** **an analogy of water flow, electrical voltage is analogous to pressure,
and electrical current analogous to the volume of water flow. Voltage pushes
current through a circuit. One volt will create a current of 1 amp through a
1-Ohm resistor. In general, current is directly proportional to voltage and
inversely proportional to resistance. (Ohms law).

The commonly used symbol for frequency in radians per second. Equal to 2π times the frequency in Hz.

**Watt**

A unit of power. A 200 watt amplifier can make the walls shake pretty well.

**Wavelength**

The distance between two crests (or valleys) of sound wave pressure. Easy
to see graphically (5.2 kb). Usually denoted by the symbol λ. Varies
from 56 feet for a 20 Hz tone to 0.7 inches for a 20 kHz tone. In general the
wavelength in inches is equal to 13,543 divided by the frequency in Hz.

**Window**

A Fourier transform of a segment of a pure 400 Hz tone will produce a spectrum
with a sharp peak at 400 Hz. But since the segment is finite, if the tone amplitude
is constant, the spectrum will have "sidelobes" that extend over a wide range
of frequencies. The sidelobes can be sharply reduced by contouring the amplitude
with a "window." An example of the spectrum with and without a window is shown
here[50 kb]

**Woofer**

A speaker driver 8-12" in diameter for reproducing low sound frequencies.

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**Zobel network**

Generally used to mean a network that compensates for the voice-coil impedance.
Without a Zobel the driver input impedance rises with frequency, which affects
the crossover response. A Zoble flattens the input impedance. See impedance
compensation in the crossover design section for further discussion and
design information.

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