This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
`A' is for "amperes", which is a measurement of current equal to one
coulomb of charge per second. You usually speak of positive current -
current which flows from the more positive potential to the more
negative potential, with respect to some reference point (usually
ground, which is designated as zero potential). The electrons in a
circuit flow in the opposite direction as the current itself. Ampere
is commonly abbreviated as "amp", not to be confused with amplifiers, of
course, which are also commonly abbreviated "amp". In computation, the
abbreviation for amps is commonly "I".
`V' is for "volts", which is a measurement of electric potential.
Voltages don't "go" or "move", they simply exist as a measurement (like
saying that there is one mile between you and some other point).
`DC' is for "direct current", which is a type of circuit. In a DC
circuit, all of the current always flows in one direction, and so it is
important to understand which points are at a high potential and which
points are at a low potential. For example, cars are typically 12VDC
(twelve volts direct current) systems, and it is important to keep
track of which wires in a circuit are attached to the +12V (positive
twelve volts) lead of the battery, and which wires are attached to the
ground (or "negative") lead of the battery. In reality, car batteries
tend to have a potential difference of slightly higher than 12V, and
the charging system can produce upwards of 14.5V when the engine is
`AC' is for "alternating current", which is a type of circuit in which
the voltage potential fluctuates so that current can flow in either
direction through the circuit. In an AC circuit, it is typically not
as important to keep track of which lead is which, which is why you can
plug household appliances into an outlet the "wrong way" and still have
a functioning device. The speaker portions of an audio system comprise
an AC circuit. In certain situations, it is indeed important to
understand which lead is "positive" and which lead is "negative"
(although these are just reference terms and not technically correct).
See below for examples. The voltage of an AC circuit is usually given
as the RMS (root mean square) voltage, which, for sinusoidal waves, is
simply the peak voltage divided by the square root of two.
`W' is for "watts", a measurement of electrical power. One watt is
equal to one volt times one amp, or one joule of energy per second. In
a DC circuit, the power is calculated as the voltage times the current
(P=V x I). In an AC circuit, the average power is calculated as the
RMS voltage times the RMS current (Prms=Vrms x Irms).
`Hz' is for "hertz", a measurement of frequency. One hertz is equal to
one inverse second (1/s); that is, one cycle per second, where a cycle
is the duration between similar portions of a wave (between two peaks,
for instance). Frequency can describe both electrical circuits and
sound waves, and sometimes both. For example, if an electrical signal
in a speaker circuit is going through one thousand cycles per second
(1000Hz, or 1kHz), the speaker will resonate at 1kHz, producing a 1kHz
sound wave. The standard range of human hearing is "twenty to twenty",
or 20Hz-20kHz, which is three decades (three tenfold changes in
frequency) or a little under ten octaves (ten twofold changes in
`dB' is for "decibel", and is a measurement for power ratios. To
measure dB, you must always measure with respect to something else.
The formula for determining these ratios is P=10^(dB/10), which can be
rewritten as dB=10log(P). For example, to gain 3dB of output compared
to your current output, you must change your current power by a factor
of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power). The other
way around, if you triple your power (say, from 20W to 60W) and want to
know the corresponding change in dB, it is dB=10log(60/20)=4.77 (that
is, an increase of 4.77dB). If you know your logarithms, you know that
a negative number simply inverts your answer, so that 3dB corresponding
to double power is the same as -3dB corresponding to half power. There
are several other dB formulas; for instance, the voltage measurement is
dB=20log(V). For example, a doubling of voltage produces 20log2 =
6.0dB more output, which makes sense since power is proportional to the
square of voltage, so a doubling in voltage produces a quadrupling in
`SPL' is for "sound pressure level" and is similar to dB. SPL
measurements are also ratios, but are always measured relative to a
constant. This constant is 0dB which is defined as the smallest level
of sound pressure that the human ear can detect. 0dB is equal to
10^-12 (ten to the negative twelfth power) W/m^2 (watts per square
meter). As such, when a speaker is rated to produce 92dB at 1m when
given 1W (92dB/Wm), you know that they mean that it is 92dB louder than
10^-12W/m^2. You also know than if you double the power (from 1W to
2W), you add 3dB, so it will produce 95dB at 1m with 2W, 98dB at 1m with
4W, 101dB at 1m with 8W, etc.
`THD' is for "total harmonic distortion", and is a measure of the how
much a certain device may distort a signal. These figures are usually
given as percentages. It is believed that THD figures below
approximately 0.1% are inaudible. However, it should be realized that
distortion adds, so that if a head unit, equalizer, signal processor,
crossover, amplifier and speaker are all rated at "no greater than
0.1%THD", together, they could produce 0.6%THD, which could be
noticeable in the output.
An "Ohm" is a measure of resistance and impedance, which tells you how
much a device will resist the flow of current in a circuit. For
example, if the same signal at the same voltage is sent into two
speakers - one of which is nominally rated at 4 ohms of impedance, the
other at 8 ohms impedance - twice as much current will flow through the
4 ohm speaker as the 8 ohm speaker, which requires twice as much power,
since power is proportional to current.