This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.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

running.

`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

frequency).

`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

power.

`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.

Continue to: