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A typical speaker or amplifier specifications list will tell these important things:

For speakers, this is the (or ) the driver will produce given one watt of power as measured from one meter away given some input frequency (usually 1kHz unless otherwise noted on the speaker).

Typical sensitivities for car audio speakers are around 90dB/Wm. Some subwoofers and piezo horns claim over 100dB/Wm. However, some manufacturers do not use true 1W tests, especially on low impedance subwoofers. Rather, they use a constant voltage test which produces more impressive sensitivity ratings.

in an amp/speaker refers to the range of frequencies which the speaker can reproduce within a certain power range, usually +/-3dB.

of the driver, typically 4 ohms, although some subwoofers are 8 ohms, some stock Delco speakers are 10 ohms, and some stock Japanese imports are 6 ohms.

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.

, or Nominal power handling is the continuous power handling of the driver. This figure tells you how much power you can put into the driver for very long periods of time without having to worry about breaking the suspension, overheating the voice coil, or other nasty things. When matching speaker output ratings to amps, make sure that you pair them up with the same impendance rating. For example, if you get speakers with an RMS rating of 45W, you will want to pair them with an amp with an output power rating of at least 45W RMS PER CHANNEL to utilise the maximum potential of the driver(s)

When configured properly, you will NOT burn the speaker drivers up should your amp be more powerful than your speakers. It is recommended that your amp's RMS rating be bigger than your speaker handling for added control and future upgrade plans.

with Peak power handling is the maximum power handling of the driver. This figure tells you how much power you can put into the driver for very brief periods of time without having to worry about destroying it.

Cheaper, not so reputable speakers/amps rate their power output according to Peak power handling. For example, Rodek's old SM-7540 amplifier rated it at 700W Max power when it only had an effective RMS output of about 45W per channel running at 12V when properly measured with scientific equipment.

FOR AMPLIFIERS:

or is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually at most 1%THD) at a certain frequency (usually 1kHz).

A complete power specification should include all of this information, e.g. "20W/ch into 4 ohms at < 0.03%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "20W/ch at < 0.03%THD". The amplifier should also be able to sustain this power level for long periods of time without difficulties such as overheating.

is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually much higher than the continuous rating level) at a certain frequency (usually 1kHz).

A complete power specification should include all of this information, e.g. "35W/ch into 4 ohms at < 10.0%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "35Wch at < 10.0%THD".

Some manufacturers state the "peak power output" rating by including the amount of power which can be drawn from "headroom", or rather, with the addition of capacitors (additional power storage)

They usually will not tell you this in the specification, however; indeed, they tend to prominently display the figure in big, bold letters on the front of the box, such as "MAXIMUM 200W PER CHANNEL!!!" when the continuous rating is 15W/ch and the unit has a 5A fuse.

To those of you who bought your 1400W amps, I can assure you that you are getting a whole lot less than what you bargained for at the price. 1400W RMS in total is a very high amount of current and is only available in VERY LOUD amplifiers/speakers.

Damping factor represents the ratio of the load being driven (that is, the speaker - usually four ohms) to the output impedance of the amplifier (that is, the output impedance of the transistors which drive the speakers).

The lower the output impedance, the higher the damping factor. Higher damping factors indicate a greater ability to help control the motion of the cone of the speaker which is being driven. When this motion is tightly controlled, a greater transient response is evident in the system, which most people refer to as a "tight" or "crisp" sound. Damping factors above 100 are generally regarded as good.

or S/N is the ratio, usually expressed in decibels, of the amount of true amplified output of the amplifier to the amount of extraneous noise injected into the signal. S/N ratios above 90 to 95dB are generally regarded as good.

On some amps, you will see a 2 OHM STABLE assurance tag or "HIGH CURRENT" amplifier.

An X-ohm stable amplifier is an amp which is able to continuously power loads of X ohms per channel without encountering difficulties such as overheating. Almost all car amplifiers are at least four ohm stable.

Some are two ohm stable, which means that you could run a pair of four ohm speakers in parallel on each channel of the amplifier, and each channel of the amp would "see" two ohms.

Some amps are referred to as high-current, which is a buzzword which indicates that the amp is able to deliver very large (relatively) amounts of current, which usually means that it is stable at very low load impedances, such as 1/4 or 1/2 of an ohm.

Note that the minimum load rating (such as "two ohm stable") is a stereo (per channel) rating.

In bridged mode (when you join the current flow of two seperate amplifiers together for more power at a very minor expense of Total Harmonic Distortion), the total stability is the sum of the individual channels' stability

Credits: Amery Reuben