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Here is a list of common definitions that you can use to help further your knowledge and help insure that you're making the proper decision when making your purchase, or to help guide you in asking the right question in here or in a shop.

Amp Classes
The "class" of an amplifier assists in determining the different performance characteristics because the "class" refers to the amplifier's internal circuitry:

Class A amplifiers are sought after for the high quality of their sound; however, because of the configuration of its transistors, a pure class A amplifier is inefficient and runs very hot.

The most widely used car amp design, Class AB amps run cooler and more efficiently than a class A. Class AB also provide low distortion levels and high reliability. These are most commonly used to power speakers but can run subs.

Class D amplifiers (usually used to power subwoofers) boast significantly higher efficiency, produce less heat, and draw less current than Class AB amps. Class D amps produce higher levels of distortion; however, this distortion occurs at high frequencies that are typically removed by a low-pass filter and therefore inaudible.

Class BD amplifiers give you the highly efficient design of a Class D amp with the clarity and sound quality of a Class B amp.

Amp Channels - the common number of channels for amplifiers are: Mono (generally for subs), 2-channel and 4-channel; however, amplifiers are manufactured in 3, 5, and 6 channels too.

THD at Rated RMS Power - the amount of change in harmonic content of the signal as it is amplified. A lower figure indicates less change and a more accurate amp. THD below 0.10% is inaudible.

Signal to Noise Ratio - this spec compares the strength of the signal to the level of background noise. A higher value indicates less background noise.

Channel Separation - Also referred to as crosstalk. The amount of interference on one stereo channel caused by the leaking of the other stereo channel. The higher the rating in decibels (dB), the better the Channel Separation.

Bass Boost - the simple switch of a button or turn of ****, this feature can allow you to boost the bass output.

Peak Power Output - measured during a brief musical burst, such as a sudden drum accent. Some manufacturers display peak power ratings on the face of their products to increase the appeal, however the RMS power rating is more accurate and is what should really be used when comparing two products.

RMS Power Output - the amount of continuous power, measured in watts, that an amplifier produces, the higher the RMS figure, the louder and cleaner your music sounds. The RMS output figure is much more accurate than the peak rating when comparing products. Generally measured in 4 ohm, 2 ohm, and 1 ohm

Peak Power Handling - the amount of power a speaker can handle during a brief musical burst. The RMS power range figure is a much more reliable rating for determining the speaker's power requirements.

RMS Power Handling - the amount of power a speaker requires to operate safely and properly. RMS Power Handling refers to the amount of power a speaker can handle on a continuous basis, giving your subwoofer 75% - 90% of its recommended RMS rating allows for optimal and safe operation.

Impedance - the load value (in ohms) that the speakers present to the amplifier, or the amount of resistance to the current flow. This is a key specification as subs wired in parallel or low impedances can create problems with your amplifier and potentially void all warranties. Always make sure that the impedance of a speaker or sub matches the amps output or capabilities (a 2 ohm sub can not be crossed over as cross overs require 4 ohms). In short CHECK YOUR OHM value before you buy anything!!!

Sensitivity - an efficiency or sensitivity rating tells you how effectively a speaker converts power into sound. The higher the number, the more efficient the speaker and the louder it will play with the same input power. An efficient speaker helps you maximize your available power. Most sensitivity ratings measure the speaker's output with 1 watt of input, 1 meter from the speaker. Some speakers' sensitivity is measured with 2.83 volts of input, instead of 1 watt.

Frequency Response - The range of frequencies the speaker will reproduce (lowest frequency to the highest). Bigger is better, meaning the wider the range, the better. The optimal range is 20 - 20,000 Hz (which is the range of human hearing).

Dual Voice Coil Subs - have more wiring flexibility than single voice coils subs; however, single voice coil subwoofers provide easier hookup options for wiring multiple subwoofers in parallel or series. You may also run a dual voice coil (DVC) sub in parallel, series, or combination.

Sealed Box Volume - the recommended interior volume in cubic feet that a sealed box needs in order for the sub to deliver its rated performance.

Ported Box Volume - the recommended interior volume in cubic feet that a ported box needs in order for the sub to deliver its rated performance.

CD Signal to Noise Ratio - is a key spec that is often overlooked by most people. The CD S/N Ratio measures how well the CD player silences background noise. Higher ratings, in decibels (dB), indicate less noise, and crisper sound.

Preamp/RCA Outputs - a vital specification as they allow you to connect 1 or more external amp(s). The subwoofer output is an important feature as it will provide much cleaner lower frequencies for your subs rather than a standard front or rear set of outputs.

Subwoofer Preamp RCA Output - an output that has a built-in low-pass crossover for specific connection to a subwoofer amplifier that powers a low frequencies driver (a subwoofer).