How much current a speaker will draw is measured by speaker impedance. Standard speaker impedance is 8 ohms, with 4 ohms being very good quality, and generally much more expensive. It's important to note that a high quality amplifier is needed if purchasing 4 ohms speakers.
Loudness in Decibels (dB)
Decibels measure loudness, which is an important factor to consider when buying speakers, especially when listening at high volume. Even small increases in decibels makes a big difference in volume - every 10 decibel rise in volume makes the sound twice as loud!
Amount of Power in Watts (W)
Electrical power is measured by watts, and therefore an amplifier's output is measured in watts. Every loudspeaker on the market has its maximum number of watts that it can accommodate, which will be specified by the manufacturer. Speakers can be severely damaged if the amp puts out more power than the speaker can handle. It's typical for manufacturers to provide 2 figures for power for the loudspeakers and the amplifiers.
Top quality speakers are very sensitive and can deliver much more sound with only a little bit of power supplied by the amplifier, as compared to mid-quality speakers. More power is needed for mid-quality speakers in order for them to deliver the same volume. Speaker sensitivity is described in terms of decibels of sound pressure level per 1 Watt of amplifier power measured at 1 meter from the speaker. Rather than specifying 'SPL/W/M', manufacturers simply say 'dB'. Most loudspeakers on the market have sound sensitivities between 85 to 91 dB, so anything less than 85 dB isn't ideal.
Total Harmonic Distortion
How accurately speakers translate what's on a hard drive or disc is measured by Total Harmonic Distortion (TDH). The lower the number, the less distortion experienced. Values generally fall within the 0.05% to 0.08% THD range for a "clean" sound system, although anything under 0.1% THD is generally pretty good.
Nominal Power refers to how much a speaker can handle over a long time span without getting damaged, while Peak Power refers to how much a speaker can handle in short bursts of time without getting damaged.
What a sound system is able to deliver in short bursts of time is measured by headroom. If you've got a home theater, then a large headroom figure is essential in order to get the most out of things like explosions and crashes in action films.
RMS refers to how much power an amplifier can put out over s long time span, while Peak refers to how much power an amplifier can put out in short bursts of time.
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