|It’s often used for in audio for speakers, guitars, or other equipment. A majority of home theater speakers are not powered, therefore an amplifier is used to power them. If you are using an A/V receiver, it will have an amplifier built into it. When choosing an amplifier or A/V receiver some factors to consider include impedance, power handling capacity and sensitivity.
Impedance is the measure of the load the speakers place on the amplifier. If you have lower impedance, you need more power to drive the speakers at the same volume. Most home theater receivers are rated to 6 or 8 ohms. It's important to remember, however, that the load is not generally one constant level. That is as the volume of a recording increase and decreases, the load will vary dynamically.
Also, lower frequency sounds place a greater load on a system then higher frequencies. So, a dramatic explosion in a movie will put much greater load on a system then the ambient noise of birds chirping and wind blowing in a quiet outdoor scene. Still, it's probably best to stay within 2 ohms of the receiver's impedance rating when selecting your speakers and not have any two sets of speakers within your system have a rated difference of greater than 2 ohms.
When you're matching a Amp to a speaker, a good rule of thumb is to pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means a speaker with a "nominal impedance" of 8 ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8-ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.