Speaker efficiency, also known as speaker sensitivity, is a measure of the speaker's output, measured in decibels, with a specified amount of amplifier power. For example, speaker efficiency is often measured with a microphone (connected to a sound level meter) placed one meter from the speaker . One watt of power is delivered to the speaker and the level meter measures the volume in decibels. The output level results in a measure of efficiency. Speakers range in efficiency or sensitivity from about 85dB (very inefficient) up to 105dB (very efficient). As a comparison, a speaker with 85 dB efficiency rating will take twice the amplifier power to reach to same volume as a speaker with 88 dB efficiency. Similarly, a speaker with a 88 dB efficiency rating will require ten times more power than a speaker with a 98 dB efficiency rating to play at the same level. If you're starting with a 100 watt/channel receiver, you would need 1000 watts (!) of power output to double the perceived volume level.