Speaker Watts Sensitivity and Volume
From Zanecorpwiki
Speaker sensitivity tells you the decibels measured at 1m from a speaker with 1 watt of power. Higher numbers mean more efficient conversion of electrical power to soundref group=notesSensitivity is related to but not synonymous with efficiency as different speakers have different impedance and response./ref. Speakers in the 90dB+ range are considered very efficient.refhttp://www.hometheaterhifi.com/volume_1_1/v1n1spk.html/ref
Since decibels are logarithmic, there's a big difference between 90 and 93dB--the latter is twice as sensitive. In general, it takes twice the power to raise the volume by 3dB (about a 25% increase in perceived volume).refhttp://www.gcaudio.com/resources/howtos/voltageloudness.html/ref To double the volume, you need 10x the power. This is why even though sensitive speakers are perfectly listenable with 1W or less input, you still need at least 15W of headroom above this in order to avoid under-driving the speakers during higher volume listening or musical peaks.
Assuming we have speakers rated at 92dB sensitivity and maximum input of 100W (standard), how loud can they get? Well, 1W gets us 92dB, 10W gets us 102dB, and 100W gets us 112 dB. Since listening to a few albums at 92dB is enough to cause damage, and 112 dB can cause damage after a few songs,refhttp://www.dangerousdecibels.org/hearingloss.cfm/ref so if your speakers are close by, you don't need much power at all to achieve as much volume as you'd ever want.
The other consideration is distance from speakers. You lose about 6dB for every doubling of distance, so if you're sitting 2m from your 92dB speakers, your ears would be presented with about 86dB.
Notice that even in a large room, set way back, you'd lose a lot of volume, but efficient speakers at 100W would still be plenty loud. Say your speakers are 8 meters--about 26ft--you'd have a drop off of 12 dB, or a little more than 1/2 the perceived volume. Assuming a listening level of 60-80dB (60dB is conversational), you'd still need less than a watt of power to each speaker.
This isn't to say that a good 50-100W amp is crazy. Maybe you've got a party going on sometime, or you want to hear something from across the house, or you just really want to pump up the jams for that one song (life is about living after all). Or maybe you've got really insensitive speakers.ref group=notesThe speaker technology influences sensitivity greatly. Horn loaded speakers can reach sensitivity into the 100dB range, requiring a 10W amp for all the sound you'd ever need even in a large space. Acoustic suspension speakers typically have sensitivity in the 80dB range./ref Generally speaking, though, unless you're doing audio for a concert, 1000W amps and speakers are just silly.
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