speakers efficiency

Just how efficient is a rating of 87 db?
1 watt @ 1 meter = 87db
2 watts = 90db
4 watt = 93db
8 watts = 96db
16 watts = 99 db
and so on.....
Every increase of 10db will give a perceived doubling in volume.
THX = 110db
Ultra THX = 112 db

With that said, 87 is not very efficient but not too bad either. You won't want to run these with a small tube amp but an average 100 watt amp should do fine as long as you don't want rock concert levels.
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Assuming a listening distance of about 10 feet in a semi-reverberant room, a pair of 89 dB/1 watt efficient speakers will probably produce about 87 dB at the listening position with a 1 watt input. I can go through the math behind that figure if anyone's interested. From there the pattern follows the trend that Elevick detailed.

Real-world, thermal compression can be a significant issue and very few home audio speakers really give you a 10 dB increase in SPL for a tenfold increase in input power. Somewhere between 7 and 9 dB is more likely, depending on the speakers and the input power level. This is one of the main arguments in favor of high efficiency speakers using high quality prosound drivers - namely, that thermal compression is negligible at typical in-home listening levels, so dynamic contrast is improved.

Anyway to get back to your question, 89 dB/1 watt efficiency is slightly above average.

Now make sure that the manufacturer is really claiming 89 dB/1 watt. If it's a 4-ohm speaker, the manufacturer may be claiming 89 dB/2.83 volts. While 2.83 volts into 8 ohms is 1 watt, 2.83 volts into 4 ohms is actually 2 watts, so in the latter case the speaker would actually be only 86 dB/1 watt efficient.

And just for the record I'm misusing the term "efficiency" - to be precise "efficiency" should be expressed as a percentage or a decimal fraction, but I'm following industry convention here.

Not to be picky, but aren't you really talking about "sensitivity" here?
Efficiency takes into consideration sensitivity AND impedance, I think.
Good listening!
If you're shopping for speakers and are concerned with efficiency, you should also consider the impedance curve of the speaker. A speaker with a flatter curve will be easier on an amp than one with more violent imepdance spikes and thus play louder with less effort.
See http://www.welbornelabs.com/recomendspeaks.htm

Scroll down to end of Recommended Speakers & before Raw Drivers.
Myraj points out that I wasn't clear in something I said.

Let me try re-writing the second to last paragraph from my post above:

"Now make sure that the manufacturer is really claiming 89 dB/1 watt EFFICIENCY. If it's a 4-ohm speaker, the manufacturer may be claiming 89 dB/2.83 volts SENSITIVITY. While 2.83 volts into 8 ohms is 1 watt, 2.83 volts into 4 ohms is actually 2 watts, so in the latter case the speaker would actually be only 86 dB/1 watt efficient even though it's 2.83 volt sensitivity is 89 dB."

Hope that helps.

Well I just wanted to point out that to have good dynamic range is essential to a good system. while you may only listen at an average 92 dbs but peaks could be much higher than this (especially on a well built system) this is the area where your amp could run into some trouble on a hard to drive speaker. These peaks could also (most probably) cold be at a frequency where impedence is also high, which is a real strain on the amp. For example using the above info (4 watt = 93db @ 1 meter) this figure probably means 90 db at listening position so in reality to listen at 93db average at listening postion you will need 8watts (with 8 ohm speakers). But for the amp to be able to handle dynamic peaks there needs to be headroom. If the peak is 12db the amp must cleanly without clipping put out 128 watts (8@93, 16@96, 32@99, 64@101, 128@103). Also factor in that this peak could be at frequency that the speakers impedence has dropped to 4 ohms or less even on a 8 ohm speaker and you need twice the 128watts or 256 watts@4ohms to cleanly play your wonderful uncompressed audio material at the average of 93 db at the listening postion. Of course good amps can also double in power as impedence goes from 8 to 4 ohms.
think i read somewhere that JA at Sterophile's database of past measurements indicate the avg sensitivity of the speakers evaluated by the mag has been 87

You seam to know a bit about pro audio as well. Do you have any information on absorptive properties of crowds and rooms with large volume? I know that Master Handbook has some info on this but I have misplaced my copy.

THX = to be THX certified, equipment must meet certain standards such as sustained volume at 110db.
Ultra THX is even harder with 112db.
Many amps/speakers can't handle these loads. If your audio gear can produce 112db continuous, it means that you can play very loud, distortion free music.
My best amps aren't THX certified even though they can blow away the criteria (keep that in mind). 120 watt tube monoblocks into 94 db speakers.
Bignerd, I don't really have any experience in pro-sound applications; I just have played around some (maybe too much) with prosound drivers.

I have no specific knowledge of the effect of a crowd on room acoustics. I'd guess a crowd of people would be more absorptive than diffusive, such that the power requirement to meet a given SPL in a full auditorium would be several dB more than if the room was empty. And since most absorbers are more effective at short wavelengths (high frequencies) than at long wavelengths (low frequencies), the spectral balance of the soundfield would be tilted downward somewhat by adding a crowd of people. But I have no idea of the specifics of such effects.