speakers efficiency

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.

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.

Duke

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.

Not to be picky, but aren't you really talking about "sensitivity" here?
Efficiency takes into consideration sensitivity AND impedance, I think.
Good listening!