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