© 2011 Steve Feinstein and Atlantic Technology
One of the speaker specifications that you see a lot is sensitivity, also referred to as the speaker’s “efficiency.” This is an industry-standard measurement designed to explain how loud a speaker will play with a given amount of much amplifier power (expressed in watts).
It’s an important specification because it allows you to compare speakers and decide how much power you need for a particular room or installation. Much of this basic information was covered in Tech Tip 24. We recommend you re-read that one. Click here to read Tech Tip 24.
OK, now we’ll give you some basic background info so you can put the rest of this Tech Tip in the proper context. (See figs 1 and 2.)
“Low” sensitivity: < 85 dB/1watt input/1meter on axis
“Average” sensitivity: 85-95 dB
“High” sensitivity: > 95 dB
Perceived Loudness Levels—
60 dB: Average conversation/office noise
90 dB: Heavy city street traffic
110 dB: Live rock concert or loud action movie in commercial theater
People can perceive loudness differences as little as 1 dB; 3 dB is an obvious step; 10 dB is considered ‘twice’ as loud. But….it takes double the amplifier power to go up 3 dB and it requires 10 times the amplifier power to get twice as loud (10 dB more).
So let’s take that info and put it into the real world, so you can determine how loud your system will play with the watts you have available.
First, consider that the sensitivity specification, as measured one meter on axis for a 1-watt input, will yield a figure of about 88dB for an ‘average’ speaker. (Note: sensitivity in not an indication of the speaker’s inherent quality; sensitivity varies mostly because different design choices by the engineer.)
Now, you’ve got two speakers in a stereo music system, so there’s some addition to the 88 dB loudness figure because of that. You are also about 8-10 feet away from the two speakers (and somewhat off axis to boot), so there’s some reduction because of that (something called the inverse square law, which tells us how fast the loudness level drops off with increasing distance). Throw in your room’s absorptive characteristics, any open walls that lead to the next room, etc, and what you’re left with is that about 4-6 dB less than the sensitivity figure is a pretty decent number to work with as to how loud two speakers will sound from your listening position.
There are a LOT of variables, obviously, but about 83 dB for 1 watt for a pair of average sensitivity speakers from your listening chair is not a bad estimate.
As we said, every doubling of power is another 3 dB of loudness. So:
2 w = 86 dB
4 w = 89 dB
8 w = 92 dB
16 w = 95 dB
32 w = 98 dB
64 w = 101 dB
128 w = 104 dB
256 w = 107 dB
As you can see, even amplifiers with relatively modest power can drive speakers of average sensitivity to pretty loud levels. But the power demands rise very quickly as you attempt to reach lifelike levels in your listening room, and you may find that both your speakers and your amplifier—not to mention your ears!—run out of steam before you get there.