How to Translate Speaker Sensitivity Ratings Into
Real-World Requirements
Common decibel levels:
- Whispering: 25dB
- Normal conversation: 60dB
- Car: 80dB
- Subway: 100dB
- Rock concert: 110dB
- Thunder: 120dB
- Pain threshold: 130dB
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One of the most commonly published loudspeaker
specifications is sensitivity, which is usually expressed like this for a speaker
of 8 ohms nominal impedance: so many decibels of sound-pressure level (dB) for 1 watt/2.83
volts input, measured at a distance of one meter. While that’s already enough to
start many peoples’ heads spinning, understanding speaker sensitivity and what it
means in terms of how powerful your amplifier needs to be can help you better match
speakers to amps. This "How To" article attempts to explain what the technotalk
means in real life.
Let’s say you like hearing your music at close to
stadium levels -- in other words, really loud. That translates, roughly, into a
sound-pressure level of 104 to 105dB. (That’s fine, as long as you don’t live in
an apartment or condo, and your house is several hundred feet from that of your nearest
neighbor.) For this exercise, we’ll use as an example the Ascend Acoustics CBM-170 loudspeaker,
which was a 2003 GoodSound! Great Buy product. Our SoundStage!
Network measurements found that the CBM-170’s sensitivity at 2.83V at 1m was
89dB, or 2dB above the average of the small speakers measured by the Network over the
years. How much power will the CBM-170 need from the power amplifier in order to output
104dB at a distance of 1m in your home?
The decibel scale is a logarithmic scale, not an arithmetic
one. This means that for every 3dB increase in speaker output, your amplifier must put out
double the power. To increase a speaker’s output from 89 to 92dB requires a
doubling of amp output, from 1 to 2 watts. To go from 92 to 95dB requires another doubling
of amplifier output, to 4W. An increase of 95 to 98dB means an increase of amp output to
8W. It continues like that: To go from 98 to 101dB means the amp must increase its output
from 8 to 16W; and from 101 to 104dB, the amp must leap from 16 to 32W.
Putting out 32W is not a big problem for most amplifiers.
Most people, however, listen to their speakers from a good bit farther away than 1m --
usually at least 2m, or about 6.5’. The rule of thumb is that every time you double
your distance from the speakers, the perceived sound level decreases by 6dB -- that is,
it’s now only one-fourth as loud. (This is an example of the
"inverse-square" or "inverse-distance" law as applied to acoustics.)
That same Ascend CBM-170 that produces 89dB at 1m produces only 83dB at 2m. This means
you’ll need an amplifier that can produce about 128Wpc, at least on momentary peaks,
to produce sound-pressure levels upward of 104dB at that listening distance. In other
words, if an increase of 3dB output requires twice the power, then an increase of 6dB
output requires four times the power. Which means that the question you should be
asking is, "Can my amplifier reliably produce 128W per channel?"
This is important: many amplifiers can’t produce
nearly that much power, and even with those that can, you have to make sure that
they’re not close to reaching their upper limits when doing so. That’s because,
when an amplifier is pushed too hard to supply adequate juice to the speakers, the
resultant overloading of its circuits can result in serious distortion, which is called
"clipping." That distortion can fry speaker components, usually tweeters. This
is why speaker manufacturers warn users that it’s more dangerous to use an amplifier
of too low a power output for its partnering speakers than one that’s too
high-powered. The results of clipping can be that severe.
If you like your music really loud and you
don’t have a super-powerful amp, look for speakers whose sensitivity is fairly high.
If your speakers are very insensitive, you’ll need a far more powerful
amplifier to produce the same output levels. Just look up your speakers’ rated
sensitivity and you’ll soon figure out what you need.
...Thom Moon
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