Understanding Loudspeaker Sensitivity
One of the most basic but useful loudspeaker
measurements is that of sensitivity. A speaker’s sensitivity is expressed in decibels
(dB), and is a measure of how loudly it will play given a certain electrical input level.
Sensitivity is not a measure of sound quality, but it can help to tell you how
powerful your amplifier needs to be in order to make your speakers really sing. This
article explains how a speaker’s sensitivity is measured, and exactly what it means
to audiophiles.
A speaker’s sensitivity can be measured in any number
of ways. However, the audio industry has established a standard way of doing this so that
the sensitivities of various loudspeakers can be easily compared.
Ideally, the best place to measure loudspeakers is in an
anechoic chamber -- a large, isolated room with big, acoustically absorptive wedges
affixed to the floor, ceiling, and walls that eliminate soundwave reflections. Basically,
an anechoic chamber mimics a free space that has no reflective boundaries: the only things
that are measured are the sounds directly produced by the speaker itself, not the
reflections of those sounds off of walls, floors, ceilings, furniture, etc.
At the SoundStage! Network, we measure speakers in the
state-of-the-art anechoic chamber at Canada’s National Research Council (NRC), in
Ottawa. To the best of our knowledge, we’re the only publication that measures
speakers in such an environment. Therefore, our speaker measurements, which can be found
at www.speakermeasurements.com,
are a great reference for anyone who wants to know exactly how a speaker performs
in the lab. And near the top of each speaker’s list of measured results, you’ll
find its sensitivity.
The standard input level is 2.83 volts (V), which
translates nicely into 1 watt (W) if the speaker presents to the amplifier an impedance,
or load, of 8 ohms. A speaker’s power output in watts is calculated by squaring the
input voltage and dividing the result by the resistance. In this case, that would be 2.83V
x 2.83V ÷ 8 ohms = 1W. If the impedance is 4 ohms, then the power output would be 2.83V x
2.83V ÷ 4 ohms = 2W. You don’t have to memorize the power stuff, though. The key
thing to remember is that the standard input is 2.83V across the frequency range
you’re measuring.
Next, you have to measure the speaker’s output, which
is often referred to as the sound-pressure level (SPL) and is expressed in
decibels. This is measured by a microphone placed in front of the speaker.
In measuring a speaker’s output, two things are critical. First, it’s important
that the output be measured at the same distance for all speakers, because the distance
the microphone is from the speaker makes a huge impact on the SPL measured. Every doubling
of this distance results in a 6dB drop in SPL. So, placing the mike 2 meters instead of 1
meter from the speaker will make a huge difference in the measured result.
The normal distance at which to measure a speaker is 1
meter (m). It’s common practice to place the mike between the midrange and tweeter
for a three-way, or between the mid-woofer and tweeter for a two-way. However, these small
variations in vertical microphone position don’t matter that much --
what’s important is the mike’s distance from the speaker’s front baffle.
Second, you can’t just measure a speaker’s
sensitivity at one frequency and say that’s that. Unlike amplifiers, speakers always
have substantial fluctuations in their frequency response -- as much as +/-3dB, or even
more, depending on the frequencies being measured. Therefore, the best way to do this is
the way the NRC does it -- by averaging a broad range of frequencies so the fluctuations
iron themselves out. The NRC averages readings taken between 300Hz and 3kHz.
By understanding this information, when you see 2.83V/m,
which is indicated in each of our own measurements, you’ll have a better idea what it
means. Translated, "2.83V/m" means that the speaker was measured with a 2.83
volt input and the microphone was placed 1 meter from the speaker’s front baffle.
"2.83V/2m" would indicate the same voltage input, but a microphone distance of 2
meters. "2.0V/m" would mean that only 2.0 volts were input and the microphone
was 1 meter away. Obviously, you could have any combination of voltages and distances, but
it’s key to remember that you must compare apples with apples. That’s why we always
use 2.83V/m.
This single measurement of sensitivity can tell you a lot
about a speaker. Normally, at a 2.83V
input and a measuring distance of 1m, most speakers will output between 80 and 90dB, the
average being about 87dB. Very few output less than 80dB, but some do put out in excess of
90dB. To give you some perspective on these figures, a conversation is about 60dB, the
level of a telephone dial tone with your ear against the handset is about 80dB, and a
truck going by is about 90dB. Therefore, 2.83V (1W if the speaker is 8 ohms) is quite a
bit of sound.
However, while 1W seems to generate considerable output, it
can lead some to think that there’s not all that much difference between a speaker
with a sensitivity of 80dB and one of 90dB. But there is. A 10dB difference equates to a
doubling of subjective volume level. So, the speaker playing at 90dB will seem as if
it’s playing twice as loud as the one playing at 80dB. That’s obviously quite a
difference in output level, easily perceivable by even novice listeners.
But to increase the output level just 3dB requires a
doubling of amplifier power. Once you do the math, you’ll realize that it takes much
more amplifier power for low-sensitivity speakers to match the output level of
higher-sensitivity ones, which is why people can get away with using low-powered
amplifiers with high-sensitivity speakers, while others need high-powered amplifiers for
low-sensitivity speakers. Thom Moon addresses this in his article "How to Translate
Speaker Sensitivity Ratings into Real-World Requirements."
The key to understanding speaker sensitivity isn’t
just to know how it’s derived, but also to know how it’s used. Sensitivity tells
you nothing about sound quality, but it does tell you a lot about how loudly a speaker
will play given a certain input level -- a key thing when it comes to matching speakers to
amplifiers, and vice versa.
...Doug Schneider
|
