Measuring Loudspeakers, Part One:
Page 3
Voltage Sensitivity
A loudspeaker's sensitivity appears to be universally confused with its efficiency. Efficiency is strictly defined [6, 7] as how much acoustic power the loudspeaker puts out for how much electrical power it is being driven with. If you feed a loudspeaker with 100 electrical watts, how many acoustic watts of sound does it produce? The answer is "not many," a typical moving-coil loudspeaker being about 1% efficient.

Efficiency is usually expressed in the form of a sound-pressure level produced by a speaker at a specific distance, 1m, for 1W input; ie, in dB/W/m. This is problematic, however, as there is no simple way of determining, for a given loudspeaker, what actually is a 1W input—it depends on both impedance and frequency. Feed a loudspeaker with an impedance of 8 ohms at 1kHz with a signal at the same frequency at a 2.83V level, and yes, you are feeding it 1W of electrical energy. (By Ohm's Law, Power = V x V/R = (2.83 x 2.83)/8 = 1W.) But if, as is very often the case, the loudspeaker has a much lower impedance at 200Hz—2 ohms, say—the loudspeaker fed the same 2.83V at 200Hz will now suck four times as much power from the amplifier. For the same sound pressure level, the speaker is four times as efficient at 1kHz as it is at 200Hz.

And why anyway does efficiency matter? While audio engineers a half-century ago were interested in the transfer of power (and telecommunications engineers still are), since the advent of solid-state devices, audio amplifiers act more-or-less as voltage-source devices—they maintain the same output voltage no matter what the load and the current drawn. What is important, therefore, is not efficiency but voltage sensitivity [8]: how loud a loudspeaker plays for a given voltage level from the amplifier. It is generally defined as the sound-pressure level produced by a loudspeaker at 1m by an input voltage of 2.83V (the voltage necessary to produce 1W dissipation in an 8 ohm resistor).

The advantage of specifying sensitivity rather than efficiency is that it remains unchanged no matter what the impedance of the loudspeaker, as it is assumed that the amplifier will always be able to provide the necessary current to maintain the 2.83V. The nearer a loudspeaker's modulus of impedance approaches that of a pure 8 ohm resistor, the closer the equivalence between the two criteria; but when a speaker has an impedance that differs significantly from 8 ohms, they can be very different, as in the case I mentioned above.

A classic example of this difference between the two terms is an electrostatic speaker I measured some years ago. It had an impedance in the bass of over 100 ohms. Its sensitivity was very low, around 79dB/2.83V/m. But consider its impedance: by comparison with a typical 8 ohm dynamic speaker, the 'stat is drawing almost no current. It is therefore very efficient at transforming electrical power into acoustic power.

A loudspeaker's voltage sensitivity is particularly important when matching amplifiers and loudspeakers. If you have a 20W amplifier, you had better use a very sensitive loudspeaker or else your music will not play very loud. Conversely, if you choose a loudspeaker with a sensitivity which is high—say, 100dB/2.83V/m—you can probably get away with a 5W amplifier, meaning that spending $10,000 on a 600W design would be a waste of money. However, even if it is agreed that voltage sensitivity is the appropriate parameter, there appears to be wild disagreement about how it should be assessed.

The problem is that loudspeakers tend not to have flat response. It is very tempting, therefore, for a speaker company's marketing department to look for a peak in that unflat response and say that, because the speaker gets that loud at that frequency, that's the sensitivity. The bandwidth of a loudspeaker will also affect the measured sensitivity if wide-band noise is used as a test signal. Two speakers may sound equally loud on music, but on noise, the model with better extension at the frequency extremes will measure as having a higher sensitivity. What is needed, therefore, is a means of producing a measured sensitivity that correlates with perceived loudness.