B&W John Bowers Silver Signature loudspeaker Sensitivity or Efficiency?
If there are areas of loudspeaker performance which are as consistently confused as efficiency and sensitivity, I've yet to discover them. Audiophiles almost universally talk about a loudspeaker's "efficiency" when what they really mean is its "sensitivity": how loud it will go for a given input level. Why is there this confusion?
Historically, dating from the days of tube amplifiers, when power outputs were very limited and the question of power transfer to the loudspeaker was more significant, there is "efficiency." Strictly speaking, efficiency is the loudspeaker's acoustic power out for electrical power in. More usually it is expressed as a sound-pressure level 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 impedance and frequency, as demonstrated in Table 1 in the measurements sidebar.
Since the advent of transistor amplifiers, which (ideally) act as voltage sources—like the Krell KSA-200S reviewed in this issue, they maintain the same output voltage no matter what the load and the current drawn—another concept has replaced efficiency: voltage "sensitivity." This is the sound-pressure level produced at 1m by a standard voltage input, defined as 2.83V. This odd value was chosen simply because it's the voltage necessary to produce 1W dissipation in an 8 ohm resistor. The advantage of specifying sensitivity 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 a pure 8 ohm resistor, the nearer the two criteria; but when a speaker has an impedance that differs significantly from 8 ohms, they can be very different. Take the B&W Silver Signature and the Velodyne DF-661: the former has a measured sensitivity of 86.5dB, the latter 87.5dB, both for 2.83V drive at a 1m distance. Given the same drive signal, the sound-pressure level produced by the Velodyne will be 1dB higher than that of the B&W. Which is the more efficient?
Those who answered "The Velodyne, because it is louder for the same input voltage," go to the back of the classroom. Remember: Efficiency involves power, and to calculate power, you need to know the speaker's impedance. The B&W averages around 20 ohms in its upper midrange and treble, the Velodyne 5 ohms. To produce a level 1dB higher than the Silver Signature in this frequency region, the DF-661 sucks four times as much power from the amplifier and is therefore less efficient. Even though it is more sensitive!
In this case, the quieter loudspeaker is the more efficient, which is why Stereophile insists on talking about sensitivity. It is the only way you can meaningfully compare loudspeaker loudnesses.—John Atkinson