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9.3 Measurements of Audio Equipment
It should be recognized that measurements are not the final arbiter of sound quality of audio
components. Often times a measurement standard has evolved because it is easily performed,
or because it is easily repeatable, or it has shown some link to certain audible characteristics.
Unquestionably, it is the latter criterion which is the most important one. After all, the listener
is not concerned with how a piece of audio equipment measures, he is only concerned with
the faithful recreation of the original musical event.
On the other hand, measurement techniques that correspond to audible effects are an
invaluable tool to the designer. However, it is the degree of correlation with the subjective
experience which is important, and anechoic bass response does not have a high correlation
with musical accuracy in the listening room. In-room frequency response and transient
accuracy are both significant factors in determining subjective quality. Nevertheless, anechoic
frequency response is by far the most prevalent measurement used to characterize speakers.
A Correlation with Amplifier Measurements
A striking parallel exists in the measurement of audio amplifiers. The power output and
distortion of an amplifier is invariably measured into an eight-ohm resistor. It is widely
acknowledged that this standard is far removed from the actual conditions in which the
amplifier will be used. One doesn't listen to resistors, one listens to loudspeakers, and the load
that the speaker presents to the amplifier is nearly always highly reactive (varies with
frequency). The eight-ohm resistive load has developed as a standard because it somewhat
approximates a speaker load, is easily reproducible by different testing facilities, and it
represents something of a lowest common denominator. That is, while everybody recognizes
that a different load should be used for amplifier testing, nobody can agree as to what that
alternative should be.
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In the last decade, there has been a growing awareness of the importance of an amplifier's
capability to drive a real-world loudspeaker. This is the reason we have seen the emergence of
amplifiers with high current output capabilities, and a lack of current-limiting or similar
protection circuitry. The ability to drive reactive loads has been accepted as having a higher
correlation with audible qualities than the traditional measurement into a load resistor.
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The cynic will also note that a resistive test load produces the most impressive measurements for
use in advertisements.