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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.
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.
(The cynic will also note that a resistive test load produces the most impressive measurements for use in
advertisements.)