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Loudspeaker Measurements
Returning to loudspeakers, a similar situation has developed. Although
nobody listens to music in an anechoic chamber, loudspeaker measurements
are commonly performed in them.
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Although various proposals have been
made for performing low frequency measurements in a more realistic setting,
there has been no agreement as to what that setting should be.
Loudspeakers continue to be measured in a test chamber that is equivalent
to the absence of any room at all.
There is a developing appreciation that this traditionally performed
measurement is not an accurate predictor of the performance actually
attained in the listener's room. Certainly, in-room frequency response is more
important than anechoic response in determining a speaker's tonal
accuracy. Placement of the speaker within a room will cause changes in the
frequency response compared to the anechoic condition.
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At lower
frequencies, the speaker's output is modified by the acoustic loading
presented by the walls and floor. However, when making measurements, it is
difficult to separate the effects of a room's bass reinforcement from standing
waves and other resonances associated with that room.
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Since an anechoic chamber which performs accurately to low frequencies is
extremely large and expensive, other measurement methods are also commonly
used. These include near-field measurements, when the microphone is extremely
close to the driver, and half-space measurements, when the speaker under test is
buried with its front baffle flush with the ground, facing upwards. Both of these
methods are equivalent to anechoic measurements below the frequency at
which the speaker baffle appreciably changes the acoustic load to the woofer,
typically between 100 and 200 Hz. Note that these conditions are also
non-representative of an actual listening situation.
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Since this discussion is concerned with the reproduction of low frequencies, we
will not delve deeply into the high-frequency variations between the anechoic
response and the in-room response of a loudspeaker. Briefly, the interaction of
the dispersion pattern of the speaker with the reflective surfaces in the room (and
the variation of both with frequency) creates an in-room frequency response that
may vary markedly from the anechoic response.