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A Series Active Monitor Speakers

E4/38 Service Date printed: 29.02.00

4.6.4 Quasi-Anechoic Measurements and Low-Frequency Limitations

Other techniques exist which provide measurement capability similar to

maximum length sequence analysis. These include Time Delay Spectrometry

(TDS) and impulse testing. They all provide quasi-anechoic frequency re-

sponse measurements of the loudspeaker alone, unaffected by room reflec-

tions. All share the limitation that this anechoic response is useful only

above a critical frequency determined by the physical dimensions of the test

environment. In the illustration below, for example, the direct signal arrives

2.91 ms after it leaves the loudspeaker and the first reflection arrives 9.2 ms

after leaving the speaker. An anechoic measurement, by any of these tech-

niques, must ignore reflections and therefore can only look at about 6.3 ms

of pure first-arrival signal before the first reflection. Meaningful amplitude

measurements cannot be made without acquiring at least one full cycle of

signal, and accurate measurements require several cycles. For the 6.3 ms

example, the frequency corresponding to this time span is about 160 Hz. If

useful frequency response is desired down to 35 Hz, for example, then the

portion of signal to be converted must be a minimum of 1/35 s (28 ms) and

ideally closer to 1/10 s (100 ms). In the example as well as in most realistic

actual rooms, this longer time necessarily includes several reflections and

the response is no longer anechoic.

True anechoic response with even moderate accuracy down to 35 Hz would

require that the first reflection path to be at least 28 ms longer than the di-

rect signal path. This requires a room with the nearest reflecting surface at

least 5 m away. This calls for a room with a 10 m high ceiling with the

speaker and microphone on 5 m tall stands!

Second reflection

(14.8 ms for wall

spaced 2.5 m)

First reflection

(9.2 ms for floor

spaced 1.5 m)

Direct path

(2.91 ms at 1 m spacing)

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