Dynaudio AIR 15, Справочное руководство

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Appendix: Acoustics
Dynaudio Professional AIR reference manual – 2014-09-28 115
Rear wall cancellation
When the monitor is set up at some distance in front of
a wall, reflections from the wall may occur and influence
the perceived frequency response. This could result in
comb filtering if
all frequencies produced by the monitor
were radiated in all directions. But monitors are typically
only omnidirectional at
low frequencies.
The result of the reflection is a single or a few dips in the
frequency response perceived in front of the monitor.
Fig. 78: A frequency (F) that has a wavelength of four
times the distance to the rear wall is canceled because
of the oppositely phased reflection.
The dip – or cancellation frequency – depends on the
distance to the wall. If the distance is 1 meter, the first
dip frequency will occur at a frequency with a wave-
length of 4 meters.
l = c / f
where
►
l = wavelength [meters or feet)
►
c = speed of sound (meters/second or feet/second)
►
f = frequency [Hz]
Hence
►
4 = 344 / f
►
f = 86 Hz
A closer position will result in a cancellation at a higher
frequency. This is then limited by the frequency where
the monitor becomes directional and does rot radiate
sound to the rear.
A farther position will result in cancellation at a lower
frequency. This is limited by the distance being so long,
that the reflected sound is attenuated due to the long
extra path travel.