ACOUSTICS - BACKGROUND INFO
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 the monitors are typically only
The result of the reflection is a single or few dips in the frequency response perceived in front of the monitor.
This frequency is cancelled because of the oppositely phased reflection
The frequency response may look like this: one cancellation at the frequency that has a wave length of four times the distance to the rear wall.
The dip - or cancellation frequency - is dependent on the distance to the wall. If the distance is 1 m, the first dip frequency will occur at a frequency with a wavelength of 4 m.
l = c / f where
l = wavelength [m] (or ft.)
c = speed of sound [m/sec] (or ft./sec.) 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.
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