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Controlling Bass Response in Control Rooms

Part 2

Parallel Walls

When experimenting with screens moved around near the rear of the room I began to notice improvements in room response when the screens were placed in parallel to the back wall. I tried to replicate this improvement by using hanging absorbers, but their effect was minimal. It seemed to me that the screens were too close to the back wall to be acting primarily as a diffuser.

Normally diffusers work in breaking up standing waves by offering a staggered surface to the incoming wavefront. Parts of the wavefront are reflected back at different times. Effectively the distance between speaker and wall varies and thus the resonant frequencies are spread over a wider band. For diffusion to be effective for a particular frequency, the reflecting surface has to vary by a minimum of a quarter wavelength. At lower frequencies this implies that the wall would have to vary in depth by say 1m to 2m to have any effect.

The thought occurred to me that a low frequency pressure wavefront arriving at a full height screen would both partially reflect off the face of it but also, more importantly, diffract around the ends of the screen and travel along behind it. The distance that the diffracted wavefront has to travel to arrive at the other end of the screen could be arranged such that the pressure maximum of the wave coincides with the arrival of the pressure minimum of the cycle, ie. a half wavelength. The result could be a partial cancellation of the wave at the ends of the screen and thus a weakening of the standing wave.

This can be imagined in say, a ferry port: Waves from the open sea in the form of a large swell would travel between two concrete sides, hit the end wall and reflect. A barrier created parallel to the end wall but narrower than the width of the port would allow the waves to travel along behind it. If the barrier was the right length the wave peak would arrive at the ends in time to meet the trough before the next wave. The result would be a reduction in the water height variation in the region of the barrier ends and a lot of dizzy fish.

With this in mind I built an MDF/REVAC/MDF barrier wall 2.1m long and placed it 0.3 m from the back wall, floor to ceiling, in front of the current finish. The length of wall was set to be a half wavelength for a frequency of about 80Hz. It was not vital to get exactly the right length. If the barrier worked as I imagined then its effect would be spread over a band of frequencies, tailing off in efficiency either side of the selected wavelength. The distance from the back wall was established by experimentation and guesswork: It had to be small in comparison to the wavelength, yet large enough to allow plenty of air movement without eating into the control room space. Listening at the desk there was an immediate improvement in the low bass region confirmed by the spectrum analyser. The barrier was designed to counteract standing waves in the 80Hz region, and the frequency range from 75 to 95 Hz measured at the mixing console had been most affected, generally raised in level some 6 to 8dB (Fig.2).

Fig.2 JPG
Fig.2 One wall in position.

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