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Controlling Bass Response in Control Rooms

Part 3

Heartened by these results, I next added a second barrier wall 1.6m long spaced 300mm from the first. This was supposed to tackle a strong narrow trough at about 100Hz. Surprisingly this frequency was largely unaffected, but a hole in the spectral analysis at 67Hz was raised by some 5dB along with a narrow band of frequencies either side. With further experimentation a third barrier wall 0.8m long largely removed the hole at 100Hz (Fig.3).

Fig.3 JPG
Fig.3 Final control room configuration

I was aware that reading a spectrum analysis can be misleading. Moving the microphone or shifting items about the room can cause completely different results, but these discrepancies tend to affect the mid to high end of the spectrum. The low frequency readings are quite constant as long as the measuring microphone is static.

The effect of these last two additions do not really tie in with the theory of the half wavelength wall. It is almost certain that the walls in front of the initial 2.1m one would effect its performance. The fact that the smaller ones seem to be acting on approximately half their designed wavelengths imply that the situation with multiple barriers is not straight forward. As the walls progress into the room space, diffusion will start to be effective. However, the results were very gratifying and quite apparent. Listening to a variety of music, the bass at the console had very clearly improved, and the bass variation about the room had largely disappeared.

It must be pointed out that the whole arrangement was still jutting no further into the room than the sofa, had been very cheap to install and caused very little disruption. This is in contrast to any other form of bass control that I know of. It is of course possible that the multiple wall arrangement is disrupting the room resonances by other mechanisms but apart from diffusion, I think that this is unlikely.

Another way to view the behaviour of the barrier wall is to regard it as a pipe open at both ends placed horizontally against the back of the room. The pipe has a natural resonance with a wavelength of just over twice the length of the tube. Any pressure front travelling inside the tube walls will find the impedance of the air outside low in comparison to that internally. Any tendency to resonate will put low pressure points at the tube ends, once again counteracting the maximum pressure variations caused by the back wall boundary.

I feel that this is a slightly different process, in that the large open ends of the walls are unlikely to create a sufficient impedance change to encourage resonance. Also the tube concept would only work half as well as the wall, as while pressure maximums are partially cancelled, minima are not.

The resonant frequency of the walls is very low, each below 10hz or so. It is unlikely that they work as just additional panel membrane absorbers.

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