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 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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