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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 One wall in position.
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