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Controlling Bass Response in Control Rooms

An article by Brian Gaylor
(appeared in Audio Media March 2001 UK)


Two years ago I finished the design and construction of the control room at Stanley House Studios in west London. There was a problem with the bass end acoustics which I know to be fairly common in other rooms. The bass varied as you moved about and most importantly there was a lack of bass at certain frequencies at the listening position seated at the mixing console. After some experimentation, I believe that I have come up with an easy solution which may benefit others with similarly shaped rooms. I am not aware of the method I employed being used now or in the past, but I could be wrong here. I would be surprised if the concept is novel as it seems too basic to be overlooked. However it seems to offer a solution that could be tried in other rooms without too much expense or effort.

The Problem

Inside any building shell the internal dimensions of the room will allow sound waves to resonate at predictable frequencies. A standing wave is created which causes areas of maximum and minimum pressure variations at set points in the room, making bass notes appear loud in some areas and weak in others. Whereas the higher frequencies are easily deflected and absorbed the lower ones, especially near the fundamental carry a relatively large amount of energy at longer wavelengths, and are difficult to control. The principal dimension of concern is the front (speaker) wall and back wall.

The Room

Fig.1 shows the microphone output at the listening position at the mixing desk when the speakers generate pink noise, before the room was modified. It should be noted that on my spectrum analyser pink noise slopes off at 3dB per octave. Peaks at 50Hz, 78Hz, 112Hz and 125Hz with troughs at 69Hz, 83Hz and 100Hz show the problem room resonances.

There is a further region between 150Hz and 180Hz which is obviously another poor area. This was improved by building screens to block off the back of the desk. The back of the J series SSL offers a large surface area just a short distance from the main monitors. It is inevitable that if the speaker wall is non absorbing, reflections between desk and wall will cause problems. Absorbent screens help dampen these resonances and prevent reflected waves passing beneath the console.

At Stanley House the back wall is totally lined with membrane absorbers tuned to the fundamental of about 50Hz. This type of absorber is constructed of a limp layer of rubberised matting pinned over an air tight cavity, like a soggy bass drum. The mass of the material and the depth of air behind define the frequency of operation, like a weight bouncing on a spring. If the material absorbs energy as it moves, and the air vibration behind it is damped then air movement at the face of the membrane will lose energy most at the oscillating frequency of the system. Lining the cavity with fibrous material both aids the damping of the air movement and detunes the absorber such that it should work over a slightly broader frequency range.

The spectrum analysis of a microphone placed inside the cavity in response to pink noise from the room showed a peak at 50Hz tailing off at about 10dB per octave. If the absorber is working properly then this graph provides an indirect suggestion that all is working as well as could be expected.

At a short distance from the front of the membranes hanging absorbers complement the deadening of reflections. These consist of a layer of roofing felt with loft insulation stuck to either side freely hanging from a top fixing. Air movement due to sound waves is impeded by the inertia of the felt matt and the damping of the fibrous material.

However the graph of the mic output at the desk still shows a peak at the fundamental frequency. A general hole from there to over 100Hz causes the listener at the desk to miss the 'kick' from bass instruments, implying still not enough absorption to control standing waves at these frequencies.

Fig.1 JPG
Fig.1 Original room configuration: Amplitude plot per frequency.

Part 2 >  Part 3 >>  Part 4 >>>


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