ROOM ACOUSTICS

Thanks to the physics of sound, the space you record in is as crucial as the gear you use, as Andy Munro explains…

MEET THE WRITER

Principle owner of Munro Acoustics, Andy co-founded Dynaudio Acoustics in 1990 and still designs custom Dynaudio monitor systems. He is currently designing many of the studios for the new BBC Broadcasting House and he is working with sE Electronics to develop a range of monitors.

BOOM BOOM SHAPE THE ROOM

All rooms have acoustic signatures that make them as unique as fingerprints. There are many complex reasons for this and a good analogy would be the tone of a rare Strad or even Strat! Interior materials and dimensions play the major part but there are other intangibles such as under-floor voids and ancient ceiling lathes that can only be assessed by audible results and measurements. Fortunately there are ways and means to make any room a good studio.

SPACE IN THE PLACE

Firstly, consider the volume of the space. For mixing and monitoring, a volume of at least 30m³ is advisable, preferably in an uneven ratio such as 3:4:5. This is to space out standing waves so they don’t form coincident patterns, leading to large dips and peaks in the room energy. If you place yourself in the areas where coincident waves are furthest apart they will actually cancel each other out. For this reason it is better not to have your listening position in the middle of the room as that is where the sound pressure will be most uneven. The same applies to the corners where the pressure waves are mostly additive. The corners also load up the bass and give it a lift so the result can be a boomy sound that makes it difficult to mix. All this applies to monitors too so it is a good idea to divide the room into thirds and place speakers and yourself on the intersections. This may be counter-intuitive as books say symmetry is important. It is but less so than a good overall tonal balance.

A MOMENT OF REFLECTION

Any strong reflections will interfere with the direct sound from the monitors. They can come from the floors, ceilings, walls and even the top surface of your console. Imagine a rubber band stretched from speaker to ear and then stretch it some more to reach the nearest big surface. The difference between the two will be half a wavelength at some frequency and that will cause a cancellation of the direct wave energy. The result is a hole and usually there will be several of these between 100 and 200Hz. At higher frequencies the holes get closer together and gradually merge into a continuous spectrum. Of course very low frequencies have long wavelengths and so the early reflections tend to stay in-phase and so add (sum) to the overall energy spectrum.

Very early reflections (small time delay) cause mid-frequency cancellations called comb filters because of their regular spacing. Putting all this together explains why some monitors work better than others in certain situations.

The bass loading and the mid filtering makes a speaker like the Yamaha NS10 work better than its measurements would seem to indicate. A difference of 20dB between 50Hz and 1kHz suddenly reduces considerably when the speaker is plonked on top of an 80-channel console.

THE TAMING GAME

Two wrongs rarely make a right so it is prudent to tame the room and give yourself a bit more confidence that what you hear is actually what goes down on the mix. The ideal way to do this is by using measurements, as there are too many variables to consider, unless a supercomputer is available and somebody can write the algorithms! There are some decent spectrum analyser programs available and there are basic rules to follow that will give a good indication of the accuracy of your system. By system, I mean the whole thing: monitors, treatment, furniture and so on.

Start by setting a reference point. Measure a good quality monitor speaker of choice by playing pink noise through it at a distance of 1m, with a microphone in the position you intend to sit. Take a note of the sound level in the 2kHz octave, the band least likely to be affected by the room. Every other lower octave band can be compared to this one and if the level at any frequency is too high or too low then action can be taken.

If the lower frequencies are rising then absorption will be required. Conversely a gradual rolloff indicates that some absorption should be removed. Low frequency absorption requires tuned absorbers that cover specific bands and these are not that easy to come by. It is a good idea to look at low frequencies with higher resolution so that notches and peaks can be spotted easily. I find third-octaves most useful as they more or less emulate the way we hear using ‘critical bands’ in the inner ear.

The subject of high frequency absorption brings me to the question of reverberation. Without specific measuring equipment it’s not easy to adjust reverb but as a rule, diffusion and good geometry go a lot further than anything else to give a room balanced acoustics.

The last part of the equation is some gentle damping to soften the sound and tighten up the stereo image in the monitors. It is possible to judge the balance of direct to reverberant ratio using some very dry audio samples of different instruments. In a good room you won’t hear any specific afterglow but you’ll be aware of being in a closed space. The average living room has a mid frequency reverb time of 0.5 seconds and the best control rooms are about half that, so a measured value of 0.2s to 0.3s is about right.

SHE'S GOT THE BODY

The diagram below shows reverb times for a large-ish demo studio control room before and after I treated it with a combination of absorber panels. It took the addition of some tuned bass modules to have any real effect on the low end and the tightening and balance of the sound improved dramatically.

It’s worth noting how uneven those original reverb times were (mainly due to bass absorption from plasterboard walls) with almost none at 250Hz.

It is hard to make prior assumptions and the effect of treatment is difficult to predict. I strongly recommend an initial room assessment before shelling out for treatment! Call the Absolute Music guys for more advice on 01202 597180.