Monday, May 21st, 2018

Andy Munro : Making the Most of Your Studio Acoustics

By editorJune 1, 2012

ACOUSTIC DESIGN

Andy Munro

Andy Munro studied Mechanical Engineering before joining Shure Brothers Inc. in 1972.  After 5 enjoyable years helping many bands, including the Stones and Led Zeppelin with their microphone and recording requirements he decided that Acoustics was the most challenging aspect of audio  and in 1980 he formed Munro Associates and hence the Munro Acoustics Group.  In 1990 he created the monitor brand Dynaudio acoustics in partnership with Danish manufacturer Dynaudio. Building company Form & Funktion followed, to offer a complete one stop shop to the recording, film and broadcast industry.

Munro Associates has recently completed a major project of 28 studios in the new BBC Broadcasting House and an exciting new monitor speaker for sE Electronics – The Egg.

“All rooms have acoustic signatures that make them as unique as fingerprints. There are many and complex reasons for this and choice of materials and dimensions play the major part but there are intangibles such as wall resonances and ceiling voids that can only be assessed by audible results and measurement. Fortunately there are ways and means to make any room a good studio or at least a workable one.

Firstly, consider the volume of the space itself. For mixing and monitoring a volume of at least 30 m3 is advisable, preferably in an uneven ratio such as 3*4*5. This is to space out standing waves (modes) so they don’t form coincident patterns. This leads to large dips and peaks in the room energy that is very position dependent. For this reason it is better not to sit 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 as well 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.

The other great fly in the proverbial ointment is any strong reflection that interferes with the direct sound from the monitors. They can come from the nearest surface such as a console or almost certainly from floors, ceilings and walls. 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.

Custom Munro monitoring system at NRG Studios Los Angeles

Very early reflections (small time delay) cause mid frequency cancellations called comb filters because of their regular spacing. Putting all this together creates an explanation as to why some monitors work better than other in certain situations. The bass loading and the mid filtering makes a speaker like the NS10 work better than its anechoic measurements would seem to indicate. So a difference of 17dB between 50Hz and 1KHz suddenly reduces considerable when the speaker is plonked on top of an 80 channel console.

However 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 measurement 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; room, treatments, furniture and monitoring. Start by setting a reference point. Measure a good quality monitor speaker of choice, using 1/3rd Octave bands of Pink Noise from a good test CD source, at a distance of 1m, with the microphone in the position you intend to sit. This is the ideal ‘nearfield’ setup. Take a note of the sound level in the 2KHz octave as this is the band least likely to be affected by the room. Every other lower 1/3rd 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 gradually rising then LF absorption will be required and conversely a gradual roll off indicates some should be removed. This appraisal is best done in conjunction with measurement of reverberation times but I will come to that later. As already mentioned deep notches can be caused by standing wave patterns or reflections or both so patience is required to ascertain exactly what is going on. Moving the microphone about will shift reflective notches to a different frequency whereas modal notches rise and fall but stay at their fundamental frequency. Low frequency absorption requires tuned absorbers that cover specific bands and these are not that easy to come by. I do intend to publish a simple computer program to enable the curious and determined to tune their rooms properly.

It is a good idea to look at low frequencies with higher resolution than 1/3rd octaves so that notches and peaks can be spotted easily. This requires an FFT or MLS analyser which is a little beyond the scope of DIY acoustics. I find 1/3 octaves most useful for monitoring setups as it more or less emulates the way we hear using ‘critical bands’ in the inner ear. Who said acoustics was simple?

sE Munro test and project studio at Sonic Distribution with modular acoustics tuned by Andy Munro

This is a good point at which to define the main definitions of acoustic space and sound-fields.

An omnidirectional small loudspeaker radiating into an open space with no significant reflections creates a free-field. All speakers are directional to some extend and several together become very directional. When in line with the speaker axis you have a direct-field and off axis there is a diffused-field and this is modified in an enclosed space by reflections and reverberation. It is the balance between the direct and the diffused sound fields that determine sound quality and that applies to the largest rock festival and the smallest project studio and that is why it is so important to grasp the concept. Single source direct sound falls off at 6dB for each doubling of distance whereas the reverberant ‘room sound’ is more or less consistent, although modified by modes and reflections. Where the two are equal in level is called the ‘critical distance’ and that is often the best place to mix an overall sound balance. Any nearer to the speaker and you are entering the near-field, a good place for solo tracking and critical stereo image placement. Beyond the critical distance is the far-field. This is where the room dominates the sound balance and musical instruments tend to become less distinct and more blended. In a concert hall this is generally a good thing and a good live recording studio will have this quality although you need a big spatial volume to achieve it! I am often asked what is the ideal reverberation time for a control room but in reality it depends on the directivity of the speakers, the distance from them and the type of music. Classical mixers go for low directivity monitors and a little more reverberation whereas electronic music is best mixed in a drier room so that all the artificial synthesizer effects can be clearly judged. Some go for a directional speaker system firing into a dead wall which helps to eliminate reflections but reduces reverberation to zero which can be uncomfortable and requires very powerful amplifiers to restore the missing energy.

The more astute among you will have noticed I haven’t mentioned high frequency absorption and this brings me back to the question of reverberation. Without specific measuring equipment it is not easy to adjust reverberation but as a rule diffusion and good geometry go a lot further than anything else to give a room good balanced acoustics. The last part of the equation is some gentle damping to soften the sound and to 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 will not hear any specific afterglow but there will be general awareness of being in an enclosed space. The average living room has a mid frequency reverberation time of half a second and the best control rooms about half that so a measured value of 0.2 to 0.3 is about right. Very small rooms should be dry anyway and good diffusion is more important to avoid sounding like a bathroom.The diagram below shows the reverberation times across the audio frequency range for a largish demo studio control room before and after I treated it with a combination of proprietary absorber panels and the final addition of some tuned bass modules. It can be seen that it took these to have any real affect on the low end and the tightening and balance of the sound improved dramatically.

Red  Trace  Original Empty Room

Blue Trace  Initial Installation of 40 Absorber Panels

Pink Trace  Final Result with Tuned Bass Panels

Please note the original reverberation times (red trace) and how uneven they are. This is why it is unwise to make assumptions as there was quite a lot of bass absorption from standard plasterboard walls and almost none at 250Hz. It is difficult to predict such effects and I strongly recommend an initial room assessment before laying out for elaborate acoustic treatment.

As a final conclusion I would emphasise that good acoustics do not happen by accident and design, measurement and experience make all the difference; egg boxes do not!

Andy Munro
Munro Acoustics Group
June 2012

 

Our thanks to Andy for a really excellent article.  You can fine out more about the work of Munro Acoustics at http://www.munro.co.uk

 

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