Adventures In Audio with Audio Masterclass
The first rule of acoustic treatment

A brief introduction to recording studio acoustics


It's going to be a long time before anyone invents a way to transfer an electronic or digital signal straight into the brain, bypassing the ears. Until then, at some stage sound must always pass through the air, and this is the most difficult and least understood part of its journey.

When sound is created, whether it is the human voice, speaking or singing, a musical instrument or plain old-fashioned noise, it travels through the air, bounces from reflecting surfaces, bounces again and mingles with its own reflection, then enters the microphone.

The same happens at the other end of the chain. Sound leaves the speakers, and although part of the energy will be transmitted directly to the listener, much of it will bounce around the room over a period of anything from half a second or less in a domestic environment up to several seconds in a large auditorium.

Compare this with an electrical signal. Once created, the signal travels in a cozy one-dimensional medium – a cable or circuit track.

The signal can't escape until it reaches its intended destination, there is nothing that it can bounce off (unless the cable is several kilometers long when it will reflect from the ends unless measures are taken), and the worst that can happen is that electrical resistance will lower the level slightly.

This is a little bit of a simplification, but it's fair to say that everything about the behavior of electrical signals is known science.

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This is not the case with acoustics. Sound travels in three dimensions, not one, and will readily reflect from almost any surface. When the reflections mingle, constructive and destructive interference effects occur which differ at every point in the room or auditorium. The number of reflections is, for all practical purposes, infinite.

Even with today's sophisticated science and computer technology, it is not possible to analyze the acoustics of a room with complete precision, accounting for every reflection.

It would rarely happen that the electrical components of a sound system of any kind would be installed (professionally of course) and then be found not to work as expected. It is normal however to complete the acoustic design of a room or auditorium, and then expect to have to make adjustments when the building work is complete.

Hopefully these adjustments will not cost more than the margin of error allowed for in the budget.

Acoustics is a complex science in practice, but in theory it's all very simple. The acoustics of a room (acousticians use the term 'room' to mean an enclosed space of any size) are determined by just three factors: the timing of reflections, the relative strengths of reflections, and the frequency balance of reflections.

Look around you at the various surfaces in the room. If you speak to a colleague, the sound of your voice will travel directly to his or her ears.

It will also bounce off the nearest surface producing a reflection that arrives at the ear after a certain number of milliseconds (sound travels just under 34 cm in a millisecond – one foot per millisecond is often used as a handy rule of thumb even though it is a little bit on the low side).

It will bounce off the next nearest surface with a slightly longer delay, then the next. Then reflections of reflections will start to arrive. At first they will be spaced apart in time but soon there will be so many reflections that they turn into a general mush of reverberation.

Some surfaces will be more absorbent, so reflections are lower in level. Some surfaces will favor certain ranges of frequencies.

These three factors almost completely determine the acoustics of a room.

David Mellor

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David Mellor