Adventures In Audio

The problems caused by standing waves in small acoustic spaces

by David Mellor
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Although acoustics is a science, the ultimate arbiter of good acoustics is human judgment. There are certain basics that must be adhered to, derived from common knowledge and experience, and also statistical tests using human subjects.

Firstly, a room that is designed for speech must maintain good intelligibility. Too much reverberation obscures the words, as do reflections that are heard by the listener more than 40 milliseconds or so after the direct sound.

Late reflections cause phonemes (the sounds that comprise speech) to overlap. Short reflections actually aid intelligibility by making unamplified speech louder.

For both speech and music there is the requirement that the reverberation time (normally defined as the time it takes for the reverberation to decrease in level by 60 dB – the RT60) is in accordance with that commonly found in rooms of a similar size.

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A small room with a long reverberation time sounds odd, as does a big room with a short reverberation time. We can thank the BBC, who probably own and operate more purpose designed acoustic spaces than any other organization in the world, for codifying this knowledge.

One of the most common problems in acoustics, that particularly affects 'room-sized' rooms, rather than concert halls and auditoria, is standing waves. The wavelength of audible sound ranges from around 17 mm to 17 m.

Suppose that the distance between two parallel reflecting surfaces is 4 m. Half a wavelength of a note of 42.5 Hz (coincidentally around the pitch of the lowest note of a standard bass guitar) will fit exactly between these surfaces. As it reflects back and forth, the pattern of high and low pressure between the surfaces will stay static – high pressure near the surfaces, low pressure halfway between.

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The room will therefore resonate at this frequency and any note of this frequency will be emphasized. The reverberation time at this frequency will also be extended. This will also happen at integral multiples of the standing wave frequency.

Smaller rooms sound worse because the frequencies where standing waves are strong are well into the sensitive range of our hearing. Standing waves don't just happen between pairs of parallel surfaces.

If you imagine a ball bouncing off all four sides of a pool table and coming back to where it started; a standing wave can easily follow this pattern in a room, or even bounce of all four walls, ceiling and floor too. Wherever there is a standing wave, there might also be a 'flutter echo'.

Next time you find yourself standing between two hard parallel surfaces, clap your hands and listen to the amazing flutter echo where all frequencies bounce repeatedly back and forth. It's not helpful either for speech or music.

Sunday February 2, 2003

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David Mellor

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

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