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Saturday, November 3, 2012

Q. Why do speakers only seem to have round diaphragms?

I’ve noticed a lot of microphones on the market lately that have odd-shaped diaphragms: for example, there’s a Pearl model with a rectangular diaphragm and an Ehrlund mic with a triangular diaphragm. Given that mics and speakers are both transducers, why don’t we see different shapes like this in speakers? I’ve only ever seen round and elliptical shapes.
Darren Ellis, via email
SOS Technical Editor Hugh Robjohns replies: In a capacitor microphone, the diaphragm barely moves, because it’s not trying to absorb sound energy, just sense the changing air pressure. As a result, there’s virtually no significant movement necessary at the edges of the diaphragm, so the ‘surround’ isn’t too difficult to deal with, even in square and triangular arrangements. The idea of non-round diaphragms, by the way, is to minimise and control the natural membrane resonances. Whereas a round diaphragm has a strong single primary resonance, a rectangular diaphragm has two, related to its different length and width dimensions. 
And, if arranged carefully, these resonances will be weaker and spread over a greater frequency range, which gives a smoother overall performance. A triangular diaphragm has no parallel surfaces, and so no strong resonances at all.

Although oddly-shaped diaphragms are used on microphones, speaker cones are required to move far more air and require very flexible surrounds. Non-circular speaker diaphragms are consequently difficult to do well.
Loudspeaker cones have similar resonant modes, but non-round diaphragms are much harder to implement. The main reason is that a loudspeaker has physically to move a lot of air and that means the diaphragm has to move a relatively long way. This ‘long throw’ diaphragm movement requires a very flexible surround, and achieving that in a non-circular shape is a serious design headache. A suitable ‘cornered’ surround would be likely to introduce all sorts of unwelcome ‘non-linearities’. It can be done: Sony manufactured flat square drive units for some of its consumer speakers many years ago (for example, the Sony APM X270). 
However, the idea was much more about quirky aesthetics than audio quality and wasn’t a great success, as the higher manufacturing costs far outweighed the dubious sonic benefits.  

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