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Last Updated: Monday, 8 September, 2003, 08:01 GMT 09:01 UK
Sound science is quackers
By Jonathan Amos
BBC News Online science staff, in Salford

The old saying that a duck's quack produces no echo is... well, just plain quackers.

Daisy, BA/Craig Strong
Daisy performs: The anechoic chamber has no sound reflections
Acoustics expert Professor Trevor Cox has deployed the most powerful techniques his science can muster to prove the bird's calls will bounce off hard surfaces in just the same way as all sounds.

But the researcher has discovered there is a kernel of truth in the myth which may explain how it arose in the first place.

He says the way a duck quacks, with a long "aaaacckkk" on the end of the call, tends to mask any echoes that are produced.

Waddle waveform

It might seem a bit bizarre that scientists should go to all the trouble of investigating quacking ducks, but Professor Cox is an expert in controlling echoes.

His work helps building designers improve the acoustics of concert halls, cinemas, and even railway stations where echoes can make it difficult to hear the original sound source - such as the tannoy making a train announcement.

If ducks had some secret trick, he would certainly want to know about it. "Many people think the duck's quack having no echo is true so we thought we would investigate it," he told the BBC.

"It's a bit of fun but it does demonstrate what we can do."

So, waddle forward Daisy the duck.

You just don't hear the echo very well and that's probably how the myth arose
Professor Trevor Cox

Cox and colleagues at Salford University's Acoustics Research Centre put the feathered animal in an anechoic chamber, which deadens all echoes, and also in a reverberation chamber, to produce as much reflected sound as possible.

They then used powerful computer tools to analyse Daisy's noises and to simulate them in different environments - such as in front of a cliff face.

"What all this shows is that the duck's quack fades away; it sounds like it quacks for a long time.

"Because the duck's quack is rather quiet anyway and the echo comes on the back of a fading sound field, it is as if the echo is being masked. You just don't hear the echo very well and that's probably how the myth arose."

Sound system

These virtual techniques - known as auralisation - are more commonly used to hear what interiors of buildings or cars will sound like during the design process, or determine how the acoustics in old spaces can be improved.

Professor Cox tries to diffuse echoes using what are called "sonic crystals".

These consist of an array of regularly spaced objects - such as rods or spheres - that can be hung in a room to produce a more favourable auditory experience.

"Rather like real crystals that will scatter X-rays, sonic crystals will scatter sound. And if you can disperse sound, you can get rid of echoes."

Professor Cox is presenting his research on Daisy and sonic crystals to the British Association's annual festival, which this year is here in Salford, Greater Manchester.




WATCH AND LISTEN
Prof. Trevor Cox
"A duck's quack does echo but is rather difficult to hear"



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