why does a ducks quack not echo

Scientists have solved one of the acoustic world's riddles. It is a myth, they will announce today, that a duck's quack has no echo. But they will also add, a trifle sheepishly, that the echo of a duck's quack is very difficult to hear. Trevor Cox, of the acoustics research centre at the University of Salford, is an expert in techniques used to diffuse echoes. These are a challenge for station announcers, concert hall conductors, cathedral choirs and sound engineers. So he was puzzled when he began to hear, on television and radio programmes, that a duck's quack did not echo and no one knew the reason why. He was galvanised into action when journalists began ringing him to find out if the recently minted myth was true. Today he will reveal the results of his tricky experiments involving Daisy the Duck. "If we are going to test a duck, we really need to know what a duck sounds like with no echoes - remembering that an echo is a late-delayed reflection from a surface like a cliff," said Prof Cox.


So first Daisy did her solo performance in an anechoic chamber - a space designed to suppress all sound reflection. The noise sounded just like a quack. Daisy then repeated her aria in a reverberation chamber, designed to produce the kind of echo that Elvis Presley exploited with Heartbreak Hotel. "The sound produced is rather sinister," Prof Cox said. "It does indeed echo, so we shouldn't be too surprised there. "
The next question was: how did the myth arise? He decided to test the hypothesis in outdoor and concert hall conditions. He could not take his web-footed friend on to the podium at the Royal Festival Hall, so he used a computer to put the duck in a virtual concert hall. The quack was prolonged, though not as drawn out as the one in the echo chamber. The next step was to hear what a duck sounded like as its quack echoed off a cliff.


This tricky topographical challenge, too, had to be solved with virtual reality. But it yielded a pleasing sound pattern, in which experts could see both the original quack and the reflection of the sound as it bounced off the "cliff". "There is a small difference, but in this room it would be very difficult to hear," said Prof Cox. " This is the reason the myth arises. A duck's quack is rather quiet, and the echo isn't heard. " Is it true that a duck's quack doesn't echo? If so, why? I'm sorry to say that it's not true about the quack of a duck. Quacks echo as much as any other sound in nature. However, there is a way to avoid an echo, the problem is that it depends on your distance from the object reflecting the sound, and not the type of sound itself. Sound travels in waves, and all of these waves have a specific wavelength (the distance from point on a wave to the exact point on the next).


If by chance, the distance between the emitter of the wave and the reflector is exactly on one of the nodes of the wave. the sound will not reflect back at all. There will just be a standing wave created between one place and another, as all points on the wave would have zero net displacement. You can try this in the lab with a strobe light and a string oscillator. Also, if you have done the experiment with the column of water and the tuning fork, you will notice dead spots. These are distances where no matter what you do with the tuning fork, you won't hear anything coming from the tube. The second way to avoid an echo, is to use a partially reflective material. This method is one of many that helps to hide aircraft from radar. If you position a half-reflective layer exactly one-quarter wavelength in front of a fully reflective layer, the wave will cancel itself out.


By separating the layers by 1/4 wavelength, half the wave bounces off the first, and the other half of the wave bounces of the second. The travel time from the first layer to the second and back again, is exactly 1/2 wavelength, which means that the positive peak displacement is balanced exactly by the negative peak displacement. Again, no net displacement = no discernable wave return. Answered by: Frank DiBonaventuro, B. S. , Air Force officer, Tinker AFB, OK. It isn't true. See I guess this guy actually tried it with a real duck and heard the echo himself. It makes sense that even the duck's quack echoes since even with the superposition of waves, the duck shouldn't be able to cancel out only the echo under totally random conditions. Answered by: Jonathan Osgood, Physics Undergrad, Wheaton College, Chicago, IL

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