Websites we've worked on: Dr Andy Moorhouse on piano ... BBC radio docs on sound:- presented by Prof Trevor Cox
"A Duck's Quack Doesn't Echo, and no-one knows the reason why?"
"A duck's quack doesn't echo" is a much quoted scientific myth. In spring 2003 it was quoted on Home Truths on BBC Radio 4 and Shooting Stars on BBC 2. You can listen to our sound files on Home Truths by going to the BBC4 web site and "listening again". Salford Acoustics was the source of the story being presented in the national and international media when we proved that a duck's quack does echo as part of the British Association Festival of Science.
Click here to hear Daisy getting into the Christmas spirit (mp3 player needed and a suspension of musical taste)
So what is an echo, and does it happen to a duck's quack?
What is an echo?
An echo is a loud sound reflection that arrives late and so is heard as a separate sound source. Echoes can sometimes be heard in large concert halls, for example. Sound takes a long time to travel from the stage to the rear wall of the concert hall, if a strong reflection comes back from the rear wall to the front of the hall, this can be heard as an echo. To prevent this problem, it is common to apply diffusers to disperse the sound. Another common place to hear echoes is in some railway stations, making announcements hard to understand.
The term echo also refers to the physical sending back of sound or other waves and the repetition of sound by reflection.
The animation below needs a flash player.
The duck with no reflections
We placed the duck in our anechoic chamber to show what would happen in a room with no reflections. Behind the duck you can see wedges of fibreglass, these are used to absorb the sound. We recorded the anechoic duck:
The duck in an "echo" chamber
A reverberation chamber is a space with cathedral like acoustics, the sound echoes around the space for a long time, and the room is used to measure the acoustic properties of building products. We recorded the reverberant duck:
The duck with one echo
Imagine the duck flying past a cliff side. It is possible to use an electronic effects box to produce the sound you might expect. In this example, the echo path is about 50m, and the echo attenuated by about 6dB.
While the echo can clearly be heard when compared to the duck with no reflections, it might be hard to say there was an echo present if you hadn't first heard the anechoic recording. Consequently, a duck's quack does echo, but in many circumstances will be hard to hear.
The mythSo a duck's quack certainly echoes around our reverberation chamber, so a duck's quack does echo. Which leads to the most interesting question, why did the myth arise? The are a few possible explanations that I can think of:
- The quack does echo, but it is usually too quiet to hear. When you want to hear an echo, you usually make a very loud noise to make sure the reflection can be heard. But a duck quacks too quietly, so the reflection is too quiet to hear.
- Ducks don't quack near reflecting surfaces. You need a large reflecting surface, a mountain or building for the sound to reflect off. Maybe ducks don't hang around reflecting surfaces.
- It is hard to hear the echo of a sound which fades in and fades out.
We think Daisy now deserves retirement, and so we are going to investigate other acoustic myths. Do you know of any worth investigating?
Where is Daisy now?Daisy was kindly lent to us by Stockley Farm in Cheshire; this is a working farm open to public. Unfortunately, Daisy has been eaten by a fox, but her daughter lives on.
Re-use of material permitted provided it is clearly labeled "(c) University of Salford, www.acoustics.salford.ac.uk"