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.
Frank DiBonaventuro, B.S., Air Force officer, Tinker AFB, OK.
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.
Jonathan Osgood, Physics Undergrad, Wheaton College, Chicago, IL
'Strange is our situation here upon the earth. Each of us comes for a short visit, not knowing why, yet sometimes seeming to divine a purpose.'