# Sound waves and fences

I hope I can explain this clearly. I regularly ride my bike on a local trail, and I’ve noticed that when the trail runs along the back of a house with a fence like this --that is, one with staggered panels–I hear a rhythmic “click-click” sound that goes away as soon as I ride past the fence. What causes that?

I’m not sure if it’s the effect you’re talking about – I wouldn’t describe it as “click click” – but there is a definite sound effect created by regular structures. I’ve heard it near iron-bar fences and near structures with corrugated steel and with poured concrete having a regular vertical pattern. You get weird sound echoes from such structures.

The regular vertical structure of the fence/corrugation/concrete acts like a diffraction grating. Instead of taking a plane wave of “white” sound and splitting it into plane waves with different frequencies (notes) heading in different directions, though, this grating takes an expanding spherical wave (generated by you and your bike, or by me stamping my foot) and turns it into reflections coming back to me (of course, it’s turned into reflections going in every direction – but I only hear the ones coming back to me). I get destructive interference in most directions and times, just as with light bouncing off a grating, but I will get almost monochromatic waves – different notes of sound – spread out in time. In essence, the result of the grating is to break up the sound I make into different notes and spread them out in time.
Stamping my foot on a hard surface near such a grating makes an impulse, like a delta function, so I get a run of impulses back spread out in time, and I hear a sound that changes up or down the tonic scale. (One professor I told this observation to called it a “tweeble”, although to me it doesn’t sound like “tweeble”. He pointed out that it’s a way to demonstrate “chirp” in a non-optical way.)
I find that this has been addressed on this Board before, back in 2003:

http://boards.straightdope.com/sdmb/archive/index.php/t-215295.html

Here’s a paper about the effect observed at Mayan Temples (!)

http://www.acoustics.org/press/136th/lubman.htm

CalMeacham is on the button, but just in case you couldn’t follow it, here’s a dumbed-down version:

You’re hearing an echo of the bicycle. You’d hear it when riding past a building too, but there it isn’t so noticable because it is so steady. And you would not hear it when riding past a cyclone fence, because the sound would just go right through and not bounce back. But the sort of fence you’re describing is just right for the staccato sound you’re describing.

I hear an echo when running past a solid wall that has ornamental “turrets” every so often. The sound of my running gets reflected off the 90 degree angle of the turrets and right back into my ears.

That’s a different effect – the “corner cube” effect, where right angles like the inside of the corner of a cube (or only two of the walls) reflect sound back directly along the way it came, but in the opposite direction. They use the analogous effect with mirrored surfaces (or total internal reflection) for optical reflectors. And solid buildings can do the same for radio waves, really screwing up radar.

Thanks for the info!