Sound Reflections

I knew about the electronic version of the effect demonstrated in this exhibit from building and repairing guitar effects pedals. For those of you familiar with guitar effects, this particular effect is called a flanger. The original name comes from when an audio engineer would stick a finger on the flange of a magnetic tape reel to slow it down slightly and then combine that signal back with the original. Perceptually, this gives an airplane-like whooshing sound. In more technical terms, the time delay will effectively alter the phase of the frequencies coming from the sound source based on the relationship between the delay time and the frequency in question. When combined with the original sound source there will be peaks where the phase is in alignment and notches where the delayed signal and original are out of phase. This creates what is known as a comb filter (http://en.wikipedia.org/wiki/Comb_filter), and altering the delay time will cause the comb to sweep through the frequency spectrum.

This can be done electronically by adding a very small amount of delay and combining it with the original 'dry' signal. The same effect can be accomplished through purely acoustic means by forcing a sound to travel in two different directions and then combine at the listener's ears.

Paul Doherty commented that he noticed this effect at the outdoor pull-up bars near the old exploratorium when an airplane went by and you were able to raise or lower yourself and hear the sound directly from the plane combine with the reflected sound off of the ground. His story reminded me that I've noticed the same thing when jogging on a smooth asphalt surface while a plane passes overhead. Even the slight bouncing up and down that happens when I jog is enough to trigger the effect. At the time I incorrectly attributed the change in sound to the doppler effect because the perceptual change can sound like an overall frequency shift up and down, but in actuality the notches in the comb filter are adding and canceling frequencies throughout the audio spectrum and this just sounds like a frequency shift.

When I first prototyped this exhibit, I placed a speaker playing white noise on a box on top of a table and moved my head up and down relative to the speaker and the table to hear the combination of the sound directly from the speaker and the sound bouncing off of the table. This demonstrated strong perceptual evidence of the effect but I couldn't shake the thought that maybe it had something more to do with the position of my ear relative to the speaker and not the filtering due to the the combination of two paths of the sound. To convince myself of the comb filter, I then kept my head and the speaker stationary and brought in a wooden plank to bounce the sound back towards my face. This worked extremely well, especially in that if I moved slightly out of the path of the bouncing sound (1-2ft to either side) the effect decreased to almost nothing, making the effect that much more convincing. I thought my work was done.

I did also listen to a sine wave at several different frequencies to see if it would cancel itself out as the plank was moved. The bouncing sound is not quite as strong, so I did not experience a completely silent node, but there was a noticeable increase and decrease in volume.

The problem that we encountered was that when I first set up the exhibit in class, the room was much smaller than where I had originally prototyped, and there were other sound-reflective surfaces in the immediate vicinity causing more reflections than just the one from the plank. This made it so that I could hear some version of the effect even when I stood several feet away from the person interacting with the exhibit. This takes away a lot of the surprise and wonder when it is your turn to try it out. In future versions, I'll ideally place the exhibit in a room that is acoustically dead so only the sound from the speaker and a single reflection off the plank can be heard.