What caused pitch-shifted echoes of the sound of a fireworks display?

July 4, 2009 8:26 PM

What acoustical phenomena explain the apparent pitch-shifted echo I heard while observing tonight's fireworks display?

My son and I were sitting in a folding chair on a hillside a little over a mile (<2km) much higher than that of the fireworks. One was higher pitched than the other, but I've already forgotten which was which. I'm sure this was not the sound of the rockets' launch/ascent. We heard the main boom from directly in front of us. From slightly behind our direct left we heard a rolling echo reflected off of a small mountain about a half-mile distant. But the zipping sound seemed to come from between our feet.

The ground in front of us sloped fairly steeply down for a short distance, was level for several hundred feet (~75m) across a neighbor's yard, then descended, slightly and smoothly, to the site from which the fireworks were being launched, no more than 100ft (30m) lower in altitude than the yard where we were seated.

What was I hearing? Some sort of acoustical mirage? A diffraction effect? Interference among echoes? Some expression of sound waves carried through the ground itself? Electrophonic sound rendered audible by the very blades of grass?

posted by Songdog to science & nature (12 comments total)

The original firework sound is not pitched, it is a dense and complex non-harmonic combination of frequencies. By subtracting some frequencies and emphasizing others you get the illusion of a shift in pitch - think the difference between adjusting the treble knob and pushing on a whammy bar (pitch shifts in nature are not (ever? usually?) caused by reflection or echo, but rather by direct manipulation of the sound producing object, usually some animal's larynx).
posted by idiopath at 9:39 PM on July 4

In my editing of my rambling comment I left some content out: every combination of reflections of a single sound source will have a tonal filtering effect, emphasizing some pitches and attenuating others (this is different from altering the pitch, mind you).
posted by idiopath at 9:41 PM on July 4

Fair enough, but the sounds were night and day. The direct sound of the fireworks had a sharp punching attack with lots of lows (the rumbling echo off of the mountains emphasized these lows, and rolled on as the echo reflected off of different parts of the slope). The zipping sounds had a slow attack; they sounded almost like they were being played in reverse. And the zips had no lows to speak of. They sounded like insects, not like the cannon-fire pyrotechnics.
posted by Songdog at 9:54 PM on July 4

The energy needed to carry a low frequency sound increases much more with distance than the energy needed to carry a high frequency sound (interestingly the math for this is just about identical to the reason that short wave radio is most efficient for long distance radio communication). In other words, an echo reflection from a more distant source will lose low frequency energy, thus sound higher in pitch.

A sound that is being reflected from a larger number of sources (ie. a mixture of a number of longer distance echoes), will also have a blurred attack, to the point that you might no longer perceive an attack to the sound. This will be particularly noticeable if there are a large number of concrete walls or other hard semi flat surfaces that efficiently reflect high frequency vibrations at irregular distances from the sound source.

Of course I cannot discount that you could have heard something not from an echo, but what you are describing does seem like a mixture of a large number of echoesto me.

Shifts or modulations in pitch are not a linear function of the audible wave, so you will be hard pressed to find a natural physical system that shifts pitch. Even in a digital system a pitch shift is pretty complicated, usually needing either a complete analytic breakdown and resynthesis of the sound (vocoder, phase-vocoder (the tech behind pitch correction)) or wavetable based reproduction of the input signal (the classic whammy pedal style pitch shift).

The electrophonic phenomenon is fascinating (I have heard of pine needles making the Aurora Borealis audible), but as far as I know, fireworks produce negligible electromagnetic energy.
posted by idiopath at 10:58 PM on July 4

I have heard something that I would describe as a fairly high pitched zip sound when I stand in a brick plaza about 40' in front of a long cement staircase and clap my hands once sharply (Red Square between the Undergraduate Library and Kane Hall at the University of Washington).

I think the sound comes from the collective effect of individual echos from each riser of the staircase. The echo from the lowest step arrives first, followed in sequence by echos from each succeeding higher riser because it is just a bit farther away than the one just below it. The frequency is 1/(the time between successive echos).

But it is not a sine wave because the clap is approximately a spike, and each echo from an individual riser is probably a somewhat blurred mirror image of that spike. I think that's an important contributor to the sensation of zzzip.

In the situation you describe, I would look for a structure or set of structures on a nearby hill facing you but on the opposite side of the fireworks that resemble the risers of a staircase, such as a set of fences going up a slope.
posted by jamjam at 11:35 PM on July 4

The energy needed to carry a low frequency sound increases much more with distance than the energy needed to carry a high frequency sound (interestingly the math for this is just about identical to the reason that short wave radio is most efficient for long distance radio communication). In other words, an echo reflection from a more distant source will lose low frequency energy, thus sound higher in pitch.

Are you sure about that? I think you've got it exactly backwards.

I could believe that short wavelengths reflect better, but long wavelengths carry better.

And when it comes to radio, longer wavelengths are better used for long distance communication, but not because of fading. It's because short wavelengths tend to be more linear, whereas longer wavelengths will tend to curve around the horizon better.

I think your memory replaced "wavelength" with "frequency".
posted by Chocolate Pickle at 12:42 AM on July 5

chocolate pickle: yeah, you are absolutely right, I had that backwards for both audio and radio. Shortwave is useful for long distance communications because it bounces off the ionosphere to communicate past the horizon, whereas longer wavelengths just keep going through the ionosphere into space. Shortwave = long distance communication was the source of my confusion.
posted by idiopath at 12:59 AM on July 5

This also will help explain what the poster heard. The lower frequency spectrum of the firework blasts went straight through something, while the higher frequency components echoed back (quite likely with the staggered zipper effect jamjam proposed).
posted by idiopath at 1:01 AM on July 5

Perhaps you heard a "culvert whistler." I can't find a sound file on the web, but here are a couple of explanations:
Home-made Thunder/Whistler Tube (scroll about 1/3 down page)
Paul Tipler's Problem 99
A valley or slope might suffice in place of the tube. The effect is similar to what jamjam describes, and gives a reflected sound that is falling in pitch.
posted by drdanger at 5:28 AM on July 5 [1 favorite has favorites]

The Exploratorium in San Francisco has an Echo Tube exhibit which demonstrates the effect drdanger mentions. You can hear it in this video of a guy beatboxing into it!
posted by moonmilk at 9:13 AM on July 5

Since you were sitting on a hillside overlooking the fireworks, I think we can simplify it a bit.

The first sound you heard was the exploding fireworks itself, since that is the shortest path between it and you (all other paths are longer, hence will be heard later). This is the CRACK or BAM or BOOM, the sound impulse generated by the explosion.

Any sounds heard after that are reflections off of various physical features surrounding the fireworks location. Hills, buildings, every single object reflects the sound that reaches it and some makes its way to you. The sound you heard at your location was the combination of all those echoes reaching you.

The Zip Sound is undoubtedly reflections from a physical feature that has regularly spaced objects, and since it seemed to come from between your feet, you could look for the physical feature down there, either between the hill and the fireworks launch point, or behind it.

I really think that's it. Nothing to do with electrophonic phenomena, sound conduction through the ground, or cylinders/tubes. You were sitting in a location receiving a wide variety of echoes, and the sounds you heard were complex and interesting to hear.
posted by exphysicist345 at 1:01 PM on July 5

Thank you all for your ideas. It's obviously not practical to arrange a controlled experiment at the site so I can't be certain which answer is best but jamjam and exphysicist345's answers strike me as closest to the truth so I'm giving them the nod. A set of close, interfering echoes could have been reflected off of fences, trees, or even ground features. That seems consistent with what I heard, and I think that's probably what was happening.

And now, on showing this question to someone standing next to me I see that I edited it into absolute incoherence just before posting, not noticing that I lost a big part of the first long paragraph after I added the <. By some miracle I still have the original browser window open and I retrieved the pre-garbled post from my history. Here's what I was trying to say:

What acoustical phenomena explain the apparent pitch-shifted echo I heard while observing tonight's fireworks display?
---
My son and I were sitting in a folding chair on a hillside a little over a mile (<2km) from our town's fireworks display. We were the only ones there, in a private yard far from the crowds. As a result it was nearly silent in between air-bursts and after I while I became aware of something peculiar: just after the sound of each detonation I heard a pair of zipping noises "vvzzzt! vvzzt!" at different pitches. Every time. Their pitch was much higher than that of the fireworks. One was higher pitched than the other, but I've already forgotten which was which. I'm sure this was not the sound of the rockets' launch/ascent. We heard the main boom from directly in front of us. From slightly behind our direct left we heard a rolling echo reflected off of a small mountain about a half-mile distant. But the zipping sound seemed to come from between our feet.

The ground in front of us sloped fairly steeply down for a short distance, was level for several hundred feet (~75m) across a neighbor's yard, then descended, slightly and smoothly, to the site from which the fireworks were being launched, no more than 100ft (30m) lower in altitude than the yard where we were seated.

What was I hearing? Some sort of acoustical mirage? A diffraction effect? Interference among echoes? Some expression of sound waves carried through the ground itself? Electrophonic sound rendered audible by the very blades of grass?

I don't know how you lot managed to make sense of this as posted. Good show.
posted by Songdog at 7:18 PM on July 6