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I encountered something strange regarding echoes and pitch.

One Fourth of July , I saw rockets reach a height its hard for me to gauge. I'd call it two to three dozen meters.

I had noticed that shortly after a rocket went off, Pop!, I'd hear a Pang! echoed from the wall of a nearby building. The Pang came about a half-second after the Pop. The Pang was little shorter in duration from the Pop and had a higher pitch than the Pop.

That night I was talking to my professor. He pointed out that reflection doesn't change pitch at an interface and said I must be mistaken.

Is it possible I wasn't? Perhaps there is some physical mechanism that would have filtered out the pitches?

Perhaps longer wave lengths were more dampened by the aluminum siding leaving higher intensities for the higher pitches? It was probably a hot day. Perhaps temperature differences could effect the pitches along the direct path differently from those from an echoed path so that there was a different balances of pitches when the echo arrived? A non-physical explanation, my left ear was facing the building the Pops were in the direction of my right ear. Maybe I've lost high pitch hearing in my right.

If this helps, the building was about 5 to 8 m high and Between 10 and 15 m long. The siding had 3cm depth by 3cm width ridges running the height of the side of the building facing the fire works show, periodically.

I'm about 168cm tall. I believe was two times the distance from the fireworks as I was from the building. I was roughly 6m from the building. I was located roughly in the blue location marked ME and the poppers were around SRC in red.

Reflecting Building.

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  • $\begingroup$ I'm not sure, so this is a comment. I suspect that there was some frequency-dependent filtering going on. Possibly because the building itself was enhancing the high frequencies, or possibly because the "pop" was getting some natural low-pass filtering from the surrounding buildings, trees, and whatnot. $\endgroup$
    – TimWescott
    Dec 17, 2019 at 0:48
  • $\begingroup$ You could roughly calibrate your ear's frequency response by snapping your fingers while rotating your head. For that matter, on a quiet night you might be able to calibrate the building's echo response by either snapping your fingers, clapping, or using a big (hence, low-pitched) slapstick. $\endgroup$
    – TimWescott
    Dec 17, 2019 at 0:50
  • $\begingroup$ In line with my comment on niels's answer, it might help to upload a rough diagram of where you, the rockets, and the building were located. $\endgroup$ Dec 17, 2019 at 2:27
  • $\begingroup$ added pic. Approximate locations. $\endgroup$
    – R. Romero
    Dec 17, 2019 at 18:26

2 Answers 2

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I've heard this before too, but I've never really thought it through. Good question.

You say:

That night I was talking to my professor. He pointed out that reflection doesn't change pitch at an interface and said I must be mistaken.

Technically he's correct for pure sinusoidal tones - the frequency won't change with reflection.


However: most sounds (and especially short, transient sounds like explosions) contain energy at many different frequencies. We expect the structure to be excited by some frequencies more than others - and thus more effectively transmitting them to the interior of the building. Acousticians commonly use plots of transmission loss as a function of frequency to describe the properties of materials as sound barriers. Here's one for a corrugated panel, using this method:

transmission loss plot

The shape of this plot is very similar for many materials - and this is intuitive - for example, music heard through a wall is mostly low-frequency components like bass and drums.

There are two very important conclusions we can draw from such a plot:

  1. The difference in transmission loss between low frequencies and high frequencies is enormous. (Remember, 40 dB is the equivalent of a 10,000-fold difference in energy!)
  2. The low-frequency "boom" of the firework explosion should be transmitted through the building, while the high-frequency "crack" component will not: it will be reflected or absorbed by the structure, instead.

That's what I believe is the best explanation for your observation: the structure is physically more likely to reflect high-frequency components of the firework sound than the low-frequency components.

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  • $\begingroup$ the high-frequency "crack" component...will be reflected or absorbed by the structure... I feel like "reflection" and "absorption" would result in two different things. If the high frequencies are reflected back to me, then I will hear them. If they are absorbed, then I will not hear them. Have I misunderstood what you are saying, and if not then why would reflection be favored over absorption? $\endgroup$ Jan 23, 2020 at 18:29
  • $\begingroup$ We'd definitely expect some of the energy to be absorbed, but you wouldn't hear any of that, would you? I agree that the reflected component isn't a perfect inverse of the transmitted component, but even if they're not we can still expect the reflected component to include less low-frequency energy. $\endgroup$ Jan 23, 2020 at 18:34
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    $\begingroup$ So are you saying that the amount of low frequency that is transmitted/absorbed is more than the amount of high frequency that is transmitted/absorbed? Thus you have more high frequency in the reflection $\endgroup$ Jan 23, 2020 at 18:43
  • $\begingroup$ Yes, that is what I am saying. $\endgroup$ Jan 23, 2020 at 19:02
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I think this is most likely due to the doppler effect. the explosion of the rocket occurs while it is moving away from you which pitch-shifts the tone of the "pop" down. Meanwhile, the sound reflection off the sides of the nearby building (which is tall) will come sideways from the exploding rocket, and hence will be doppler-shifted less. This means the reflected sound will be higher in pitch.

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  • $\begingroup$ If it was due to the Doppler shift then wouldn't you expect to hear higher and lower pitches (like a distribution of pitches)? Certainly not all of the rockets were heading in the same direction/speed relative to the OP. It might also help to determine from the OP the location of the wall relative to the rockets and the OP. $\endgroup$ Dec 17, 2019 at 1:53
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    $\begingroup$ all of that extra info would be helpful. $\endgroup$ Dec 17, 2019 at 2:00

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