[Karl]

Has anyone noticed a squeaking noise as fireworks go off, especially in built up areas? Check out this video, when the first shell bursts you hear a squeak too.

Anyone know what causes this?

This post has been edited by Karl: 26 September 2010 - 05:51 PM

[Potassium chlorate]

Can't it be the stars being propelled very fast through the air, almost like bullets, the first second or so directly after the burst?

[Arthur Brown]

I'm guessing that this is whistle still whistling after the hemis have broken.

[Mortartube]

It's not the whistle Arthur. It is definitely an audio effect caused by sound reflection as I have heard the same effect when using H.E at an airshow and the sound was reflected by buildings. I believe it may be caused when the burst sound is of certain duration and contains certain frequencies, and that sound is reflected from a hard surface.

I surmise that the first part of the soundwave hits a wall for instance and is reflected, the end part of the sound wave is still travelling towards the wall and compresses the first part of the waves causing a frequency shift.

[helix]

Its been 20 years since I studied acoustics so I could be wrong but it seems to be some form of end reflection attenuation. end reflection attenuation is more effective at reducing the lower frequency noise. The amplitude of the reflected wave is similar to the original wave but out of phase when the two meet they cancel each other out. The high frequency noise would not be attenuated which could be why you hear the high pitched sound. If it was reflection attenuation then there would need to be a lot of hard/ reflective surfaces about to cause it.

[MDH]

This sound can be heard clearly over a flat area with large, metallic buildings nearby and very few (large) trees. I have never heard it in any other sort of area.

This post has been edited by MDH: 27 September 2010 - 04:38 AM

[Karl]

Sounds like a valid theory to me Helix. As I said i've heard it in built up areas when people have let fireworks off but not all the time, it depends how loud the burst is.

[PyroCreationZ]

I have heared this noise often as well but only in videos, not in real life. At first I thought people just had crappy cams lol but yeah I agree it has to do with the kind of buildings nearby.