Destructive Interference in the sound of multiple firecrackers set off at once?

[MarvinBotts]

TL;DR Summary

There is a strange phenomenon that occurs which no one has adequately explained in my opinion.

The more firecrackers in a single group that we set off, the less noise is produced after a certain point. Or if they are fused to go off all at once, less db of sound occurs than if progressively lit from a few to many at the end.

Some have speculated that the heat is suppressing volume. I suspect destructive interference.

There is a massive and continuous fireball as fire crackers are exploding, generating thousands of shockwaves. Perhaps they are bouncing off each other and cancelling each other out like noise cancelling headphones,

 

[tech99]

Are there some measurements to show this effect?

 

[MarvinBotts]

There are numbers published. I will get a hold of them. However, I am not sure of the methods used. I know sound waves travel .6 meter per second for every degree temp change but since its rhousands of 35 ms event over several minutes, its a real mind bender than if a single event

 

[hutchphd]

Sound transmitted near the ground is notoriously (because of broken houses) dependent upon local air density and the resultant refraction and reflection of the sound. The folks in Huntsville AL learned this as I recall. Considering the heat produced I feel certain it will generate refractive effects, perhaps causing more sound to go skyward before reaching the bystanders.

 

[Vanadium 50]

I'd like to see measurements, because as described this seems really, really unlikely.
For one reason, the energy lost in destructive interference always shows up as constructive interference somewhere else.

 

[MarvinBotts]

Sound transmitted near the ground is notoriously (because of broken houses) dependent upon local air density and the resultant refraction and reflection of the sound. The folks in Huntsville AL learned this as I recall. Considering the heat produced I feel certain it will generate refractive effects, perhaps causing more sound to go skyward before reaching the bystanders.

Well, it is a very unusual experience. The first mega superstring I witnessed was overwhelming. I was 300ft away and I could not tell which direction the sound was coming from even though I could plainly see it

 

[MarvinBotts]

I'd like to see measurements, because as described this seems really, really unlikely.
For one reason, the energy lost in destructive interference always shows up as constructive interference somewhere else.

So I get your scepticism but as a professional pyrotechnician I have witnessed it many times. The PGI, Pyrotechnics Guild International has journaled the effect and db measurements. I will get them. I thought noise cancelling headphones zeroed out out of phaze sound waves but produce heat as a buproduct

 

[tech99]

The ears have a compensating (AGC) mechanism whereby loud repetitive sounds gradually sound weaker. That is why bands turn up the volume as the evening progresses. It depends whether we are relying on subjective effect or measured effect. Normally we expect sounds such as this to be de-corellated, having random phase, and so will add on a power basis. Ten fireworks ignited simultaneously will give ten times the power, which is 10dB increase, and so will sound about twice as loud.

 

[Lord Crc]

Wouldn't each firecracker going off produce a volume of low density around it for a short while? If so, a firecracker going off next to it "immediately" afterwards will have less air to push against, hence make less of a pressure wave?

 

[hutchphd]

"Tis the sound of one hand clapping?

 

[berkeman]

The PGI, Pyrotechnics Guild International has journaled the effect and db measurements. I will get them.

I don't think you want to analyze the "dB readings". Instead, you want to get the recording of the sound from a single microphone system that does not include any AGC circuits. You want to analyze the time-domain sound pickup waveform both in the time domain and the frequency domain. Do you have access to such recordings?

 

[hutchphd]

It might be intersting to look at the dB readings although I don't think they will be definitive. One could get a handle on the magnitude of the effect maybe.

 

[MarvinBotts]

The ears have a compensating (AGC) mechanism whereby loud repetitive sounds gradually sound weaker. That is why bands turn up the volume as the evening progresses. It depends whether we are relying on subjective effect or measured effect. Normally we expect sounds such as this to be de-corellated, having random phase, and so will add on a power basis. Ten fireworks ignited simultaneously will give ten times the power, which is 10dB increase, and so will sound about twice as loud.

Sorry, you are mistaken. Db's are logarithmic. 10 X the fireworks DEFINITELY will not increase the power 10X. a 1 Db increase is about 30% louder, and so on. A similar phenomenon is in play with earthquakes. The difference between a 6 and 7 is 32X

 

[sophiecentaur]

Sorry, you are mistaken. Db's are logarithmic. 10 X the fireworks DEFINITELY will not increase the power 10X. a 1 Db increase is about 30% louder, and so on. A similar phenomenon is in play with earthquakes. The difference between a 6 and 7 is 32X

I have to re-correct this. dB is the log₁₀ of the ratio of the powers. Whilst your square root treatment works for sound pressure (Volt and Current too), +10dB represents ten times the power.
However, they are not all going off at once (which would definitely correspond to log₁₀(number of crackers) so the details of our hearing - frequency response and time constants etc etc - will affect out subjective appreciation and also the measured ratio where the Power would be measured in Joules per Second when they all explode within one second.

Wouldn't each firecracker going off produce a volume of low density around it for a short while? If so, a firecracker going off next to it "immediately" afterwards will have less air to push against, hence make less of a pressure wave?

I think that air can be considered as a linear medium except under very high sound levels and, where the sound has been produced by an explosion. the majority of the pressure is supplied by the shock wave from expanding gunpowder gases so any 'lack of air' after a bang would hardly affect the net sound pressure. Once you're a few metres from the crackers, the actual sound pressure level won't be very high so the sounds would 'add up'.

 

[MarvinBotts]

I plan on doing addtional testing to ensure readings are correct and controlled. You know the actual pop of the firecracker is a 35ms event that requires a type II weighted sound meter

 

[sophiecentaur]

The weighted measurement from a meter is designed to give results which compare well with subjective experience with a range of ‘typical’ sounds.
Imo, fire cracker sounds are not typical and i’d suggest that it would be risky to rely on a meter.
It may well be true that the firecracker effect is ‘a thing’ but objective experimental data needs to be Raw and not tinkered with by any weighting. That data would contain the information you want. I’d predict that there would be no strange mechanism at work and that it’s all down to human hearing.

I write this from the standpoint of a hearing impaired observer. What I hear under some circumstances can be absolute nonsense to me. (Not the words lol)

 

[MarvinBotts]

Be that for what it is... Just a reminder that I am looking for a causal factor for a clump of explosions verses one spread out. We found a practical solution... By spreading the same number of firecrackers over a fence, the sound inhibiting paradox is dramatically reduced. See 2nd video
It was also observed by DB meter and perception that seperating the string down a fence increased sound levels for an equal number of firecrackers bunched up

 

[Lord Crc]

Wouldn't each firecracker going off produce a volume of low density around it for a short while? If so, a firecracker going off next to it "immediately" afterwards will have less air to push against, hence make less of a pressure wave?

I think that air can be considered as a linear medium except under very high sound levels and, where the sound has been produced by an explosion. the majority of the pressure is supplied by the shock wave from expanding gunpowder gases so any 'lack of air' after a bang would hardly affect the net sound pressure. Once you're a few metres from the crackers, the actual sound pressure level won't be very high so the sounds would 'add up'.

I still think I was on to something, but got it the wrong way around.

If a firecracker A goes off, the gasses will form an expanding high-pressure region around the firecracker. As the gasses expand, the pressure drops. If another firecracker B next to it goes off shortly after, B's gasses will expand into the already higher-than-ambient pressure. Surely this lowers and broadens the pressure peak of B's gasses, leading to a lower perceived "crack".

Regardless, I found the following paper illuminating: Blast pressures and waveforms of consumer firecrackers

 

[DaveC426913]

Well, it is a very unusual experience. The first mega superstring I witnessed was overwhelming. I was 300ft away and I could not tell which direction the sound was coming from even though I could plainly see it

That makes sense. Your brain determines direction based largely on the time delay for a sound to reach each ear. A zillion virtually identical sounds going off over top of each other would make it impossible to distinguish a single pop from another which would completely confound echolocation. Thus, it would sound like it's all around you.