How many decibels in the crack of a beer can

Exactly that
Campbell CampbellAsked:
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d-glitchCommented:
I have a Radio Shack microphone (a clone of the Shure SM58) and an oscilloscope.  I finally got around to doing the test on the holiday.

The microphone sensitivity is  -54 dB V/Pa.  This converts to 1.85 mV at 94 dB SPL

First a trace of a balloon pop at 1 meter:BalloonPeak amplitude is 80 mV  ==>  20*log(80/1.85)  ==>  32.7 dB
This gives an SPL of 127 dB which agrees with the published value.

And now for a Pepsi can at 10 cm:PepsiHere the peak amplitude is 18 mV  ==>  20*log(18/1.85)  ==>  19.8 dB
This gives an SPL of 114 dB.  

To find the SPL at 1 meter, you have to subtract 40 dB.  So popping a can comes in at 74 dB.
127 - 74 ==> 53 dB  and the balloon is 450 times louder than the can.

Mal Osborne's "reasonable guess" was on the money, and much better than mine.
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CompProbSolvCommented:
To start with, you need to specify at what distance.

I would think it depends on the design of the can and also on the reaction of the expanding gases when the top is opened.  It also would depend on how quickly the top was opened.

The simple answer is that I don't think there is a simple answer!
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Dr. KlahnPrincipal Software EngineerCommented:
CompProbSolv is correct.  This is not a simple question until the parameters are pinned down, e.g. "measured at a distance of 1 meter from directly above the top of the can, using a conventional sound meter of at least IEC 61672-1 Type 2 with F weighting ..." and it goes on and on.
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d-glitchCommented:
Here is a chart showing a range of dB levels for a variety of phenomena.
     http://www.noisehelp.com/noise-level-chart.html

I would take a the popping balloon [125 dB] as a starting point: both sounds are due to a sudden release of pressure.  The stored energy [volume * pressure] is similar in each case.

But the balloon is louder because it fails catastrophically and releases all of its energy in about a millisecond.  I would estimate the time constant of a Pepsi can is 10 ms, and the peak sound level at 115 dB.
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Mal OsborneAlpha GeekCommented:
Also the type of can, way it was opened, temperature, how much it had been shaken, and the weighting used.

A reasonable guess would be maybe 50 - 80 at one metre.
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CompProbSolvCommented:
Interesting!  Shall we debate whether Pepsi is different from beer in this case?

I'm curious about the very different waveforms.  With 1ms/div it appears that the fundamental of the balloon pop is around 4ms or 250Hz.  That seems a lot lower than I'd have expected.  There is certainly a lot of much higher frequency energy on the graph, so that is likely what we notice more so than the fundamental.

It also surprises me that the balloon take so much longer to release its energy.  There is still significant sound pressure 6ms after the initial pop.

Great stuff!  Thanks......
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d-glitchCommented:
The balloon releases all of its energy at once in a mini shock wave.  You get a 1ms positive pulse followed by a 1ms negative pulse.  Everything after that is reverb from the lab bench and the room.  The balloon diameter is 1 foot, and the speed of sound is close to 1 ft/ms,  which is where the 1ms features come from.

The can is a very different story.  When you open the tab, the can "rings" at approx 6 kHz.  It is essentially a tubular bell with two characteristic modes of vibration: around the cylinder and from top to bottom.  It doesn't ring for very long because it is full of fluid and heavily damped.

I also know why my original estimate for cracking the can was so far off:  The can is smaller than the balloon by a factor of 10, and at higher pressure by a factor of 10 or more.  But the stored energies aren't close to being the same, because sugar and water are not compressible and don't store any energy.  I should have only considered the gas volume in the can.
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CompProbSolvCommented:
What?  You didn't do this test in an anechoic chamber?  <G>

To be clear, you're suggesting that everything after about the 3ms point (for the balloon) is echoing/reverb in the room?

Also, does it seem interesting to you that the negative pulse for the balloon has significantly more area than the positive pulse?

Your description of the can resonating seems quite reasonable.  Initially I'd have expected more of an initial pulse from the escaping gas, but I suspect that the small volume (as you mention) significantly limits this.
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d-glitchCommented:
On second thought (or 4th or 5th), I probably shouldn't be speculating so much on a couple of scope traces.

I'm pretty sure you get a 1 ms pressure feature from a 1 foot balloon.  And I am very sure there is a lot of reverberation.  But I can't explain the negative pressure signals at all, and I haven't characterized the sensor very well either.

A microphone is not a pressure sensor.  For example, it is AC coupled, so the ambient pressure always reads as zero.  The dominant negative feature in the scope photo could be the microphone recovering from a strictly positive pressure wave.
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d-glitchCommented:
An interesting experiment.
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