New type of mid air laser show

[buffo]

You are making the mistake of assuming that all of the sound created by each little blast plasma is at 30Khz. It's not. If you can hear the formation of a single ball of plasma (which, obviously you can, as shown in the video), then the more of them you have per second, the louder the sound will get.

Remember, it's not creating a 30Khz noise, it's creating a broad-spectrum white-noise type sound at a RATE of 30Khz. They will overlap, interfere, and in general mix together to form a steady roar that will most definitely be audible.

Adam

[Xytrell]

Yes, I know it's white noise, but if you get a bunch of them together, the audio pitch distribution tends to a normal distribution around the repetition rate, doesn't it? Yes, there will be many pitches lower and higher, but it will make a general sound at the frequency of repetition. I'm just extrapolating that if a laser at 60hz repetition makes a low pitched noise, and if the rep rate is increased to 300hz, the noise increases in pitch as well... wouldn't a high frequency laser make a high pitched noise? Sure, some of the noise will be in the audible range, but there must be a point where the laser gets quieter with a higher rep rate. Then again, I could be totally wrong. But if I am, what would a laser start to sound like as the rep rate increases?

Does anyone have any experience in practice rather than theory?

[buffo]

I have a good bit of experience, both from my days as a submariner (where noise is the enemy) and from some unofficial experiments (don't call the BATF on me - they were purchased legally!) with fireworks in my backyard. I'll start with the fireworks example, as it is far easier to understand.

I once set off 16,000 firecrackers more or less at the same time. I had purchased a *huge* roll of firecrackers, but instead of rolling it out in a long line, I lit the bundle while it was still rolled up tightly. (Don't ever do this, BTW...) This particular string of firecrackers, like most, consisted of a central "tree" fuse with individual firecrackers hanging off each side. Their fuses were woven together with the main fuse, so that if you looked at the roll edge on, the main fuse was coiled around in the center with firecrackers sticking up and down, the fuse looking like the cream filling of an Oreo cookie. Thus, when I lit the main fuse, it began to burn radially, but also burned inwards from the outside to the center at about the same time. As it burned inwards, those fuse ends also began to burn radially, creating a geometric progression that rapidly exceeded the number of firecrackers in the roll.

This meant that instead of hearing perhaps 10 or 15 cracks per second for several minutes, we were treated to an astonishingly *deafening* roar accompanied by a blinding ball of white flame as 16,000 firecrackers detonated in about 5 to 6 seconds. The noise was unbearably loud and absolutely terrifying. But to say that it was centered at something close to 3Khz (16,000 explosions divided by 6 seconds = 2666 Hz) would be false in the extreme. The sound had no discernable pitch at all.

Also, given the physics of the event, if there *was* a discernable pitch, it would have risen and fallen over the 6 second interval, since the rate of explosions would have rapidly increased and then decreased significantly over the period. Yet this was not what we heard. There were a few initial pops, then multiple cracks, and within a second it had become a roar which only got louder and louder, right up until the end, when only a few lame pops signified that the show was over. (And everyone could remove their hands from their ears.)

As for submarine references, that will take a lot more time to explain, and the results may not be very intuitive for you anyway. (Unless you've spend time doing vibration analysis, that is.) But suffice to say that broad-spectrum noise will interfere with itself to create all sorts of strange audio effects, and multiple broad-spectrum noise sources will interfere with each other to create a veritable cacophony of sound from which it is nearly impossible to extract a fundamental frequency. (Which is why it's nearly impossible to get a blade-count on a single ship in a crowded sea lane without directional hydrophones. If you can't isolate the source, it's hopeless.)

In short, if this laser display device was making 30,000 tiny balls of plasma per second, you would definitely hear it, and it would most certainly not be quiet. Yes, I understand your argument that in theory, the beat frequency of 30,000 Hz would be inaudible, but you are forgetting that this is a broad-spectrum sound source with significant amplitude, so it will be creating multiple sound pressure peaks at various frequencies far removed from 30,000 Hz. And those peaks will be the major (not minor) component of the total sound pressure created by the event.

Adam

[Xytrell]

I meant experience with lasers, not firecrackers. The firecrackers can't be detonated at the same precise interval as a laser can because of the relatively unpredictable nature of a firecracker explosion.

The laser show posted HAS a pitch. according to my graphic equalizer, somewhere in the 8khz area. A winamp screen shot is the best I can do, but you can confirm it for yourself if you have better software. I've also included a shot of the waveform from a program called sound forge, and there is something of a pattern there.

[ElektroFreak]

The detonation of a firecracker and the detonation of the air as it breaks down have very distinct similarities, including the fact that the sound emitted is broad-spectrum white noise. The two are directly comparable, and will have the same effect. If the sound waves emitted by the air breaking down was confined to 30hz, it would be inaudible, but since the sound emitted is emitted across the whole spectrum of sound, it is very much audible. A broadband snapping sound will still be quite clearly audible even if it occurs at a frequency of only 30Hz. You would hear a snap 30 times a second, quite loudly. It's the fact that each individual snap consists of a broad range of frequencies that is the issue here. That fact will make the sound audible regardless of whether it is at 1Hz or 100kHz or anywhere in between.. The broadband sound waves emitted by the detonations themselves are what you are hearing.

While you are correct, the sound has a pitch, it is not a pure pitch. The sound consists of both the range of frequencies emitted by the individual detonations and the frequency at which the detonations occur.

[mixedgas]

Having had a 4 nanosecond 20 hz yag at one time (forgot to unload it for demo at SELEM) , you get a white noise snap from the air plasma and a snap plus ringing from the flashlamp/cavity. At 8 Khz your hearing the pump laser fundamental rate.

Qswitches can hum too, although weakly.

Steve

[buffo]

I'm ready to give it up guys - it's clear he's not listening.

Apparently my experience in audio isn't sufficient. He only accepts audio experience as it pertains to lasers... (As if white noise from laser-induced plasma is any different than white noise from some other source!)

Adam

[Xytrell]

So if it's supposed to be equally distributed in the audio range, Why, then, does the laser show posted seem to have a pitch? Why do I hear a tone? Why does a graphic equalizer show a pitch? Why do I see a pattern in the wave form? Does the particular microphone picking up the sound in the video filter out all but one pitch because it's broken? Am I imagining it? Are these not legitimate questions?

I AM listening. I'm just speculating that you didn't hear a pitch when you were lighting firecrackers because the interval of the explosions might not have been as precise as a series of laser pulses. Isn't this a possibility?

At 8 Khz your hearing the pump laser fundamental rate.

So you're saying the pump laser is louder than the plasma bursts? What is the "fundamental rate?"

[ElektroFreak]

Allow me to quote myself:

While you are correct, the sound has a pitch, it is not a pure pitch. The sound consists of both the range of frequencies emitted by the individual detonations and the frequency at which the detonations occur.

..

[buffo]

So if it's supposed to be equally distributed in the audio range, Why, then, does the laser show posted seem to have a pitch?

Because, as we said several times above, a white-noise sound source creates all sorts of chaotic interferences. In this case, those interferences have combined to form a fundamental frequency of around 8Khz. However, if you had read the .pdf linked above, you would see that the images are made up of just 128 points which are scanned rapidly over a period of .2 seconds. (Which is the length of time the plasma balls remain visible.) So divide # of points by time, and you get a "plasma-dot-frequency" of 640 hz. The system supports a maximum of 200 points per image, which yields a frequency of 1Khz.

Notice how none of these answers are even close to your perceived fundamental frequency of 8Khz? So where did 8Khz come from then? As mentioned above, it's probably from the myriad interferences of 128 (or even 200) white-noise point sources all mixing together. Or it could be some harmonic created by the sound reflecting off the baseplate and interfering with itself. (Or maybe even from some artifact of the digitizing process when the audio was recorded. Way too many variables to nail down for sure.) In any case, your own data shows you that the beat frequency is largely irrelevant when dealing with multiple loud white-noise sound sources.

As an aside, the calculations needed to predict which frequency will be dominant when you have multiple broad-spectrum sound sources are incredibly complex. The dominant frequency can change based on trivially small changes to the environment. Study audio in any detail, and sooner or later you'll learn all about this. (Study chaos theory, and you'll get there faster.)

Are these not legitimate questions?

They would be, if they were only questions. You ceased asking questions quite a while ago, and since then have been arguing from a position of ignorance. That is why we are getting testy about it.

When you start arguing about the answers (when you don't seem to even understand them), then your questions are no longer legitimate. For example, you said:

But no matter how loud the noise is, if the frequency is 30kHz, you simply cannot hear it.

This is not a question, it's a statement. And it ignores the fact (which we had already stated twice) that the sound created by the tiny balls of plasma is not a pure frequency, but a cacophony of different tones which interfere with each other to create multiple frequency peaks. Then you said:

I know it's white noise, but if you get a bunch of them together, the audio pitch distribution tends to a normal distribution around the repetition rate, doesn't it?

Ok, admittedly this is a question, but by saying "I know it's white noise", you claim to understand what white noise is. To which we again explained that no, the pitch could be just about anywhere, and even if there was a detectable pitch at the repetition rate, the other frequency peaks (from the white noise interfering with itself) would be just as loud, if not louder. I even provided you with a real world example to illustrate this in a manner that should have been easy to understand - even intuitive. To which you replied:

The firecrackers can't be detonated at the same precise interval as a laser can because of the relatively unpredictable nature of a firecracker explosion.

Thus demonstrating that you didn't bother to consider the example, and chose instead to seize on the difference in repetition rate precision, as if it had any bearing on the example. (Hint: it doesn't.) At this point you're arguing with the explanation even though you don't understand the physics. And this is when I started to wonder if you were trolling or not.

The point was that even with a clear example of something that should have had discernable pitch, there was no fundamental pitch to be heard - just a tremendous roar of white noise. But you were too concentrated on continuing the argument to see this point. You then go on to state that:

The laser show posted HAS a pitch. according to my graphic equalizer, somewhere in the 8khz area.

But if you had done a little reading, you would see that this is NOT what you'd expect, since none of the components of the laser display system operate at anything close to 8Khz. So you just disproved your own theory, but you don't even know it yet, and you are still arguing about it. Now do you see why we're upset?

I'm just speculating that you didn't hear a pitch when you were lighting firecrackers because the interval of the explosions might not have been as precise as a series of laser pulses.

This is the first time you've given voice to your speculation. And if it were true, then at the very least I should have heard a variable pitch. Or a rising pitch, followed by a falling one. SOMETHING. But no, all I heard was a tremendous roar.

Tell me - what is the "pitch" of the exhaust note of the Saturn V rocket? Oh sure, if you do a fourier transform on a recording of the Apollo 11 launch I'm sure you can tell me which frequency was the loudest, but is that the same as pitch? NO, it's not.

Any broad-spectrum sound source BY DEFINITION doesn't have a fundamental pitch. Yes, you *may* be able to detect a beat frequency if you have multiple sources going off, but that beat frequency may just as well be overwhelmed by the rest of the sound. Also, you may miss the beat frequency because you key in on a particularly strong harmonic that is produced by the multiple interferences going on. All of this is highly unpredictable, however.

And none of it changes the fact that you *will* be able to hear one of these free-air-plasma laser display units even if it's running at 30Khz.

Adam