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Sound waves and resonant frequency

 
 
Reply Mon 12 Mar, 2007 12:03 am
Resonant frequency is the frequency at which an object oscillates, and if you bombard an object with sound waves that are at the same frenquency as the resonat frequncy of the object, the object will shatter and implode.

I know that when you try to use sound waves on a segmented object to impact it, it doesn't work well because the in the segmented object, the sound waves tend to dissipate or overlap. Is there a way to modify sound waves so that the segmented object can implode?

Can resonant frequency be calculated and measured for any object? Are there any tables of resonant frequencies?

Can sound waves bend metals?
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Type: Discussion • Score: 0 • Views: 4,358 • Replies: 29
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Dedshaw
 
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Reply Mon 12 Mar, 2007 08:07 pm
well if there hasnt been a chart created yet, im sure its possible to figure out...mathmaticians are really good at this kinda stuff, just take a bunch of samples...figure the results for them, and then they calculate an equation for it ...play with numbers ect...
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JGoldman10
 
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Reply Mon 12 Mar, 2007 09:07 pm
Hello Dedshaw. Thank you for responding. Can sound waves be modified to affect segmented objects?
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Chumly
 
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Reply Mon 12 Mar, 2007 09:22 pm
Re: Sound waves and resonant frequency
Simply because an object is oscillating at its resonant frequency from "sound waves that are at the same frenquency as the resonat frequncy of the object" is no reason to presuppose that it must "shatter and implode" regardless of the shape of that object.
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JGoldman10
 
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Reply Wed 14 Mar, 2007 10:34 pm
How would you modify sound waves so that segmented objects can be impacted?
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Chumly
 
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Reply Thu 15 Mar, 2007 02:08 am
Do you mean segmented as in discrete segments?

Do you mean impacted as in break apart? If so I've already informed you that simply because an object is oscillating at its resonant frequency from "sound waves that are at the same frequency as the resonant frequency of the object" is no reason to presuppose that it must "shatter and implode" regardless of the shape of that object.
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Phoenix32890
 
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Reply Thu 15 Mar, 2007 03:42 am
There is a very simple equation that will allow you to determine a resonant frequency:

http://www.ham-radio.com/lc.html

You might want to take a look at this:

http://www.ibiblio.org/obp/electricCircuits/AC/AC_6.html
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Chumly
 
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Reply Thu 15 Mar, 2007 03:46 am
JGoldman10 is asking about mechanical resonances, not the RLC circuits you refer to. In actuality though, I am not sure precisely what he wants to know, and I don't think he does either, what the hell, it adds to the fun!
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Phoenix32890
 
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Reply Thu 15 Mar, 2007 03:48 am
Years ago, we had a very powerful hi-fi system. The room that it was in was about 30' long. At the far end of the room we had a pinball machine. We had one record that had some extremely low frequencies on it. The first time that we played it, the glass on the top of the pinball machine started to resonate.

I quickly put a heavy book on it, which stopped the resonating. I would suspect though, that if that particular frequency were constant, the glass might have eventually shattered.
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Chumly
 
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Reply Thu 15 Mar, 2007 03:55 am
RLC = resistive inductive capacitive

Big capacitors are used to correct lagging power factor on inductive loads. Most industrial / commercial / residential loads are resistive-inductive.
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Chumly
 
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Reply Thu 15 Mar, 2007 04:02 am
I've heard (but not confirmed) that Tesla believed the correct application of low resonant frequencies (even when the power output was rather small) could bring down whole buildings or perhaps even destroy the earth.

MythBusters had a show where they presumably dispelled Tesla's claims for an earthquake machine.

In my mind, there are two factors that argue against an earthquake machine, one is the natural damping of large bodies, and the other is the need for a huge amount of stored energy as supplied by the modest source.

Glass OTOH is a rather more coherent and brittle substance.
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JGoldman10
 
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Reply Thu 15 Mar, 2007 05:47 pm
DO you know how wine glasses have different frequencies at which they oscilliate, and when someone sings a musical note thats at the same frequency as the oscillation, or resonant/resonaning frequency of the wine glass? the glass breaks and shatters.

What I'm asking is how can I determine at what frequency will a sound wave cause any object to implode? ANd how can I modify sound waves sound that a segmented object will implode, since sound waves don't resonate well in a segmented object?
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ebrown p
 
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Reply Thu 15 Mar, 2007 06:06 pm
Tesla was a brilliant physicist-- but somehow his work got translated and distorted to make him into some kind of conspiracy spawning demi-god. About 10% of the things said about Tesla are things he actually said or did.

Take anything you hear about Tesla with a huge grain of salt.

Now for the physics...

There are two basic properties of a simple sound waves... frequency and amplitude. The amplitude is common known as the sounds volume.

If you want a sound wave to destroy things.... the easiest way to do this is to simply turn up the volume.

Another way to think about this is to think about what a sound wave is made up of-- pressure. As a sound wave passes you... it is an area of low pressure followed by an area of high pressure. If the amplitude (volume) is very high... then it will be an area of very low pressure followed by an aree of very high pressure.

If the low pressure areas are low enough... and the high pressure area is high enough... it will cause glass to break or buildings to crumble or people to die.

The good news (I hope you think this is good news) is that this takes an extremely high amount of energy and is thus not a very easy thing to do.

Now the other factor in a simple sound wave is frequency.

Because objects oscillate, a the amplitude needed to break an object is lower at its resonant frequency.... but the amplitude is still the deciding factor - you are just lowering the threshold.

The amount of oscillation, and the behavior of objects at resonance, is a very interesting study. It gets very quickly into advanced math (differential calculous) especially when you are dealing with compound objects.

Calulating the resonate frequency of simple objects (like regular glass tubes) is an interest exercise that is done in every high school physics classroom. In this case, the tubes will amplify the sound when the frequency is correct. This is a very interesting effect (but not dangerous since the volume is nowhere near enough even at resonance to shatter the tubes).

Of course the nature of the material an object is made out of also impacts how easy it is to break. Glass is probably easier than stone which is probably easier than stone.

Damping oscillations is also a very important and interesting problem. This technique uses mathematics to make cars smoother and to stop rattles from occuring.

But I am sorry to report... if you had a way to generate enough energy for a truely damaging sound wave... there are a lot simpler and more efficient ways to use this energy destructively if that is what you want to do.
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Chumly
 
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Reply Thu 15 Mar, 2007 06:07 pm
One way to hopefully discover the resonant frequencies of an object would be to whack it with a hammer while recording the resulting sound waves with a microphone and frequency analyzer. It is possible however that the resonant frequencies of the object could be above or below what the microphone and frequency analyzer can process.

Do you have a tuning fork? This would be a good first step into discovering and using resonant frequencies.
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ebrown p
 
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Reply Thu 15 Mar, 2007 06:09 pm
A frequency generator is easier than a frequency analyzer. When I was teaching... we simply moved the knob on the generator to change the frequency while listening for the telltale increase in volume.
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Chumly
 
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Reply Thu 15 Mar, 2007 06:19 pm
A frequency analyzer isn't a big deal anymore they run on a laptop with a refence mic or rack mount devices. Pro-audio stuff like this is common now.

I agree with what you say but I take exception that "the tubes will amplify the sound when the frequency is correct" because to me the term amplicafation implies a higher net signal power output than the total gross signal power input. Again coming from the perspective of pro-audio.
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ebrown p
 
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Reply Thu 15 Mar, 2007 06:22 pm
Chumly wrote:
A frequency analyzer isn’t a big deal anymore they run on a laptop with a refence mic or rack mount devices. Pro-audio stuff like this is common now.

I agree with what you say but I take exception that “the tubes will amplify the sound when the frequency is correct” because to me the term amplicafation implies a higher net power output than the total gross signal input. Again coming from the perspective of pro-audio.


Thanks for the correction and what I said was incorrect (and not what I was trying to say).

Let's just say... the tubes will make the sound seem to be louder.
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georgeob1
 
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Reply Thu 15 Mar, 2007 06:35 pm
There is another factor so far missing from this discussion. That is the energy dissipating characteristic of the object/oscillation in question. Any system subject to constant energy input at or near its resonant frequency - even at low amplitudes - will experience a sustained in phase input of oscillatory energy. If the rate of energy addition is greater than the rate of dissipation then eventually something will happen -- either structural failure or the excitation of other oscillatory modes which themselves increase the dissipation rate.

"Flutter" is the term used to describe the oscillations of the wing and structure of an aircraft in response to turbulent gusts and disturbances. Static ground-level) flutter tests generally involve a very small motor driving a piston at a wingtip, pushing the wingtip up and down (a few inches) at different frequencies. The only way the aircraft structure can dissipate this energy is through structural heat, and by pushing the surrounding air around through its movements. I have seen entire tail sections of large airliners oscillating left and right five feet or more in response to a 1/2 HP motor oscillating the wingtip. The design trick is to give the entire structure a resonant frequency very far from what it will experience in real use.
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Chumly
 
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Reply Thu 15 Mar, 2007 06:41 pm
ebrown_p,
Right! We surf into the topic of psychoacoustics, a topic I find fascinating and one I would like to learn more about.

georgeob1,
When you say "If the rate of energy addition is greater than the rate of dissipation then eventually something will happen -- either structural failure or the excitation of other oscillatory modes which themselves increase the dissipation rate."

Another possibility is simply an increase in retained heat energy until the input energy equals the output energy i.e. the temp will go up until thermal equilibrium. I am not sure "the excitation of other oscillatory modes which themselves increase the dissipation rate." is an essential component of this in every case however. For example a highly damped structure may not be capable of these secondary harmonics.

I understand exactly what you are talking about when you say "The design trick is to give the entire structure a resonant frequency very far from what it will experience in real use" as the exact same principles are applied to pro-audio gear to either enhance or damp depending on the application
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georgeob1
 
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Reply Thu 15 Mar, 2007 07:00 pm
Chumly wrote:
When you say "If the rate of energy addition is greater than the rate of dissipation then eventually something will happen -- either structural failure or the excitation of other oscillatory modes which themselves increase the dissipation rate."

Another possibility is simply an increase in retained heat energy until the input energy equals the output energy i.e. the temp will go up until thermal equilibrium. I am not sure "the excitation of other oscillatory modes which themselves increase the dissipation rate." is an essential component of this in every case however.


Correct. I should have written ".... rate of energy addition is greater than the rate of dissipation from the oscillatory motion then eventually...."

The flexing of a monocoque aircraft structure doesn't generate much heat.

However the underlying principle has general applicability -- consider an underdamped oscillatory electronic circuit.
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