Passive Intensity-Dependent Acoustic Transmission (Power Limiter) 2

[joedunai]

I am looking for a solution that would limit the transmitted sound intensity/power in a passive way. There actually already exists such natural limiter caused by the nonlinear sound propagation at high intensities in waveguides. But the required intensity to reach that limit is very high.

I would like to achieve such limitation at lower arbitrary sound intensities. The required component would transmit the input sound waves with little attenuation that are below a chosen threshold intensity, but limit the transmitted signal to a constant max. intensity, if the input signal’s intensity is above the threshold. The component does not have to be an existing commercial component. I am just looking for research subjects that have found phenomena and materials, which could be used for the design of the component.

It is also possible to do this by electronic means, but for my purpose the limiter should be a passive component, as simple as possible.

If anybody has any suggestions about existing solutions, or phenomena that could be exploited to design such power limiter, please let me know.

Thanks in advance.

 

[MikeHalloran]

A few of the large yachts I have worked on have a ventilation grillage between the engine room and an adjacent compartment. The grillage allows free flow of air, and appears to stop sound entirely, or damn near.

The grillage comprises multiple parallel slots an inch or two wide, and a foot or so tall, and having an axial length through the bulkhead of a foot or so. The web between slots is an inch or two thick. The whole thing appears to be made of, or lined with, the sort of black foam that you find in acoustic installations.

I assume someone is making them commercially, but I have no idea who, or what the product is called.


Mike Halloran
Pembroke Pines, FL, USA

 

[joedunai]

Thanks Mike for the suggestion. What you are describing is actually a type of silencer that reflects/absorbs sound regardless of its intensity. Although this is not what I was looking for originally, I would be interested to see a drawing of your silencer with dimensions, if there is one.

The component I am looking for would for example transmit a specific single frequency sound without attenuation, say up to 100 dB (input=100 dB, output~100 dB). When the input sound intensity goes above this threshold to say 120 dB then it would attenuate the signal to the maximum output of 100 dB (input=120 dB, output~100 dB). These numbers are arbitrary; it should be possible to choose a specific threshold input intensity where the attenuation kicks in.

 

[GregLocock]

Imagine an impermeable diaphragm with limit stops on its motion. At low amplitudes the diaphragm is free to vibrate and so transmit the incoming pressure waves to the other side. At high amplitudes it will smash into its limit stops and generate lost of higher frequencies, but the fundamental would be attenuated.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Strong]

Joe,

I have been working in acoustics and industrial noise control since 1968. I wish you all the best for your venture/adventure.

Walt

 

[GregLocock]

Another approach would be to insert a tube, sealed at each end with light diaphragms, into the duct. Extract most of the air from the sealed chamber. It will be able to transmit low amplitude signals but not high amplitude ones, as the molecules won't be able to move fast enough.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

Star for Greg Locock. Certainly worth investigating.


Mike Halloran
Pembroke Pines, FL, USA

 

[joedunai]

Good idea Greg! I have thought of that one already with a slight difference. Membranes transmit sound nonlinearly at high intensities and naturally limit the transmitted sound intensity. Take a look at Figure 2-10, Page 92 in the book “Nonlinear Acoustics” by Robert T. Beyer which can be downloaded from:

http://www.dtic.mil/dtic/tr/fulltext/u2/a098556.pdf

The curve of membrane displacement vs. incident sound pressure level curves up to vertical when the input SPL reaches the threshold, which is characteristic for that specific membrane. This means that a properly designed resonant membrane alone would do the trick.

There are only two problems with this solution. The major one is that the membrane breaks when the incident SPL reaches the threshold. This could be prevented by placing a grid of rigid (perhaps covered with rubber) stopper blades (parallel with the walls of the waveguide). But there still remains the second problem, which is the required high precision manufacturing of the membrane and stopper at lower SPL thresholds.

For example the membrane represented by the top curve on the above figure limits and breaks at around 0.15 inches displacement. This is fine and easy to manufacture the stopper in this case, but the SPL is around 145 dB which is very high. But if I would want to limit the threshold to say 90 dB instead, then the stopper would have to be placed at about 0.003 inched away from the membrane. So we would have to implement a manufacturing precision in the range of micrometers, which is quite expensive, especially if we want the threshold to be manually adjustable.

This solution is still on the table for the case nothing better would be found, but we are still looking for some more robust alternative. Something like using a nonlinear metamaterial, or any other solution that would be cheaper to produce.

Thanks Greg for the great suggestion, and thanks to Walt for the encouragement.

 

[joedunai]

I have just seen your second reply Greg after submitting my last post.

“Another approach would be to insert a tube, sealed at each end with light diaphragms, into the duct. Extract most of the air from the sealed chamber. It will be able to transmit low amplitude signals but not high amplitude ones, as the molecules won't be able to move fast enough.”

This is an interesting approach, but this closed chamber with partial vacuum would reflect most of the sound even at low intensities. Or am I missing something? We can add a quarter wave transformer to match the impedances though, but I am still unsure why would it limit the transmitted SPL. Could you explain this in more detail? Will give it some more thought.

 

[joedunai]

Greg, did you mean that the SPL would be limited by the nonlinear shock wave formation that would manifest at lower SPL at lower ambient pressure?

 

[GregLocock]

Yes, to your last. I don't see a way of getting a non linear broadband transfer function without any attenuation at all, but I must admit I'm not putting any huge amount of effort into thinking about it. Perhaps if you add a passive amplifier you could do it for a small frequency range, as you suggest.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[joedunai]

Greg, the device has to work only at one single frequency. It does not need to have a broadband transfer function without attenuation. It is OK to have a small attenuation at low intensities something like below 10%, while having a nonlinear increase of attenuation much greater than 10% at high intensities.

What kind of passive amplifier are you suggesting? Thanks for your comments.

 

[GregLocock]

An organ pipe?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?