Material most likely to transmit vibrations

Dear backon:

Watch how you bond the transducer, as this can give spurious results, and provide high losses. A crimped design mght be superior. SIlver solder will be better than epoxy, if the transducer can take a little heat (and you base material can wet silver solder). Metals have the lowest "absorption coefficient". The hardest metals usually have the lowest of the absorption coefficients. Thickness represents the ability to absorb, and geometry to deflect the signal, so keep the thickness short. I'd pick a metal that can be implanted. You'll know those better than me.

And engineers occasionally get to make noise too.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)
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I'm in internal medicine and for research at the medical school (we want to model coronary flow) we need to epoxy a loudspeaker or ultrasonic transducer to the outside of a metal or plastic trough. I realize that in your field you want to dampen vibrations: we need the opposite. We are looking for a material that will transmit virtually all the vibrations from the transducer into the liquid in the trough. Should we use metal, plastic ? And if metal or plastic, which type ?

Thanks in advance.

Josh

Reply to
backon

I work for a company that manufactures ultrasonic handpieces as well as ultrasonic bubble detectors. Metal will transmit more than plastic, but we use both depending upon the application. For metal handpieces we use both titanium alloy and stainless steel. As far as plastics, we use ABS, polycarbonate, and some others. The really critical thing for you will be coupling the ultrasonic transducer to the wall of the trough in order to get maximum transmission. Any air gap or poor coupling between surfaces will reduce your signal.

Reply to
ms

This is a stock concern of sailors wanting to apply a depth sounder transducer without making a hole in the hull. There are various methods adopted there. While exploring the optimal position for a through hull transducer, people sometimes use a water balloon as the transfer medium conforming the transducer to the hull shape. Another approach is the dam wall of any material at hand which will trap a small pool of oil into which the transducer dips. People report favorable results with silicone rubber, even polyester resin or epoxy resin as the interface in permanent mountings. A metal interface would be somewhat stiff to pass the acoustic signal at all well, I'd think.

Brian Whatcott Altus OK

Reply to
Brian Whatcott

I should have also said that we usually use epoxy to bond the transducer to the substrate. DP-190 from 3M works well--it is what we use in many bubble detector sensors. But most any epoxy (even the 5 minute kind from the hardware store) will provide adequate results, depending on the nature of the application. I have used 5-minute epoxy quite often for prototyping.

Reply to
ms

Thanks !

Josh

Reply to
backon

Thanks !!

Josh

Reply to
backon

An engineering approach to the problem involves the design of a "1/4-wave" transformer that smoothly makes the transition between the acoustic impedance of the transducer (very high) to the acoustic impedance of the driven medium (liquid in trough). As the medical ultrasound people understand, this makes a tremendous difference. It's analogous to the anti-reflection coating on camera lenses. Anyway, a "half-fast" approach involves putting materials of intermediate density between the transducer and the driven medium. As mentioned by others, the worst choice is air. Loaded epoxies might be a good choice. As the name suggests, the thickness of the intermediate layer is ideally

1/4-wavelength, as measured in the intermediate material. Paul Mathews
Reply to
Paul Mathews

Dear backon:

It refers to the amount of material (thickness in conduction path), the conduction speed (speed of sound) of the material, and the frequency you intend driving it at.

thickness ~= 1/4 * conduction_speed / frequency

... based on the context supplied.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

Our hospital has a biomedical engineering unit. I assume they would know what a 1/4 wave transformer is. Just in case: could you elaborate?

Thanks

Josh

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Reply to
backon

Reflections at the boundary between different materials are minimized by the incorporation of some kind of more gradual transition. (The general term for such a structure is 'impedance transformer'.) There are many ways to achieve this transformer action. For example, 'horn' structures are often used. However, a particularly simple kind of transformer that works well at a single frequency is the 'quarter wave' type, described briefly in the other posts. Make one this way:

1) choose suitable material of intermediate density or known intermediate acoustic impedance at the frequency of operation 2) determine speed of sound in the material and calculate thickness required for 1/4 wave 3) build using calculated thickness

If your layer is more than 1/4 wave thick, it will still provide substantial benefits, just not optimal.

Other examples of impedance transformers include: 'horn' structures including 'bells' on wind instruments, sloping beaches, and electrical baluns.

Paul Mathews

Reply to
Paul Mathews

Backon, Maybe you could describe your application in a little more detail for us. While the 1/4 wavelength layer thickness is important for high power ultrasonics (e.g., cataract aspiration handpieces or localized energy transfer to tissues), it is less critical for a more forgiving application (such as air bubble detection in liquid).

Reply to
ms

Thanks for all your help. It was greatly appreciated.

Josh

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Reply to
backon

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