Re: [Phys-l] Plastics for soundboards

[curtis osterhoudt <flutzpah@yahoo.com>]

Thank you for that link, bc. It looks to be very helpful for our purposes! Unfortunately, I've done much of the same nodal analysis, and have asked for -- and received, from the box-maker -- some sound files which indicate that the _real_ boxes, what with their mixed boundary conditions and strange sound posts mounted in strange places, really screw up the otherwise pretty resonance patterns. 

   Thank you, too, for the insights. I've asked the box maker to try some experiments with the CFRPs; I agree that the volume density may be a problem, though with a thin panel as a resonator, the areal density may be just fine. There _will_ be some definite changes due to the acoustic impedance mismatches, but we're not sure yet whether the changes will be favorable or un-. 

    From what the makers have shown me, they really *do* want resonances, though of course they're going to have to put up with either very sharp resonances at some frequencies and little sound at others, or a broadband resonance (relatively low Q). Remember that at very, very high Qs, the resonator will lose only a very tiny fraction of its energy per cycle, and thus be almost inaudible. I've done some preliminary measurements on a different oscillator in and out of a vacuum chamber; the Q changes by a factor of several hundred due to damping from the air. They're wanting notes to last for quite a while, which of course means that a given note can't be very loud (conservation of the mechanical energy which is released from the vibration of a tine). At a typical frequency of 1 kHz, they want a Q of something like 3000 (this is roughly what I found from the sound files they sent); I hope to keep Q very roughly constant at all of their frequencies, with
 maybe a few twiddles here and there, depending on what their "expert" wants for his compositions. It may only be possible with some tricky engineering.

               Best wishes, 
                               C.O.



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Down with categorical imperative!
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________________________________
From: Bernard Cleyet <bernardcleyet@redshift.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, March 17, 2009 5:30:41 PM
Subject: Re: [Phys-l] Plastics for soundboards

I disagree!  material for sound boards is very much a teaching  
subject -- acoustics, modulus, density, hysteric dissipation, etc.  
all physical subjects.  If not directly in the class room, then as  
projects.

I found your second link fascinating.  My only experience w/ sound  
boards is building a coupala Zuckerman kit harpsichords and would,  
therefore, think  a thin plywood would be suitable, what I suppose is  
already used.  From your reference, I think CFRPs are too dense, and  
too stiff;  would require very thin sheet, perhaps just what desired?

"... high enough Q to resonate, but not so high that no sound can be  
audible; ..."

  I think you have this backwards -- Resonance not wanted, but also  
not so heavily damped there is no vibration and, therefore, no sound  
radiated.  i.e. a single digit Q.

In searching I found this fascinating article:

http://www.speech.kth.se/prod/publications/files/qpsr/ 
1992/1992_33_4_087-096.pdf

bc


On 2009, Mar 17, , at 10:05, curtis osterhoudt wrote:

> Dear List,
>
>    I have a side-project going in which I am a consultant to a  
> maker of music boxes. I have considerable experience in acoustics,  
> and some in materials, but would appreciate any help which might be  
> given here (and I know that this forum, despite its wonderful  
> tangents, really isn't meant for such questions; I'd appreciate any  
> off-list messages or pointers to more appropriate forums).
>
>    One of the questions a particular manufacturer has is about  
> appropriate materials for sounding boards in their boxes (the  
> bottom of music boxes usually acts as a sounding board). Though  
> various woods give good results (high enough Q to resonate, but not  
> so high that no sound can be audible; good looks; long lifetimes;  
> ease of machining), a question has been raised as to certain  
> plastics. Now, I know that plastics, almost by definition, are not  
> going to be entirely suitable for such a use (they're _plastic_,  
> can become brittle with age, light exposure, and ozone, warp under  
> pressure, etc.). However, CFRPs seem almost ideal from my viewpoint  
> (except for, perhaps, looks and sunlight resistance) http:// 
> en.wikipedia.org/wiki/Carbon_fiber_reinforced_plastic has a few  
> details, as does this article: http:// 
> chambermusictoday.blogspot.com/2009/01/bow-kinematics-carbon-fiber- 
> vs.html .
>     Does anyone here have experience with materials who could  
> upload their brain to mine?
>
>                    Best wishes,
>                                Curtis O.
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