Stress waves (was violin bridge)

[Richard Brekne]

Phillip L Ford wrote:

> On Sun, 6 Jan 2002 10:01:56
>  John Delacour wrote:
>
> > Robin's latest post brings us back
> >to the essential starting point, which is the termination of the
> >string at the bridge, where the transfer of energy begins, where the
> >inital stress is set up.  I want to get a clear idea of what happens
> >here.  At the moment I see a molecular disturbance occurring here
> >which radiates into the bridge; and I use the word 'radiate' in a
> >strict sense, in that the pressure waves travel outwards from the
> >source as the rays of the sun or the spokes of a wheel from the hub.
> >These waves travel just as much along the bridge as down through it,
> >though not necessarily with exactly the same speed or amplitude,
> >since various factors will modify these.  Before we even get to the
> >soundboard, there is plenty to consider.
> >
> >
> >JD
> >
> I haven't read Robin's post yet but I will.  The propogation of stress
> waves, compression waves, or sound waves (however you choose to
> call them) from the point of contact between string and bridge seems
> likely.  However, I don't see why that would prevent the string(s) from
> physically moving the bridge and soundboard which would move air
> which we would hear.  As I see it both things can be going on at once.
> It doesn't have to be either/or.

I think at the moment I would agree with you Phil. I am not sure I see the
conflict either.  It appears to me that surface waves are very transverse like.
I have been reading a bit in Benades book and has me pondering the different
modes for vibration for a 2 dimensional plate... how that fits in terms of
simple transverse vibration visa vi this transverse like action of the surface
componet of compression waves. Stephen said right out that these two were
different puppei and I'd like to hear more.

>
> But I'm still mystified as to how these compression waves get
> transformed into transverse vibration of the soundboard (I think from
> what I have read in these posts that everyone is agreed that transverse
> vibration of the soundboard has to take place to move air so that we can
> hear the sound).

Im not sure that they do. Surface waves seem certainly transverse like, and
certainly could produce sound. How much is another discussion.

> Why wouldn't the compression wave remain a compression
> wave?  Why wouldn't it travel through the bridge, into the board, to the rim
> and back all the while remaining a compression wave?

I would think that it does, along with whatever else it does, for example
intereacting with different medium types or different densities in the same
type.

>
> I've ordered a couple of books on wave propagation.  I'll see what the
> people who are supposed to be authorities have to say on the subject.

Well, have fun. Its thick reading... for those of us without lots of practice on
this level of math and physics anyways. But I trod on through and pick up on
things little by little.  Benades book is the easiest of three best I have to
relate to so I would recommend this. If you find any other good reading please
give me a wink.

>
>
> Phil F




--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no