Sound Waves Physics

[Richard Brekne]

Subject:
        NEWTON BBS Piano Sound ---Physics



Hi List:

I dont have any URL's to show anyone, but I posed the
following question to the NEWTON Physics Website, a
government supported site that you can write physics
questions in and get replies from bonified educated physics
doctors and masters. Don, you mentioned that the most simple
answer was probably the right one. Simple answers to complex
questions always put me on guard. Ron, JD, and Robin, I have
a few comments, questions and observations about some of the
posts written the last couple days, but I will take them in
a seperate post later on. Just wanted to send this off for
your edification. Seems pretty clear to me he makes a strong
point out of compression (sound) waves being transfered all
over the place as well as going good for some of the effects
of transverse waves Ron and Del talk about.  His answer
points (to me) in the direction of both the lists present
theories being involved in the production of sound waves
from the instrument.

RicB


> > >    Question -   When the hammer strikes a string in a
piano, the string
> > > is set in vibration which in turn has an effect on the
bridge attached to
> > > the sound board. Is this affect such that the string
physically moves the
> > > bridge and then in turn the bridge moves the sound
board, or is it such
> > > that there is a transfer of energy in the form of
internal compression
> > > waves through the bridge and delivered to the sound
board which sets up
> > > the movement in the sound board which in turn moves
the bridge? Also,
> > > just how much air displacement is necessary in the
form of a compression
> > > wave to become audible ? Thank you.


Nathan A. Unterman wrote:

>This is a complex question. When the hammer strikes a piano
string, both
>transverse and longitudinal waves are induced, both the
fundamental
>frequency and overtones of varying intensity that depend
upon the
>construction of the instrument. These vibrations are
transferred to the
>bridge and to the rest of the piano. The transverse waves,
where the motion
>of the string is perpendicular to the string can also be
transferred to the
>piano frame directly, especially in a "grand piano" with
its large area
>resonator. To indicate how complicated this whole process
is, the piano
>transfers its sound waves to the floor upon which the
instrument is placed,
>so the "quality" of the sound will depend upon the
composition of the floor.
>A piano on a carpet produces sounds different than the same
instrument on a
>bare wood floor.
>
>The longitudinal waves have an amplitude of the order of
millimeters, or
>fractions thereof. The intensity of the sound is
proportional to the square
>of the displacement, and different ears hear sounds of
different frequencies
>and intensities. The commonly used scale for measuring
sound intensity is
>the logarithmic "decibel" scale. One decibel is taken to be
the onset of
>hearing by humans, but that is quite variable.
>
>The amplitude of an audible sound depends upon the
sensitivity of the ear,
>but also on the medium transmitting the sound. A classic
example is a rail
>road. It is possible to hear an oncoming train a much
further distance away
>by placing ones ear on the track, rather than listening for
the train sound
>traveling through air alone.
>
>So you see, your inquiry is really quite complicated. Sorry
I cannot give a
>simple answer.
>
>Vince Calder





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