Re: Overtones


Subject: Re: Overtones
From: Richard Wentk (richard@skydancer.com)
Date: Mon Jun 28 2004 - 14:45:37 EDT


At 11:16 28/06/2004 -0700, you wrote:
>Hi,
>I was wondering if you could elaborate for me:
>When you say:
>"Instruments with more than one sound generator (e.g. anything with multiple
> >strings, or any ensemble) produce complex phase cancellation effects in the
> >overtone structure which also affect the perceived sound."
>
>do you mean that if more than one note is playing(on purpose or by
>accident) then there will be phase cancellation?

I mean anything with more than one physical noise source (e.g. the
different strings in a guitar, piano, violin) OR any collection of
instruments playing in an ensemble.

For ensemble playing you get a simple chorus effect where minor mistunings
create movement in the overtone structure.

With physical strings and some other instruments there's also a feedback
process via physical resonance which makes the overtone structure more
complicated than for a single string. The sound energy isn't just created
by the strings, it's also picked up and bounced around among them. If
there's a soundboard, that will also contribute to the process. That
creates some very complex and interesting variations in the overtone spectrum.

This is why sampled pianos always sound so dead. You can hear it's a piano,
kind of, but there's none of the movement in the sound you'd get from a
real piano.

If you want to hear this to best effect, either make any noise into a piano
with the damper held down, or find a hammer dulcimer or zither - which has
tens of strings and no damping of any sort.

>and also when you say that pitch is important too, did you mean the note
>being played, as in the frequency of the note?

Yes. Every note will have a slightly different overtone structure, because
instrument bodies tend to act like resonant filters, damping some frequency
ranges and enhancing others.

In physical modelling synthesis you model this by using one process to
create an excitation signal, and then another to simulate the resonance of
a physical body. At the risk of oversimplifying, it's a bit like putting
the raw sound through a graphic EQ.

It's very easy to hear this. Go find a music store and try out a selection
of acoustic guitars. Some will be quite bass heavy, otherwise will favour
the mid more, some will have more bite in the treble. That's all down to
the resonant properties of the body, which in turn depend on the wood used,
the thickness of the soundboard, the finish and also the shape and size.
The body acts like an acoustic EQ, emphasising some of the overtones from
the raw string sound more than others.

Master luthiers also understand how to control the amount of string
resonance to make guitars that really sing.

Richard



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