Re: wind instrument +- filtering of broadband noise

[Leigh Palmer <palmer@SFU.CA>]

> At 03:45 PM 12/8/00 -0500, Chuck Britton wrote:
>  > ... resonances that are driven by sympathetic
> > vibrations and those that are driven by more esoteric inputs such as
> > 'delta function' hammer blows or chaotic turbulence of wind
> > instruments.
>
> That looks like yet another misconception.
>
> In fact, chaotic turbulence is a broadband noise source.  It would be quite
> misleading to think that a wind instrument is a resonant filter applied to
> a broadband noise source.  The musician does not _hiss_ into a flute or a
> trumpet.
>
> In a flute, in normal operation, mode-locking occurs.  That means that one
> frequency "wins" while other frequencies that could have occurred (and
> would have occurred with broadband excitation) do not occur.  One
> observable consequence of this is that the higher harmonics are
> phase-locked to the fundamental;  you could not possibly explain this in
> terms of resonant filtering of a broadband source.

A similar phenomenon occurs when one blows a note on a bottle*. The
blower adjusts his embouchure to make the whole system - bottle, lips,
oral cavity, etc. - oscillate at the resonant frequency. This
phenomenon is called "centering" by musicians and it corresponds to
finding an acoustical impedance maximum (as a function of frequency).
At this embouchure "setting" one can elicit a nice note from a beer
bottle with only the slightest volume flow. A similar phenomenon is
seen in the very different excitation of a horn, a lip read
instrument. In this case the lips actually open and close at the
frequency of the impedance maximum when properly adjusted. The scale
of notes accessible on, say, a bugle are successive acoustical
impedance maxima. A much different scale is found for any longer horn.
The "natural horn", an ancestor of the French horn, illustrates well
why "improvements" were needed in adjusting the frequencies of these
maxima.

All of these are examples, in John's apt term, of mode locking.

Leigh

*The same is true for a flute, of course, but it is one of several
instruments that have defeated me in my lifetime. When I play a beer
bottle no one laughs *at* me!