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#55 ttonon

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Posted 20 June 2005 - 07:09 PM

I don't know which model is the closest to a real concertina reed... (snip)


Hi Clive,
If you investigate this topic, I think you'll conclude that the free reed tongue vibrates only at its fundamental bending mode (with or without weighted tips), with next to nil contribution from higher modes, measurable if at all (by sensitive electronic instruments) only at higher than normal blowing pressures, and the same goes for twisting (torsional) modes. All these modes, by the way, are sinusoidal. If there were significant contributions from more than one mode, the result would be noise, since their frequencies are not, as a rule, close enough to integer multiples of each other. Thus, one cannot expect to encounter these modes in any instrument that produces acceptable musical tones.

Best regards,
Tom
http://bluesbox.biz

Edited by ttonon, 20 June 2005 - 10:10 PM.


#56 JimLucas

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Posted 21 June 2005 - 02:18 AM

I don't know which model is the closest to a real concertina reed... (snip)

If you investigate this topic, I think you'll conclude that the free reed tongue vibrates only at its fundamental bending mode...
...
If there were significant contributions from more than one mode, the result would be noise, since their frequencies are not, as a rule, close enough to integer multiples of each other. Thus, one cannot expect to encounter these modes in any instrument that produces acceptable musical tones.

<{POST_SNAPBACK}>

Tom, are you claiming that (bell-shaped) bells don't produce "acceptable musical tones"?

(I was going to wait until I had read Tom's PICA paper before commenting further, but since others have not done so, I'm returning to the "fray".)

I don't think that anyone here is claiming that the reed itself vibrates at any but its fundamental bending frequency. But the frequency spectrum that is responsible for the timbre doesn't come from that vibration. It comes from the Fourier decomposition into sinusoidal frequencies of the very non-sinusoidal pressure wave generated when the vibrating reed chops the flowing airstream, and by the differential resonance and absorption of those frequencies in the reed cavity. (Probably also from differential transmission/absorption outside the cavity -- e.g., through the end, -- but I won't try to treat that here).

The integer-multiples criterion may be valid for chords -- i.e., where the strengths of the different frequencies are of similar magnitudes, -- but modes which are much weaker than the fundamental, though still perceptible, should be able to add "flavor" without masking/disrupting the fundamental pitch or even causing an unpleasant sensation, no matter what their ratio to the fundamental. I suspect that descriptions of the timbre of different instruments as more or less "harsh", "brassy", "rich", "mellow", etc. have more to do with the relations among these relatively weaker higher frequencies than any clash or harmony between them and the fundamental. The fundamental questions of timbre are then: What are the frequencies (or frequency ratios) of these "harmonics", and what are their (relative) intensities?

#57 Clive Thorne

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Posted 21 June 2005 - 05:31 AM

I don't know which model is the closest to a real concertina reed... (snip)


Hi Clive,
If you investigate this topic, I think you'll conclude that the free reed tongue vibrates only at its fundamental bending mode (with or without weighted tips), with next to nil contribution from higher modes

<{POST_SNAPBACK}>



Tom,

What you say agrees with what my gut feel told me, but I didn't like to state is as a fact based only on my gut feel. If anything adding weight at the end would push it further to the dominant fundamental mode but you're probably right that this the dominant mode in an unweighted reed anyway.

Clive

#58 tony

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Posted 22 June 2005 - 10:22 AM

Tom,

Thank you for your contribution, I found it very informative and most helpful.

I am inspired to experiment further.

#59 tony

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Posted 29 June 2005 - 11:49 AM

So, I thought I would have a good look at the baffles to see if that could be the cause of my problem.

The baffles are made of linen and are in two parts, leaving space to get at the screws to the handles I expect. They are mounted on small pieces of cork (the baffles, that is). They seem to have collapsed, or distorted, somewhat in places.

I removed the baffles to see what difference it would make. I expected some change in timbre and volume but was pleasantly surprised to find the timbre on the treble side now matched the timbre on the bass side very closely.

I had intended to replace the baffles, possibly with leather ones, but having played it this way for a few days I am beginning to think in terms of a 'brighter' sound rather than a 'harsher' one. I will probably leave it this way and hope my wife doesn't complain of the increased volume. She hasn't yet.




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