Klarinet Archive - Posting 000168.txt from 2010/08

From: "Keith Bowen" <keith.bowen@-----.com>
Subj: Re: [kl] About clarinet acoustics
Date: Mon, 16 Aug 2010 17:40:25 -0400

Tony

There's pressure measurements inside the bore, but they need fancy
equipment.

You are right that Diego's calculations are only valid with a cylindrical
pipe in which the sound speed is constant.

The clarinet of course plays notes other than low E :-). The method I chose
for the determination of the pitch of a bass clarinet that I could not play,
was to pick a note far enough away from the bell that the bore is still
cylindrical (on a bassoon form or German bore bass clarinet this is easy; on
a French bore one would need to go up to Bb or C in the chalumeau). One
measures this length, and also the bore diameter, tone hole diameter and
wall thickness of the next open hole, and calculates the 'end effect' of the
open hole (how much longer the tube effectively is, compared with the actual
length of the tube). The end effect must be taken into account as it can be
more than the distance between tone holes. The correction formula is on page
450 of Benade (Fundamentals of Musical Acoustics) and is also given in my
masters' thesis that I sent you recently. Since here we are in a cylindrical
portion of the bore, the speed of sound is identical to that in open air.

So measuring the low E on a French clarinet won't help much.

Keith

-----Original Message-----
From: Tony Pay [mailto:tony.p@-----.org]
Sent: 16 August 2010 22:13
To: The Klarinet Mailing List
Subject: Re: [kl] About clarinet acoustics

On 16 Aug 2010, at 21:29, Jennifer Jones wrote:

> Mike McIntyre wrote to Tony Pay:
>
>> The only way out I can see is that the textbook "cylindrical bore"
>> is at best an approximation -- especially near the mouthpiece?
>> If the bore cross-section varies along its length, then that
>> shifts the node-antinode pattern, of course, as
>> with the extreme case of oboes and saxophones. If you tell me how the
>> cross-section varies I could think about this a bit more.

What you subsequently write, applied to mouthpieces -- and oboe reeds -- I
would say has only a small effect, which is why I didn't pursue it. (I
couldn't see that it would reduce 8cm very much.) The main thing about
oboes and saxophones is that the bore REMAINS conical as you continue down
the instrument.

So I should have read Mike's '?' more seriously. He doesn't have to do with
clarinets on a daily basis like us -- well, ME:-( -- you have proper jobs --
and so is not so immediately aware of the fact that the major departure from
cylindricality in a clarinet is at the bottom end.

I think I shall find tomorrow that a precise determination of the pitch of a
fingered low E without the bell gives a more plausible calculated position
for the top node. (I have a suitable app on my iPhone:-)

Of course, really we need another method for determining it. If I said, for
example, that I could remove the bottom flare with blutak and THEN do the
experiment, you might say, well, hang on a minute, that's not a CLARINET.
We'd need to know further that the postion of the top node doesn't change
much with such a modification.

The fact remains, if I understand things correctly now, that having
cylindricality is the only way that Diego's calculation method works.

What other methods are there for determining node positions?

Tony
--
Tony Pay
79 Southmoor Rd
Oxford OX2 6RE
tel/fax +44 1865 553339
mobile +44 7790 532980
tony.p@-----.org

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