Klarinet Archive - Posting 000115.txt from 2010/08

From: Diego Casadei <casadei.diego@-----.com>
Subj: Re: [kl] Location of antinodes of vibration in an air column
Date: Sat, 14 Aug 2010 15:46:44 -0400

Hemm... I must admit to have reached the limit of my understanding and
mastering of the English language. I'm not able to fully understand
most of the quoted message :-(

Anyway, I can try to make some additional remark. First, though
implicitly, I've always being thinking about pressure variations (never
about displacements). This is because the ear reacts to them, not to
the fluid displacement. I know that strings and water, the models for
wave propagation, are better described in terms of displacements, but
sound waves in air are best described in terms of pressure variations.

[BTW, in my reply to Richard's email my language was a bit imprecise,
and at the end it seems that I speak about variations in pressure and
(variations in) velocity. The second "variations" should not be
implicitly inserted. Don't know if this was unclear but reading it
again I had this doubt...]

Second, I never used the noun "antinode" in my life, before having found
it on the paper which was at the origin of my comments. I kept it in my
email hoping to make it easier to everybody to follow it... I should
have spoken about "relative amplitude maxima" instead.

Again, I never thought in terms of "displacement antinodes" (nor nodes):
I was thinking about pressure variations instead.

I really cannot understand how my sentences could be used to conclude
that "the pipe length is not only a half instead of a quarter
wavelength, but also _exactly_ a half wavelength." Perhaps you could
explain this? In any case, the correct unit is one quarter of the
wavelength, not one half.

The last comment is about the statement that the "reed can support a
helluva pressure amplitude, even if not quite maximal unless you bite
the reed down." This statement has nothing to do with acoustics.
First, sound waves describe the propagation of pressure _variations_,
not the pressure itself. Second, the pressure here is the air pressure,
not the static mechanical pressure exerted on the reed by biting it
(which is not transmitted to the air).

Anyway, my wording was indeed somewhat imprecise, though I made my best
to avoid this problem. Please continue to feel free to criticize it.

Cheers,
Diego

Tony Pay wrote:
> On 14 Aug 2010, at 03:40, Richard Sankovich wrote, in part:
>
>> Diego, thanks for your detailed explanation of sound production by vibrating strings and air columns. I must take issue, however, with your conclusion that there will be a vibration antinode at the energy source, or, in your words, a point of maximum amplitude.
>
> What Richard subsequently says fits in with what 'my guy':-) Michael McIntyre writes to me. (I wrote to him asking whether in his opinion the 8cm is theoretically sustainable, not having yet checked your actual experiment.)
>
> I quote:
>
>> ...havoc over elementary acoustics :-[ due largely to the usual failure to follow (forgive me)
>> lucidity principles, even though I see you're trying to set a good example, which of course no-one notices.
>>
>> I find the 8 cm plausible enough. I check the wavelength calculation even though I can't check the length of a B flat clarinet just now. The end effect adding
>>
>>> about half a centimeter outside the bell
>>
>> seems about right. But the following might repay a moment's attention, regarding bottom written E:
>>
>>> we have an antinode at the end of the bell,... also an antinode at the reed
>>
>> which is plainly wrong whether "antinode" means "pressure antinode" or "displacement antinode", as well as in failing to replace "at" by "near". That creates the double absurdity that the pipe length is not only a half instead of a quarter wavelength, but also _exactly_ a half wavelength.
>>
>> So, just through slight carelessness, we have a deadly combination of inexplicitness with incongruous repetition, with the one word "antinode" meaning two quite different things", namely
>>
>> (1) the displacement antinode near the end of the bell, and
>>
>> (2) the displacement near-node in the mouthpiece adjacent to the reed,
>>
>> which the writer calls an "antinode" perhaps because he's suddenly become amnesic about the distinction between "some displacement" and "maximal displacement" and about the fact that the reed can support a helluva pressure amplitude, even if not quite maximal unless you bite the reed down.
>>
>> love M
>>
>> PS Please urge everyone concerned to take two minutes on www.atm.damtp.cam.ac.uk/people/mem/lucidity-in-brief/
>
>
> Anyway, I am very surprised about the 8 cm. It throws off most of my rule-of-thumb assumptions about where to modify bore dimensions in order to change twelfth relations. Perhaps I'll do better in future:-)
>
> Thank you Diego; and apologies for reading you far too superficially at first.
>
> It just goes to show, again, that being specific about criticism (as you were) is worthwhile.
>
> Tony

--

Diego Casadei
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