The Oboe BBoard

The Oboe BBoard The C4 standard

New Topic | Go to Top | Go to Topic | Search | Help/Rules | Smileys/Notes | Log In

Re: Two reed cutting questions

Author: jhoyla
Date: 2008-09-02 13:26

Hi Mike, next installment

<<I also never thought of back as freeing the heart to vibrate. I always thought of the heart more as a relatively stationary fulcrum that allows for independent vibration of both the back and the tip.>>

I like this! It sounds very plausible. Why, though, do we need to scrape the heart at all? Obviously, because it does play some vibratory part in the sound production and is not a "stationary fulcrum" - at least, not in an American scrape. I can see how the barky part of an European scrape can fulfill this function.

<< I see the function of the back as a sympathetic resonator that filters out the higher frequencies from the tip and supports only the lower frequencies. >>

I like this too - I like it a lot! I think your "balloon" analogy is a bit over the top, but I can see how a sympathetic resonator can absorb the higher partials. Also explains why a broader, deeper back makes the sound more mellow (absorbs more partials).

<< But, as I said, I don't think anybody has a rock-solid understanding of just how reeds work....>>

TOTALLY agree with you here.

<<I also don't really get the "longitudinal" vs. "lateral" vibrations argument. >>

I hope I explained it well enough in my last post? All vibrations in the cane itself will be reflected in the vibrating air column. If sections of the reed are vibrating laterally at a different frequency to the fundamental, you will hear this as a buzz or chirp.

<<What you have at the top of the staple is an oscillation of air pressure as a function of time. This is caused primarily by the opening and closing of the reed, with the pressure increasing as the reed opens to allow air from the mouth (at a higher pressure than the atmosphere) to enter the tube. Secondarily the volume of the reed cavity expands and contracts in response to the pressure changes caused by the opening and closing of the reed. I think this is where the back of the reed has the greatest effect; the sharp pressure changes caused by the sudden opening and closing of the reed essentially translate into high-frequency energy (if you know physics, think about the Fourier transform of a delta function...),>>

well, a square wave, but yes your point is made ..

<< and the flexibility of the back of the reed essentially acts as a damper, expanding ballon-like to reduce the suddenness of this change. The balloon-like expansion of the sides of the back would in fact have a "lateral component" of vibration relative to the comparatively immobile spine, >>

Not if the reed is well balanced, it won't. Any lateral movement will cancel out as they will be in mutual counter-phase.

<<but this doesn't matter--the derivative of the air pressure as a function of time at the tip of the staple is lowered, and this should damp out higher frequency overtones. >>

Agreed - I really like this analysis!

<<I think what frequently makes a good reed are precisely those things that "complicate" the above analysis. Blending the tip with the heart means that it is harder to define where the tip ends--and thus harder to define the fundamental frequency of the tip/airgate. >>

Hmm .. Is there a "fundamental frequency of the tip/airgate"? I don't think so. The forcing function of the air-column dimensions is what governs the fundamental, much more than the physical properties of the reed. Otherwise, I agree that the "blend" is extremely important, in that it "drives" the heart, making the entire tip-plus-heart vibrate at the fundamental. The heavier the heart, the fewer high-frequency components it can generate but more energy is needed to set it in motion.

<The result is an "airgate" that can function at any fundamental frequency you choose by the fingering of the oboe. Blending the tip in with the sides, around the heart, allows the back to "couple" with the tip, perhaps helping the back to amplify the fundamental frequencies and filter out the overtones.>

Sounds good.

<But all this is speculation; I am afraid we are stuck in a poorly-controlled experimental environment, and theory isn't going to help us much.>

sigh.

Thanks for giving such a great response!

J.

Reply To Message | Avail. Forums | Flat View

Newer Topic | Older Topic

Topics