 The Clarinet BBoard
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Author: Jason M.
Date: 2001-06-11 03:41
I have recently become very interested in the physics of sound. After doing a fair amount of research I got up in front of my highschool physics class to do a presentation on various acoustical topics. The presentation went over well, but at the end a friend of mine from marching band really stumped me. Her question: When you loosen your embouchure, why does it cause you to go flat?
The best I could do was stumble around with the concept of the standing wave set up in the instrument caused by the reflected wave due to the abrupt pressure change at the end of the air column. That's a moutful. Normally the reflected pressure wave will hit the reed and cause it to begin to vibrate sympathetically with the fundamental of the pitch you are fingering. My guess is that by loosening, you are forcibly slowing the vibration of the reed and thus fighting against the natural standing wave in order to flatten the pitch. How far off am I?
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Author: Gordon (NZ)
Date: 2001-06-11 14:36
If you loosen you effectively make the reed longer (from tip to where it is in permanent conatct with the mouthpiece) Longer things vibrate slower. Add that to you postulations. It seems to fit.
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Author: William
Date: 2001-06-11 15:46
The slower the vibration, the lower the pitch. Actually, it is the length of the string or air colume that determines the pitch--longer =lower, shorter=higher--but with the lips and reeds, we are artificially establishing a vibration for a basic sound that we alter by shortening or lengthening the length of our instruments via different fingerings and accoustical node-interrupters (register keys). However, when you slow that basic sound source (lip buzz or reed vibration) by loosening your lips (lengthening its span of action) the pitch center of the instrument that you are playing also lowers, or "it goes flat." When you tighten the embouchure, an opposite effect occurs in that the pitch goes up, or "sharpens." Tension in strings and drum heads is also used to speed or slow the vibrating medium to alter the pitch--high tension=high pitch, low tension=low pitch. So, pitch control and intonation should as simple as length and tension. Maybe that is why the longer the gig, the more nervous I get. Good tuning, all!!!!!! :>)
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Author: Gordon (NZ)
Date: 2001-06-12 12:00
I would follow that better if I could understand what is meant by "pitch centre" which I presume is a technical term in acoustics. Any enlightenment?
I understand about the intreraction between the standing wave and how it determines the frequency of vibration of the reed However isn't this an interactive process - the reed and the standing wave? So is it not true that if we alter the lips to cause the reed to have an INCLINATION to vibrate slower this will have a SMALL effect on the pitch of the standing wave?
Just asking. Sorry I cannot put this in acoustics jargon.
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Author: Suzy
Date: 2001-06-13 18:52
The pitch center is the place where each pitch, or the sound created at that certain pitch, is vibrating at its optimal speed. To keep a note "centered" is to have an equal balance of air pressure and embouchure resistance or grip.
If we "alter the lips to cause the reed to have an inclination to vibrate slower" we are basically overblowing the embouchure resistance, causing the reed to vibrate at a wider, and thus, lower, pitch.
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Author: Jason M.
Date: 2001-06-13 21:01
First of all, I'd like to thank everyone for trying to help me out with this concept. Some of what you've said makes sense to me, but some of it simply doesn't.
First of all, the whole concept of pitch center still seems to be a bit of a stretch to me. As far as I know, a reed setup acts as what is essentially a "white-noise drive." At least until the point that the standing wave is set up, it produces sound evenly across a large range of frequencies. By loosening the embouchure I can see how it would lower this band a little, but most of the frequencies would still be in there, it's just that the very top end would be missing, maybe some harmonics would be lost. The overall length of the instrument should still be the same, so shouldn't the pitch still be the same? One argument that did make sense to me was that by loosening the embouchure you are making the reed longer, which is what gordon said. It would make more sense to me though if he had said that by loosening the embouchure you are making the instrument longer as that is the part that should control the pitch, right?
In Suzy's response, there was again talk about pitch center. Unfortunately, it just served to confuse me more. What does it mean for a pitch to be vibrating at it's optimal speed. There is only one "speed" for a specific pitch to vibrate at. The term speed to me seems a bit misleading. Does it mean "frequency" or does it really mean the speed at which the reed is traveling at a given moment, which I believe would be related to both amplitude (loudness) and frequency. When you speak about overblowing the embouchure and allowing the reed to vibrate wider, it does not, at least to me, follow that the pitch should be lowered. For the pitch to be lowered, the wave must oscillate more times per second, but if the reed were just vibrating wider then it would signal a change in amplitude rather than pitch. It's a difference between longer in time and wider in space.
Finally, to address the "tension" argument, it doesn't really make sense to me to talk about tension with an air column. I can understand talking about pressure, but for something to have tension it would seem to me that it would have to have the ability to actively pull on something. When talking about a gas, the only equivalent I can think of is pressure
I'm sorry to kinda snipe at other people's theories when I myself am still somewhat uneducated in the subject. It seems that this is the way science progresses. In any case, I would appreciate anyone that could address these issues, or perhaps point me to a website that answers these questions. Please make sure the website actually answers the questions though... I've been to a lot of great physics websites that don't. Thanks for your interest; as an eager student I await a teacher.
~Jason M.
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Author: Mark Charette
Date: 2001-06-13 22:17
Jason,
Instead of guessing at what happens, check out Benade's "Fundamentals of Musical Acoustics".
There's places in that book that relate to some of your guesses and theories. It doesn't explain everything, but it'll give you a solid theoretical start.
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Author: Gordon (NZ)
Date: 2001-06-14 14:15
I really like your response, Jason, and it made me think of Bernade too. You seem to have the right sort of mind to carefully wade through relevant areas of this excellent but technical book.
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