The Clarinet BBoard

RE: why does clarinet overblow at a12th?

Author: JMcAulay  Date:   2001-11-05 05:41 Mark, I have fallen into a universal trap: trying to provide a simple explanation for a complex topic. So now I add a few comments: The even-numbered harmonics of a clarinet's fundamental tone are very weak *in the chalumeau register*. After all, that's the one that we start with, before overblowing. If the clarinet is played in the clarion or altissimo registers, the instrument is being overblown, so the harmonic picture changes quite dramatically. In fact, the differences in the harmonic patterns give the instrument its different sounds in the different registers, especially noticeable between the chalumeau and the upper registers. And the "presence" of even harmonics does *not* mean that they have appreciable intensity. It is a bit amusing that in the the Ohio State U charts which you linked, every single one is labeled "even harmonics... are clearly present," even though those in the chalumeau register (F3 and E4) show the second harmonics vanishingly small. Of course, they're present; it would require a piece of laboratory electronic equipment to generate a near-perfect square wave. For that matter, odd harmonics are clearly present, too. It is as if whoever made the charts wanted to arrive at a specific conclusion, so the charts are simply labeled with that conclusion, regardless of the significance of the harmonics. F3, for example, shows the second harmonic approximately 40 dB below the fundamental frequency, a power intensity difference of about *ten thousand times*. E4's second harmonic appears to be perhaps 35 dB down (a difference of exactly 35 dB would represent a power ratio of 3,162 to one). In neither case, I suggest, is the second harmonic sufficient to have any noticeable effect on what's happening. And on both these charts, there are only three even-numbered harmonics (the eighth, tenth, and twelfth of F3) which have a power intensity greater than a hundredth of the fundamental. Perhaps my comment that the second harmonic is "almost totally absent" may be a bit pushy, so may I alter that to "a few thousand times lower in power intensity" in order to offer less arm-waving and a more definitive statement. Again, this applies in the chalumeau register. I do beg the indulgence of all in my comment regarding even-numbered harmonics that "the clarinet has very few." This, of course, is a misstatement. In truth, harmonics, if present at all, go on into the ultrasonic range. I should have said, rather, that (in the chalumeau register) the clarinet's even-numbered harmonics are all very weak. In the clarion and altissimo, as we are no longer at the chalumeau's fundamental frequency as the predominant tone, the harmonic situation differs considerably. Here, the harmonics appear generally to decrease in intensity more or less regularly, from the second harmonic on through the higher-order partials. From a physics standpoint, it appears as if opening the "speaker" key gives us an effect closer to that of a completely open pipe. I do appreciate the OSU charts, because they make quite clear the reason the clarinet does not overblow an octave, but rather a twelfth. As a harmonic, the octave is barely produced in the clarion register, while the twelfth is far stronger. Looking at the charts for F3 and E4, it is seen that in each case, the twelfth (third harmonic) is perhaps 13 dB down from the fundamental, a power difference of only twenty times. Again, the octaves (second harmonics) are *thousands* of times weaker. The F3 chart also shows the fifth and seventh harmonics at levels almost equal in intensity to the fundamental frequency, offering clear evidence of something we all know: it's pretty easy to get a high-pitched "squeak" when overblowing notes such as this one. Mark, thanks for keeping me honest. Regards, John

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