Posted by John Swensen on September 13, 1999 at 18:53:14:
In Reply to: Re: Re: Physics of mouthpieces and buzzing posted by Lonny on September 13, 1999 at 17:24:54:
When you buzz, your lips periodically open and close, interrupting the stream of air, producing a series of pressure pulses at some frequency. When you buzz into your horn, each pulse travels down the horn (at the speed of sound) until it reaches the vicinity of the bell. At this point most of the pulse is actually reflected back down the horn, and only a small amount radiates into the room. When the reflected pulse reaches your lips, the higher pressure of the pulse tends to make your lips close just then; if they were about to close anyway, the reflected pulse helps them to close and reinforces the buzz at that point. If the frequency of your buzz is such that the period between closings happens to be an integral multiple of the round-trip time from your lips, to the bell, and back again, that buzzing frequency will be strongly reinforced, and the note sounds strongly.
If you change the integral multiplier (buzz pulses per round-trip-time) you change the partial you are playing on. If you change valves, you change the length of the acoustical path, the round-trip time and, hence, the set of frequencies that will be strongly reinforced.
If you start buzzing a note a half step sharp or flat, relative to what your horn/valve combination wants to reinforce, the reinforcing action of reflected pulses will tend to pull your buzzing pitch to the correct frequency after a few round trip times (unless you really fight to bend the buzzing pitch up or down). During these first few round trip times, your tone will not be stable and you will not have a clean attack; if you are unlucky, the frequency may oscillate for a while and you will get the dreaded "double buzz". If, however, you buzz the precise frequency that your horn "wants" to play, the buzz will be reinforced from the start, and you will have a clean attack, no matter what dynamic level you are playing.
I think that is the simple story about what is going on with brass instruments, although the longer story is much more complicated and subtle. If you want to understand much more about the physics of brass instruments, read by Arthur Benade's "Fundamentals of Musical Acoustics"--- almost no math, lucid examples and explanations, from a physicist and serious amateur musician.