Posted by Gerald J. on August 17, 2001 at 16:18:07:
In Reply to: Re: Re: Re: Vibration, damping, and weight posted by Chuck(G) on August 17, 2001 at 14:53:04:
Sure, drive the horn with a speaker diaphragm at the mouthpiece (fitted). Measure the output at the bell, vary the frequency, measure the half power points, divide center frequency by bandwidth, you have Q. Or with a really good spectrum analyzer, drive the mouthpiece with noise and look at the bandwidth of the spectrum from the bell.
Another measure (but not necessarily quantitative) would be to see how far one can lip a pitc. The higher the horn's Q the narrower the lipping range. My old Eb with worn misaligned valves will lip 3/4 step, but every note has to be lipped into tune.
Like the strings of a double bass which have a much higher loaded Q than those of a violin, the strings are harder to start, and harder to stop, and when hit with an impulse function, they ring longer. So to keep in time with a none spastic conductor the bass player has to anticipate. That's true of tuba also because of the finite propagation time through the horn. And when the conductor goes off arrhythmic, bass and tuba alike get yelled at for being late or early.
A pizzacato note on a bass may ring for several seconds while on a fiddle its a "plink" and its done.
Which bringers to mind another technique for measuring Q. Excite the resonator with a dirac delta function (infinitely tall, infinitely narrow pulse) and measure the decay rate of the sound that comes out the bell. The trouble with this on a horn with many resonances is that every harmonic will contribute to the sound at the bell, and each will have a different Q and rate of decay. Besides which slapping the mouthpiece tends to make it more than a little bother to remove.
Gerald J.