Acoustic "coupling" -- how to make it happen?

[Jon Snow]

Hi folks,

I am a bagpiper and I wonder if you could help me find some resources to both better understand my instrument and possibly make some improvements.

First, my instrument has three drones which must remain in tune with each other. There is a certain amount of acoustic interaction that takes place between the three reeds, sort of like if you strike up a tuning fork and hold it near another tuning fork, the second fork will begin to sound as well.

This phenomenon does not seem to be well understood in the piping world, although there is some anecdotal information floating around. Usually it's in the form of "pipes made by So-And-So are magically stable and can play for days without going out of tune".

I have been trying to modify my instrument to try to find ways to improve the coupling between the drone reeds, with sporadic but unreproducible success. Can anybody suggest some resources that might help me understand what I should try next?

My second question is really just about understanding how my instrument works. The drones in a bagpipe are much shorter than they should be to produce the frequency they produce. This is due (I assume) to the change in bore diameter between the sections of the pipe. Can anyone recommend a book or something that would explain how this works?

Thanks!

[Chuck Morrison]

Hi Jon,
You don't say what the 3 notes in the drone actually are, but I'm going to assume that you're dealing with sympathetic resonances that may have something to do with the harmonics of the reeds as related to the pipe sections. The way I'd approach this (since I don't know anything about it) is to figure out what the actual reed fundamentals are, the fundamental of the pipe(s) in question and see what the relationships are. Sympathetic resonances will occur when you get the notes (fundamentals or harmonics) in question close to alignment. The harmonic series for the elements in question (reed and pipe) need to be understood. My guess is that the reeds, being fixed at one end will only support the odd harmonics. I'm not sure if the pipe will be odd harmonics or all harmonics, but that depends on whether the pipes are open at one end or both ends. It could all be worked out with a little experimentation.

As far as information on the basics, "Fundamentals of Musical Acoustics" by Arthur H. Benade is a pretty good book on the subject. Pipes are a bit of a hybrid instrument as they rely on reeds and fingered pipes in combination. You get to research two distinct types of instruments in one.