I'm back in my office now. Michael Edmiston is a great resource to have
here, but I have some others to recommend. (Incidentally Michael, you
neglected mentioning that the french horn is played with the hand in the
bell, a very important point as you know, but the naive reader might
assume that valve fingering is the important hand operation here.) One
that comes to hand is "The Physics of Musical Instruments" by Neville H.
Fletcher and Thomas D. Rossing (yes, the same Rossing). It refreshes my
memory; the instrument presents sharp maxima in the impedance curve to
enable intonation. The argument is intended to explain that.
The concept of impedance is very important, and it is, in my opinion,
greatly underplayed in elementary physics courses. The bicycle example
is one which can be readily understood by most students. Only a small
force is necessary to impel a bicycle on a horizontal road. The human
musculature is best suited to exerting large forces, but it cannot do
so at the speed of a bicycle's motion. Thus a matching device which
reduces force but increases speed as a consequence facilitates the
transfer of power = force x velocity. The ratio force/velocity can be
called the mechanical impedance of the system. A man alone has a higher
mechanical output impedance than a man-bicycle system, and the latter
system is better adapted to generating power in horizontal locomotion.
The bicycle is said to be an impedance matching device between the man
and the task at hand. It is a variable impedance matching device if it
has a derailleur, and it can be adapted to exert larger forces in
exchange for doing so at lower speeds. For more see "Bicycling Science"
by Frank Rowland Whitt and David Gordon Wilson.
Similarly, a musician is capable of generating very high amplitude
pressure pulses *internally*, but at very low volume amplitude, when
performing the exercise of lipping music without a trumpet. If that
pressure amplitude were present in a sound wave in your acoustical
environment you would be seriously damaged! The instrument is an
impedance matching device which enables the coupling of the high
pressure amlitude (NB: not the pressure itself; only the AC component
of the pressure) at low airflow volume (again, the AC component only)
to a lower pressure amplitude, higher AC airflow volume suitable for
propagation in the acoustic environment. Again, the product of the
two quantities is power, and the ratio is acoustic impedance.