Klarinet Archive - Posting 000333.txt from 1999/04

From: "Mark Charette" <charette@-----.org>
Subj: [kl] Benade & Wall Material (very long)
Date: Tue, 6 Apr 1999 06:29:08 -0400

The following is unedited. I think I'm at the outer limit of what I may
safely copy for illustrative purposes ...
----------------------------------------
>From A. H. Benade, "Fundamentals of Musical Acoustics":

22. 7. The Effect of Wall Material on the Playing Properties of Wind
Instruments

The question of whether or not the playing properties of a wind
instrument are influenced by the material from which it is made has been
the subject of curiously bitter controversy for at least 150 years. We
at once recognize the influence of dimensional changes that may be
forced on an instrument maker when, for example, he makes a wooden
rather than a silver flute, or those inadvertent ones which come about
from the different ways in which a bore reamer cuts wood and plastic. At
a subtler level, many kinds of things can lead to questions concerning
the influence of materials: when identical woodwind air columns are made
using wall materials of different porosity or rigidity, the resulting
sounding pitch of the instrument may vary by as much as twenty cents;
thin-walled instruments on which one can feel vibrations are often
improved (but sometimes spoiled) by putting layers of adhesive tape on
the outer surface at an empirically chosen spot; repairmen and players
alike are aware of the quite noticeable changes in the playing
properties of an instrument when pads of differing material are
installed for use in covering the tone holes, or when the bore is oiled.

Theory also poses questions, some of which are not hard to answer. For
instance, it has been known for many decades that the walls of a
perfectly round pipe cannot vibrate enough to radiate audible sounds
into the room. When such a pipe is slightly out-of-round (elliptical),
it can be excited much more strongly by internal pressure variations,
but even so it cannot radiate sound into the room with sufficient
amplitude to be heard in the presence of the other sources of
excitation. Because of this, changes in the material or the thickness of
the walls cannot detectably alter the sound of an instrument insofar as
it depends on radiation by the walls.

The vibration of wind-instrument walls can sometimes influence the
playing behavior significantly for a different reason. Just as
vibrations of the piano soundboard can alter the natural frequencies of
the string modes, so also wall vibrations can alter the frequencies of
the air column. The air column "looks" oversize at points of large wall
vibration if the natural frequency of the wall lies above that of the
air mode which drives it, and undersize when this frequency relationship
is reversed. I have seen instruments (thin-walled metal flutes in
particular) whose behavior seems perfectly insane unless the complex
influence of wall vibrations on regimes of oscillation throughout the
scale is suitably damped out.

In wind instruments, the vibrational damping provided at the walls by
air friction, oscillatory temperature effects, porosity, etc., far
outweighs the damping produced by the escape of sound through the tone
holes into the room. Because the player and his reed are in the business
of maintaining an oscillation in the air column, it is clear that the
major portion of his physical effort is devoted to the feeding of
acoustic energy directly to the walls of his instrument. Since different
materials provide varying amounts of damping, it is logical to wonder
about the magnitude of this effect.

Since 1958 I have made several studies of the possible differences in
damping that can be made by using copper, silver, brass, nickel silver,
or various kinds of wood as the air-column wall material. If the walls
are thick enough not to vibrate and if they are smooth and nonporous,
experiment and theory agree that switching materials will make changes
in the damping that are generally less than the two-percent change that
most musicians are able to detect.

Turbulence in the vibrating air is another phenomenon that can be
influenced by the nature of the wall material, though the relation is
indirect, since it comes from the ability of the material to take and
keep a sharp edge. The presence of sharp edges brings about airstream
turbulence at blowing pressures lower than those that would elicit it in
the absence of such edges. As one plays louder than mezzo forte on a
flute, oboe, or clarinet, the sound level becomes high enough for new
phenomena to appear. At mezzo forte, the oscillatory flow through the
first one or two open tone holes is no longer of the simple,
smooth-flowing type found in soft-playing, but neither is it quite of
the fully developed turbulent type that causes the rushing and roaring
noises from a strong wind. Two undesirable things happen in the tone
holes when turbulence starts: (a) the damping rises greatly, even before
the tone holes begin to hiss in a manner that is familiar to many
players, and (b) the nature of the flow through the holes is such as to
make them act as though their sizes had been changed, thus spoiling the
careful voicing adjustments of the instrument. From this point it is
only a short path to the realization that rounding the corners of the
tone holes and of any other projections or angles in the air column
(e.g., at the junctions of its various parts) will postpone and
regularize the onset of turbulence and so make the instrument play well
over an extended dynamic range, provided that these corner roundings are
carried out in a way that preserves the acoustical alignment of the
instrument. In 1972 John Cuddeback worked with me to measure the damping
of a clarinet air column over a wide range of excitations, before and
after its corners were carefully rounded. The results confirmed these
conclusions and resolved certain discrepancies in some earlier
experiments.

The connection between sharp edges on the corners of an instrument and
the material from which it is made is not difficult to find: instruments
normally come out with their corners sharper when plastic or metal is
used than when wood is the material of choice. It is the instinctive
tendency of a skilled craftsman to show his competence by producing
crisp clean edges for all his tone holes and joints, and the degree of
sharpness of these corners depends very much on the nature of the
materials with which he works and the sort of tools he uses. I have
found historical and contemporary examples of instruments made by the
best workmen in which the corners were deliberately rounded, as well as
those (much more common today) which are left with sharp corners. In
every case players prefer the ones with rounded corners. In the normal
course of traditional instrument-making, rounded corners are most often
produced on wooden instruments. A number of metal and plastic
instruments which I have reworked have prompted musicians to remark in
public that they play just like good wooden ones; players have also
remarked that instruments I have worked on have good "personalities"
which are independent of the materials from which they are made. One
must of course be sure that the mouthpiece, the air column, and the tone
holes are properly adjusted to one another, since mere postponement of
turbulent damping does not by itself
give an attractive instrument.

It is interesting to consider the history of woodwind-making in the
light of the turbulence phenomena. In the old days when undercutting of
tone holes was prevalent, the general sharpness of corners in all
handmade woodwinds was slight enough that turbulence effects were
usually not very noticeable. By the mid nineteenth century, woodwinds
were beginning to be made with accurate jigs for drilling precisely
sized holes in their exact position along the instrument. Corners became
sharp and neat as harder materials came into use and also as an
indication of fine workmanship—all of which provided the perfect
conditions for
nurturing turbulence troubles! Instrument makers found that the more
"scientific" they became in their mechanical methods, the more
unsatisfactory the older designs became and the more sensitive the
instruments became to the material from which they were made.
Essentially all of today's mass-produced woodwinds would benefit from a
careful rounding of tone-hole corners. Our growing understanding of
these things suggests simple solutions to many problems that have
heretofore seemed to require an alteration in the tone-hole design and
the key mechanism. (Note: the embouchure hole of a flute cannot be
tinkered with in simple application of these suggestions.)
----
Mark Charette@-----.org/clarinet
"Cards by Aimee", http://www.sneezy.org/Aimee
"The phenomenon is too variable for proper study" often
translates from "I don't know how to get musicians to do
anything twice the same" - A. H. Benade

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