THE ASIAN FREE REED MOUTH
ORGANS
James P. Cottingham
Physics Department, Coe
College
Cedar Rapids, IA 52402, USA
jcotting@coe.edu
Abstract
Mouth-blown instruments using a free reed coupled to
a pipe resonator have a long history in China, Japan, and throughout Southeast
Asia. The sheng employs a free reed at one end of a closed tube with a
conical-cylindrical cross section. The
khaen employs an open tube of effective length L, with the reed located at
approximately L/4. The bawu is a closed
cylindrical pipe with the free reed at one end, in which the effective
acoustical length is varied by the use of tone holes. The playing frequency of
each pipe of the sheng or the khaen is typically slightly above both the
resonant frequency of the pipe and the natural frequency of the reed. In the bawu, on the other hand, both the
pipe resonance and sounding frequency are normally well above the natural reed
frequency, resulting in a striking difference in tone quality. Experimental
studies have been made of the sheng, khaen, and bawu as well as simulated
instrument tubes constructed from PVC pipe. Acoustical measurements made
include studies of reed vibration using a laser vibrometer
system, measurements of sound spectra produced when the pipes are played, and impedance
measurements of the pipes. Comparisons
have been made with theoretical work on the coupling of the reed vibration with
the pipe resonator. The results are
generally consistent with both experimental results from earlier studies and
with theoretical considerations regarding reed-pipe coupling.
INTRODUCTION
Unlike the free reeds found in Western instruments
such as the reed organ, accordion, and harmonica, the reeds of the Asian free
reed mouth organs are not only coupled to pipe resonators, but are
approximately symmetric, so that the same reed can operate on both vacuum and
pressure (inhaling and exhaling).
Figure 1 shows typical reeds from a sheng and an American reed
organ. The sheng, sho,
and khaen all employ one reed per pipe, thus requiring a separate pipe
for each pitch. Also common in various
parts of South and Southeast Asia are free reed pipes which allow change of
pitch using finger holes. One version
of this type of instrument now common in China is the bawu. Details on these instruments are available
in the article by Miller [2].
FIGURE 1. Reeds from a sheng
(left) and an American reed organ (right) from Gellerman [1].
The free
reeds used in the Asian mouth organs are cut from a single piece of thin metal,
typically brass or a bronze alloy, and set into a bamboo pipe. In the single
note per pipe instruments, a finger hole is drilled at a point that destroys
the pipe resonance and prevents the reed from sounding unless the hole is
closed. Wind is provided by blowing
either in or out through the mouthpiece which forms the opening of the air
chamber that surrounds the reeds. The instrument is held upright with the air
chamber supported by the hands. Fingers
and thumbs of both hands are available to close the holes and sound notes. It is typical in playing the instruments
that several notes are sounded simultaneously, some of them serving as
drones. See References 2 and 3 for more
detailed information.
The
frequency of reed vibration is determined by both the reed and the pipe. One or two tuning slots (usually two for the
khaen) are cut into the back of the pipe, determining the effective acoustical
length. The vibrating frequency of the
blown reed can within certain limits be pulled to match the pipe resonance, so
that fine tuning is done by means of the position of the tuning slots. In the
khaen, reed length only approximately corresponds to sounding frequency, with
pipe length apparently used as the prime means for tuning. Measurements taken on a sheng, on the other
hand, indicate that the individual reeds are much more carefully tuned to match
the pipe resonance [4].
A
characteristic shared by the Asian free reed mouth organs with other free reed
instruments is that the sounding frequency drops as blowing pressure is
increased. Data on this has been
reported elsewhere [3,4]. The pitch
change encountered in practice does not generally seem to cause musical
problems.
IMPEDANCE CURVES
Impedance curves have been measured for the bawu,
the khaen, and the sheng following the method of Benade and Ibisi [5]. For khaen pipe impedance measurements, the
pipes used were constructed from PVC tubing with dimensions similar to actual
khaen pipes. For the sheng, bamboo pipes from a real instrument were used. In the case of the bawu, measurements were
made on a “bawu” constructed from a piece of PVC pipe with a bawu reed attached
as well as on actual instruments. A few
results for the khaen and sheng are given below. Impedance curves for the bawu have been reported elsewhere
[4]. The impedance curve measurements
for the PVC khaen pipes, of which two are shown in Figure 2, are apparently the first such measurements
on khaen pipes.
FIGURE 2. Impedance curves (dashed) and sound
spectra (solid) of a 59-cm PVC khaen tube: (left) with end sections of lengths
11 cm and 35 cm, (right) with no end sections.
Khaen pipes
The fundamental frequency of the sound spectrum (275
Hz) in Figure 2 is just above the frequency of the first impedance peak (270 Hz
), which confirms previous observations that such a free reed pipe will sound
at a frequency above the reed frequency and close to a resonant frequency of
the pipe. If the length of the tube
places the fundamental below the reed frequency, the reed-pipe can sound near
the second harmonic of the pipe [3,4].
The difference in tone quality observed in comparing khaen pipes with
and without the end sections seems closely related to the differences in the
impedance curves for the two cases, as illustrated in the figure.
Sheng pipes
Sheng pipes are cylindrical over most of their
length, but the lower portion in which the reed is mounted is conical. The frequencies of the resonances identified
by the impedance peaks are not harmonic.
The pattern observed for the sheng is that the sounding frequency of the
pipe is above that of the first impedance peak, with the second harmonic of the
sound spectrum apparently reinforced by the strong nearby peak in the impedance
curve.
FIGURE 3. Impedance curve
(dashed) and sound spectrum (solid) of
a sheng pipe.
PLAYING
FREQUENCY
Figure 4 shows the general relationship between frequency
and pipe length for a khaen pipe. The
pipe length was gradually shortened by cutting lengths from the original
pipe. As can be seen in the graph, the
sounding frequencies follow closely the fundamental pipe frequency, always
remaining slightly above it. At the
shortest pipe length in this example, the reed did not sound at normal blowing
pressure (0.8 kPa), but could be made to sound at +1.7 kPa. At this same
length, underblowing (0.3 kPa) caused the reed-pipe to sound at a frequency
very close to the reed frequency. The
results presented in Figure 4 show that the sounding frequency of the reed-pipe
combination is higher than the natural resonance frequencies of either the reed
or the pipe taken alone.
FIGURE 4. Sounding frequency of a khaen tube
as a function of length at 0.8 kPa, except as noted. (Note that this figure is a correction of a result reported earlier
in Reference 3.)
FIGURE 5. Playing frequency of a bawu as a function
of pipe resonance frequency. Solid
elliptical data points from underblowing are near the reed frequency
(horizontal line).
Figure 5 shows the playing frequency of a bawu as a
function of pipe resonance frequency
The normally blown notes, obtained both manually and mechanically at
around 2.0 kPa, are generally close to, but slightly above, the measured
impedance frequency of the pipe.
Underblown notes are close to the natural frequency of the reed. Notes played so high above the reed
frequency have a dramatic difference in tone quality compared to those near the
reed frequency.
SUMMARY
The free reeds coupled to pipe resonators in the
instruments under consideration seem to behave as "opening" or
"outward striking" reeds as discussed by Fletcher [6], with the
sounding frequency of the reed-pipe combination is above the natural
frequencies of both the reed and the pipe. For the bawu, a free reed pipe with
finger holes, the sounding frequency can be about an octave above the reed
frequency; for the cut-off khaen pipe it can be about half an octave. Previous
measurements of reed motion using a laser vibrometer verify that for these
instruments the free reed tongue makes only slight excursions in the upstream
direction, thus approximating the outward striking reed model [7].
ACKNOWLEDGMENTS
The author is grateful to Terry E. Miller of Kent State University for providing examples
of high quality khaen, as well as expert advice and opinion on the Asian free
reed mouth organs. Thanks are also expressed
to Andrew Horner of the Hong Kong
University of Science and Technology for obtaining examples of bawu for
laboratory use, and to Thomas Rossing of Northern Illinois University for the
long-term loan of his sheng, and the use of the NIU laser vibrometer system.
REFERENCES
1. Gellerman R.F., The American Reed Organ and the Harmonium, Vestal Press, 1996.
2. Miller, T.E., "Free-Reed Instruments in Asia: A Preliminary Classification," in Music East and West: Essays in Honor of Walter Kaufmann, Pendragon, New York, 1981, pp.63-99.
3. Cottingham, J.P., and Fetzer C.A., "The Acoustics of the Khaen," Proc. International Symposium on Musical Acoustics (ISMA98), Leavenworth, Washington USA, pp. 261-266, 1998.
4. Cottingham, J.P., "Acoustics of a symmetric free reed coupled to a pipe resonator," Proc. Seventh International Congress on Sound and Vibration (ICSV7), Garmisch-Partenkirchen, Germany, pp. 1825-1832, 2000.
5. Benade, A.H., and M.I. Ibisi, "Survey of impedance methods and a new piezo-disk-driven impedance head for air columns," J. Acoust. Soc. Amer. 81, 1152-1167 (1987).
6. Fletcher, N.H. "Excitation Mechanisms in Woodwind and Brass Instruments," Acustica 43, 63-72 (1979).
7. Busha, M. and Cottingham, J.P., “Experimental investigation of air-driven free reeds using a laser vibrometer system,” J. Acoust. Soc. Amer. 106, 2288, (1999).