Page 52 - Spring 2018
P. 52

 Joe Wolfe
School of Physics University of New South Wales Sydney, New South Wales 2052 Australia
The Acoustics of Woodwind Musical Instruments
The oldest known instrument family produces a wide range of tone colors and pitch using a range of interesting physics.
Introduction and Overview
Woodwind instruments have been played by humans, possibly including Neander- thals, for more than 40,000 years. Tubular pieces of bone, pierced with holes, and similarly shaped fragments carved from mammoth ivory have been discovered in caves in Europe; they are flutes, perhaps end-blown like a Japanese shakuhachi (Atema, 2014). Artificial musical instruments may be even older, but those made of perishable materials, including possibly drums, lyres, and didgeridoos, cannot compete archaeologically.
Woodwind instruments are classified by the sound source: air-jet instruments (e.g., flutes; Figures 1a and 2), single-reed instruments (clarinet, saxophone; Figures 1a and 3), or double reeds (oboe, bassoon; Figure 1a). Because of their diversity, the woodwind family provides a wide range of timbre, and orchestral composers often contrast their tone colors by passing a theme between different woodwind instru- ments (see examples at Beyond orchestras, wood- winds from folk traditions extend the timbre range further. Organ pipes (Angster et al., 2017) are also arguably woodwinds; they are excited either by an air jet or a single brass reed.
Like brass instruments (Moore, 2016), woodwind instruments convert energy in the steady flow of high-pressure air from the lungs into that of oscillations and sound waves. Also like brass, woodwinds have a sound source whose properties are strongly nonlinear. For brass, the source is the player’s lips; for woodwinds, it is either an air jet or one or a pair of reeds. This source interacts with a resonator (largely linear), which is usually an acoustic duct, called the bore. In woodwinds, the effective length of the duct is varied when tone holes in its wall are opened or closed by the player. Usually, the instrument plays at a frequency near one of the duct resonances. Under the player’s control, interactions between source and reso- nator determine pitch, loudness, and timbre.
The player’s mouth provides air with a pressure of typically a few kilopascals and a flow rate of tenths of a liter per second, both varying with instrument, loudness, and pitch; this gives an input power of ~0.1 to a few watts. Oscillations of the air jet or reed modulate airflow into the bore, where resonant standing waves in turn produce fluctuating flow or pressure at the mouthpiece. These, in turn, control the input, a process called auto-oscillation discussed in Sound Production with an Air Jet and Sound Production with Reeds. Only a small fraction (order of 1%) of the input energy is radiated as sound from any open tone holes and from the remote end. Most of the input energy is lost as heat to the walls. This inefficiency is fine, however; players typically input a few hundred milliwatts, and an output of even a few milliwatts can produce about 90 dB at the player’s ears. Most wood- winds are not as loud as brass. However, the piccolo, whose playing range includes the ear’s most sensitive range, is readily heard above the orchestra.
50 | Acoustics Today | Spring 2018 | volume 14, issue 1 ©2018 Acoustical Society of America. All rights reserved.

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