Thomas D. Rossing- Rossing@physics.niu.edu
Physics Department
Northern Illinois University
DeKalb, IL 60115
Uwe J. Hansen- Phhanse@scifac.indstate.edu
Physics Department
Indiana State University
Terre Haute, IN 47809
Popular version of paper 5aMU3
Presented Friday morning, June 8, 2001
141st ASA Meeting, Chicago, IL
The Caribbean steelpan is probably the most important new acoustical musical instrument to develop in the 20th century. In addition to being the foremost musical instrument in its home country, Trinidad and Tobago, steel bands are becoming increasingly popular in Europe, North America and some Asian countries as well. The modern family of steelpans now covers a 5-octave range, and steel bands of today use them to play calypso, popular, jazz, and Western classical music.
Although the development of steelpans took place in the years following the end of World War II, when the annual celebration of Carnival was resumed with great enthusiasm, only recently have scientific studies of the steelpan been made.
Bass pans are among the most important instruments in the steel band but also among the least studied acoustically. The bass player generally plays on four to nine pans, each having three or four notes. Bass pans cover approximately the range G1 to C4 (an approximately 2.5-octave range that goes up to middle C). The skirts are the full height of the original barrel (87 cm), and the pans are supported on soft blocks so that the barrel and the opening around the bottom form sort of a Helmholtz resonator (an object which produces tones proportional to the width of the opening).
The complex vibrations of a steelpan can be described in terms of normal modes of vibration, which represent the ways in which a pan can vibrate and produce sound. When the pan is played, many different normal modes are excited, and a rich sound results. The spectrum of the sound is a recipe telling us what normal modes are present in each note and at what strength.
Of the several techniques for observing normal modes of vibration, holographic interferometry offers by far the best spatial resolution. As in all optical holograms, laser light is split into two beams, one of which illuminates the vibrating object, and the other goes directly to the photographic film or other detector in order to create an interference pattern. In the case of a vibrating object, these interference lines give a contour map of the vibrating surface.
Normal modes of vibration of both the playing surfaces and the skirts of the bass pans have been recorded using electronic TV holography. Recording holograms on photographic film tends to be rather time consuming, but electronic TV holography, offers one the opportunity to observe vibration motion nearly in real time and a fast, convenient way to determine the normal modes. A TV camera is used as the detector rather than photographic film, and the holographic image is calculated pixel by pixel and displayed on a TV monitor.
In all of the bass pans tested, the notes are tuned so that the entire note area moves as a unit at the fundamental frequency. In the second mode, tuned an octave above the fundamental, a nodal line bisects the note area, so the two halves move in opposite directions, somewhat like a see-saw. The third mode is generally tuned an octave plus a fifth above the fundamental, while a fourth mode with two intersecting nodal lines is nearly two octaves above it. The harmonic tuning of these modes gives the pan a clear ring with a strong sense of pitch.
Holographic studies of the skirt (the untuned cylinder of the original oil drum) reveals a variety of vibrational patterns made up of bending waves that propagate around the circumference of the drum. The vertical wavelength is more than 3 times greater than the lateral wavelength, due to the greater stiffness. Although the vibration amplitude level in the skirt is about 40 dB lower than that of the struck note, the large surface area of the skirt still results in audible sound radiation. While the skirt may not contribute much to the overall sound level, it appears to influence the timbre of the sound.
A bass pan is generally supported on blocks a few centimeters off the floor. The air volume of the drum, along with the opening at the bottom, constitutes a resonator that also influences the sound.
Fine violins have been handcrafted for several hundred years, and many parallels can be drawn between violin making and pan making. In the case of the violin, science arrived on the scene very late, and the development of violin making proceeded almost entirely by trial and error and the insight of fine craftsmen such as Stradivari and Guenari de Jesu. Only in the last fifty years has scientific research caught up with the art and craft of the violin makers, and we at last have a pretty good understanding of how the violin makes sound.
We have probably arrived at that same point with steelpans. Pan makers and scientists now collaborate on controlled experiments that detail the different stages of construction in order to understand sound production in steelpans.