Brian Copeland- Bcopeland56@hotmail.com
Department of Electrical and Computer Engineering
The University of the West Indies
Trinidad and Tobago, West Indies
868-662-2002
Andrew Morrison
Thomas D. Rossing - Rossing@physics.niu.edu,
815-753-6493
Physics Department
Northern Illinois University
DeKalb, IL 60115
Popular version of paper 2pMU7
Presented Tuesday afternoon, December 4,2001
142nd ASA Meeting, Fort Lauderdale, FL
The Caribbean steelpan is probably the most important new acoustical 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.
Previous studies [Acoust. Soc. Am. 108, 803-812 (2000); also Science of Percussion Instruments by T.D.Rossing (World Scientific, 2000)] have shown exactly how individual note areas on a steelpan, as well as the whole pan, vibrate when played. The purpose of this study was to determine exactly how a vibrating steelpan radiates sound. This information is of great value, not only to the pan-maker and tuner, but to performers, concert hall designers, and especially to sound engineers who wish to record the steelpan along with other instruments.
The best way to describe sound radiation from a source as complex as the Caribbean steelpan is by mapping the sound intensity field. Sound intensity is the rate at which sound energy flows outward from various points on the instrument. The sound intensity field represents the direction and the magnitude of the sound intensity at every point in the space around the source.
A single microphone measures the sound pressure at a point, but it does not provide information about the direction of the sound energy flow. In order to determine the sound intensity (which gives information about both magnitude and direction) it is necessary to compare the signals from two identical microphones spaced a small distance apart. The resulting "pressure gradient" can be used to determine sound intensity by means of a computer. When this is done at a large number of locations, a map of the sound intensity field results.
The first two graphs show sound intensity maps in a plane passing through a double second steelpan. A single note (F#3) has been excited at its fundamental frequency. The first graph of the "active intensity" represents the outward flow of sound energy, while the second graph of "reactive intensity" represents the energy that is stored in the sound field near the instrument. While the active intensity is the most significant field in a concert hall, both active and reactive intensity fields have to be considered in recording a steelpan.
The third graph is a map of the sound pressure in the vicinity of the steelpan. It uses color to show "hot spots" in the sound field, mostly above and below the pan and to a slightly lesser degree, around the skirt of the pan, which also vibrates when it is played.
It is well known that steelpans radiate a rich sound, consisting of several prominent harmonics in addition to the fundamental sound of each note. Comparing sound intensity and sound pressure maps of the individual harmonics (including the fundamental) allows us to determine the timbre or tone color of sound radiated in different directions. For example, the sound radiated from the back side of the pan (where microphones are often placed in recording studios) generally emphasizes the second harmonic (octave), and thus appears to be "brighter" than the sound from the front side of the pan.
It is our intent to extend these sound radiation studies to other members of the popular steelpan family of musical instruments.
