Good Vibrations: The Interplay of Music and Physics
Laurie McNeil
September 12, 2021
Laurie McNeil is a professor of Physics at the University of North Carolina. Her talk was titled, “Good Vibrations – the Interplay of Music and Physics.” Actually an explanation of how music works in terms of the physics.
She started with the question, what is musical sound? For the musician, it’s about things like notes and pitch. For the physicist, sound waves and frequency. Whereas musicians speak of timbre, physicists would put it in terms of frequency spectrum. A musical sound consists of multiple frequencies, the amounts of each creating what we call timbre. Different instruments playing the same notes are differentiated by their timbre.
McNeil discussed in turn various categories of musical instruments, delving into the physical aspects of how they produce sounds. Stringed instruments do so by modulating speed as determined by the tension and mass density in the strings, producing different sound wave patterns. A shorter string produces a higher note; also achievable by tightening the string (increasing tension) or using a thinner string (decreasing mass density).
She noted that as a wave propagates across a string, portions of it move up and down while others (nodes) are stationary. The frequencies are in whole number ratios; required for the ear and brain to perceive a distinct pitch. Each standing wave has its own frequency, the ratios between them corresponding to musical intervals, or octaves. For example, a 3:2 ratio is a perfect fifth; a 4:30 a perfect fourth.
For wind instruments, it’s a matter of pressure waves in an air column. Note that a flute is open at both ends, a clarinet closed at one end, thus creating different resonances in their air columns. The shape of the tube matters too. A brass instrument has one closed end and a conical bore. Its valves are used to change the tube length.
In the case of percussion, it’s vibrations of rods and membranes. McNeil pointed out that an ordinary drum doesn’t produce pitch, but a kettle drum does, because the air in the basin changes the resonant frequencies to make them closer to whole number ratios.
Understanding all of this is, of course, not necessary for the enjoyment of music.
The Q&A included some discussion of how differences among instruments in the same class can produce different sound quality, the classic case in point being the Stradivarius violin. But Fred Levine observed that the quality of the player can matter more than the quality of the instrument, and a skilled player can make even a lousy instrument sound good.
And, in concluding her talk, McNeil quoted Duke Ellington that “if it sounds good, it IS good.”