A Physics Digest—Part 4: Sound

Sound begins when something vibrates—sending a sound wave through the air. The ear then gets stimulated and electrical impulses reach the brain. So does a tree make a sound when it falls? By this definition, only if an ear and a brain are present to sense the air’s vibrations.

While light and heat waves can travel through a vacuum, sound waves need a medium—a solid, liquid, or gas. (So a tree falling in outer space would be silent.) Sound waves travel at different speeds in different materials (they move faster in solids than air). A sound wave is transmitted within a medium when something makes a part of that medium vibrate. That part jostles neighboring parts, and the disturbance moves along—just like heat. Throw a stone into a pond and watch the circular vibration waves move outward. While the disturbance (energy) flows along, the material itself doesn’t. Watch a cork on the surface of the water bob up and down and notice that the cork doesn’t move outward with the wave.

The rate of vibration of sound is defined as its frequency. Our ears can register frequencies as low as 20 vibrations per second (Hertz), up to as high as 20,000 Hertz (although my aging ears quit at about 12,000). Sound waves transmit energy, which is often measured as loudness. Our remarkable ear can sense loudness levels of greater than a range of a factor of a trillion, so we squeeze that down into a workable logarithmic range of about 120 decibels.

Sound can become particularly attractive when we talk about music—although it can be very subjective. One person’s music is another’s noise. As a less subjective evaluation, we’d pretty much all of us define a jet engine as making a lot of noise.

Qualities of musical sounds that describe their properties are pitch (the same as frequency) and timbre. When a musical instrument makes a sound, it is composed of a fundamental tone (the basic pitch) and various overtones, which are whole-number multiples of the fundamental. The particular mix of these multiple tones defines the timbre of the instrument—its unique sound quality.

Musical instruments make sounds in three ways: by vibrating strings, vibrating air columns (horns and woodwinds) and vibrating surfaces (cymbals and drums). All of these sounds are simulated in a stereo system when an electrical signal causes a speaker surface to vibrate.

Our ear loves regular mathematical relationships between musical pitches. When the pitch of one sound is twice that of another, we hear an octave. When the ratios of pitches are whole number fractions—like 5/4 and 4/3—we hear musical intervals of a third and a fourth, respectively. That’s harmony!