Physics and Technology – Physics – Acoustics
Acoustics is the study of sound waves traveling through air, liquids, and solid objects. Just as hearing is one of our most relied upon senses, speech is the basis of the growth of human society.
Acoustics could be said to have contributed to art, science, and society.
Physics and Technology – Physics – Waves in the Air
Among the varieties of waves occurring in nature, sound waves are especially familiar to us, since we possess a sense organ—the ear—that can pick up an astonishing amount of information from them.
Waves are vibrations in motion. What is actually vibrating, though, as a sound wave travels? It is the atoms and molecules of the medium through which the sound is moving. Vibrating around a resting point, they cause their neighbors to vibrate as well.
These vibrations—and thus sound—can move through nearly anything: a piano string, a container of air, the ocean, and even our Milky Way galaxy. Sound waves in the air have a special characteristic: air molecules vibrate only in the direction in which the wave is traveling.
This is called a longitudinal wave. In contrast, waves moving through solid objects, such as the Earth, can also have vibrations perpendicular to the direction of travel (transverse waves).
Sound, music, noise The concepts of wave physics find their counterparts in acoustics: for example, a sound wave of a particular frequency is perceived as a specific tone. The amount of energy carried by a sound wave is registered by the listener as loudness or volume. Whether a sound is heard as music, speech, noise, or static depends on the various sine waves that combine to make up the sound.
The term “tone color” is used to describe this phenomenon. Sine waves with frequencies related to each other in simple ratios—such as 1 :2, 2:3, or 3:5-are perceived as harmonious. Frequency relationships such as 4:17 or 97:111, on the other hand, are less pleasant to the ear. If many different frequencies with similar energy levels are emitted at the same time, we hear static or “white noise.”
The speed of sound depends on the substance through which it is moving, but it is usually greater than 621 miles per hour (1,000 km/h). This is still much lower than the speed of light, however, as is evident when we see a flash of lightning long before the sound of the thunder arrives.
CREATING TONES WITH MUSICAL INSTRUMENTS
In stringed musical instruments (such as the violin, guitar, or piano), sounds are produced when metal strings are set in motion, vibrating separately from each other. The frequency of the vibration—and thus the particular note produced—depends on the length of the string.
With brass and woodwind instruments, on the other hand, the musician’s blowing action causes vibrations in a column of air inside the instrument. By pressing different valves, the player can change the length of the air passages and thus the pitch of the note.