Sound and Hearing

Sound — a longitudinal wave produced by a vibrating object, travelling as compressions and rarefactions through a medium.

Sound needs a medium. It cannot travel through a vacuum, because there are no particles to compress.

The human hearing range is approximately 20 Hz to 20 000 Hz (20 kHz). The upper limit falls with age, which is why some ringtones are audible to teenagers and not to their teachers.

Ultrasound — sound above 20 kHz, beyond human hearing. Used in medical scanning, cleaning delicate objects and measuring depth by sonar.

Speed of sound in air is about 340 m/s. It travels faster in liquids and faster still in solids, because the particles are closer together and pass the disturbance on more quickly. This is the opposite of light, which slows down in denser media.

What the two properties of the wave do:

• Amplitude determines loudness. A bigger amplitude is a louder sound.
• Frequency determines pitch. A higher frequency is a higher note.

Oscilloscope traces. A microphone converts sound into an electrical signal that an oscilloscope displays as a transverse-looking wave — a convenient picture, not a claim that sound is transverse. A louder sound gives a taller trace; a higher-pitched sound gives waves that are closer together.

Measuring the speed of sound. Stand a measured distance from a large wall, make a sharp sound, and time the echo's return. The sound has travelled twice the distance to the wall:

speed = 2 × distance to wall / time for echo

Forgetting the factor of two halves the answer, and is the usual mistake. Timing several echoes and averaging reduces the effect of reaction time, which is the dominant error.

Example: a clap 85 m from a cliff returns after 0.50 s. The sound covered 170 m in that time, so the speed is 170 ÷ 0.50 = 340 m/s. Using 85 m alone would give 170 m/s, which is half the true value.