Sound and Waves
Sound and Waves tests wave terminology, the conditions for sound propagation and echo calculations. The key is to separate frequency, amplitude and speed and to use the echo distance correctly.
Wave basics
- A wave transfers energy without transferring matter. Sound is a longitudinal mechanical wave: particles vibrate along the direction of propagation.
- Frequency f is the number of vibrations per second (hertz); wavelength lambda is the distance between successive compressions; speed v = f x lambda.
- Amplitude controls loudness; frequency controls pitch; waveform or quality distinguishes sounds of the same pitch and loudness.
Sound propagation
- Sound needs a material medium; it cannot travel through vacuum. It travels faster in solids than in liquids and faster in liquids than in gases under ordinary conditions.
- Audible range for a normal human ear is about 20 Hz to 20,000 Hz. Sound above 20 kHz is ultrasound; sonar uses ultrasound echoes underwater.
- An echo is reflected sound. If a sound travels to a reflector and back in time t, then 2d = v x t, so d = vt/2.
Worked example
A person claps and hears an echo after 0.8 s. If the speed of sound is 340 m/s, find the distance to the reflecting wall. The sound travels to the wall and back, so total distance = 340 x 0.8 = 272 m. The one-way distance d = 272/2 = 136 m.
Traps
- Forgetting to divide the echo distance by 2.
- Treating frequency as loudness; amplitude is the loudness factor.
- Claiming sound travels through vacuum because light does.
- Using v = f x lambda with frequency in kHz without converting to Hz.