How vibrations travel as sound and how we hear them
Sound is produced by vibrating objects, such as a guitar string, a drum skin or our vocal cords. It travels as a longitudinal wave of compressions and rarefactions and needs a medium, so it cannot pass through a vacuum. Sound travels fastest in solids, slower in liquids and slowest in gases; in air it moves at about 340 m/s. Humans can hear frequencies from about 20 Hz to 20,000 Hz.
Frequency decides pitch: higher frequency gives a shriller sound. Amplitude decides loudness, measured in decibels. Wave speed, frequency and wavelength are linked by v = fλ. When sound reflects from a distant surface we hear an echo, which needs the reflecting surface to be at least about 17 m away in air. Repeated reflections in a hall cause reverberation, reduced by curtains and soft panels. Sounds above 20 kHz are ultrasound and below 20 Hz are infrasound.
Why it matters
Doctors use ultrasound to see babies before birth and to examine organs, and ships use SONAR to measure ocean depth and find shoals of fish. Concert halls are designed to control echoes, and noise pollution limits protect hearing. Musical instruments and speakers rely on frequency and amplitude, and wave ideas here lead to oscillations and waves in Class 11.
Key terms
- Frequency
- The number of vibrations or waves produced per second, measured in hertz.
- Amplitude
- The maximum disturbance of a wave from its rest position, which decides loudness.
- Echo
- A sound heard again after it reflects from a distant surface.
- Ultrasound
- Sound waves with frequencies above the human hearing limit of 20,000 Hz.