6.4 Sound

Sound as a longitudinal wave

Sound is a longitudinal pressure wave that travels through a medium as particles compress and rarefy in the same direction the wave moves. It needs a medium (gas, liquid, or solid) and cannot travel through a vacuum. Speed depends on the medium's properties, generally fastest in solids, slower in liquids, slowest in gases.

Frequency, wavelength, and speed

Sound obeys v = fλ, where v is speed of sound in the medium, f is frequency (perceived as pitch), and λ is wavelength. Frequency is set by the source and stays constant across media; speed and wavelength change together when sound enters a new medium. Loudness relates to amplitude (intensity), not frequency.

Beats and the Doppler effect

Beats occur when two waves of slightly different frequencies interfere, producing a beat frequency equal to |f1 − f2|. The Doppler effect describes an observed frequency shift when a source and observer move relative to each other: frequency increases as they approach and decreases as they move apart, due to compression/stretching of wavefronts.

Key terms

Sound wave type

Longitudinal pressure wave requiring a medium; cannot travel through a vacuum.

v = fλ (sound)

Speed equals frequency times wavelength; frequency is set by source and constant across media, so speed and wavelength change together in a new medium.

Beat frequency

f_beat = |f1 − f2|, the periodic loudness variation from interference of two close frequencies.

Doppler effect

Observed frequency shift due to relative motion between source and observer — higher pitch approaching, lower pitch receding.

Speed of sound by medium

Generally fastest in solids, slower in liquids, slowest in gases, due to differences in particle spacing and interactions.

Try an AP-style question

A sound's intensity increases by a factor of 10. How does its intensity level change?

1. It increases by 100 dB
2. It increases by 10 dB
3. It doubles
4. It increases by 1 dB