Physics » Mechanical Waves and Sound » Summary of Sound

Summary and Main Ideas


  • Sound waves are longitudinal waves

  • The frequency of a sound is an indication of how high or low the pitch of the sound is.

  • The human ear can hear frequencies from 20 to \(\text{20 000}\) \(\text{Hz}\). Infrasound waves have frequencies lower than \(\text{20}\) \(\text{Hz}\). Ultrasound waves have frequencies higher than \(\text{20 000}\) \(\text{Hz}\).

  • The amplitude of a sound determines its loudness or volume.

  • The tone is a measure of the quality of a sound wave.

  • The speed of sound in air is around \(\text{340}\) \(\text{m·s$^{-1}$}\). It is dependent on the temperature, height above sea level and the phase of the medium through which it is travelling.

  • Sound travels faster when the medium is hot.

  • Sound travels faster in a solid than a liquid and faster in a liquid than in a gas.

  • Sound travels faster at sea level where the air pressure is higher.

  • The intensity of a sound is the energy transmitted over a certain area. Intensity is a measure of frequency.

  • Ultrasound can be used to form pictures of things we cannot see, like unborn babies or tumours.

  • Echolocation is used by animals such as dolphins and bats to “see” their surroundings by using ultrasound.

  • Ships use sonar to determine how deep the ocean is or to locate shoals of fish.

Table: Units used in sound

Physical Quantities


Unit name

Unit symbol

Velocity (v)

metre per second


Wavelength (λ)



Amplitude (A)



Period (T)



Frequency (f)


Hz (\(\text{s$^{-1}$}\))

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