Sound is produced by vibrating bodies. It is a mechanical, longitudinal wave and needs a medium (solid, liquid or gas); it cannot travel in a vacuum.
Longitudinal wave: particles vibrate parallel to the direction of the wave, forming compressions (high pressure) and rarefactions (low pressure).
Transverse wave: particles vibrate perpendicular to the wave (light, waves on a string) with crests and troughs.
Sound is a longitudinal wave in air.
Characteristics of a wave
Term
Meaning
Unit
Frequency (f)
Vibrations per second
hertz (Hz)
Time period (T)
Time for one vibration; T = 1 / f
second
Wavelength
Distance between two successive compressions or crests
metre
Amplitude
Maximum displacement from the mean position
metre
Speed
v = f x wavelength
m/s
Characteristics of sound
Property
Depends on
Notes
Loudness
Amplitude
Measured in decibel (dB). Normal talk about 60 dB; above 120 dB is painful
Pitch
Frequency
High frequency = high pitch (shrill); low = grave. A woman's voice has a higher pitch than a man's
Quality (timbre)
Waveform
Lets us tell two instruments or voices apart
Speed of sound
Fastest in solids, then liquids, slowest in gases: solid > liquid > gas.
In air at 25 degrees C: about 346 m/s (about 331 m/s at 0 degrees C). It rises with temperature.
In water about 1500 m/s; in iron about 5000 m/s.
Light is far faster (3 x 10^8 m/s), so lightning is seen before thunder is heard.
Range of hearing
Range
Frequency
Examples
Infrasonic
Below 20 Hz
Earthquakes, whales, elephants, rhinoceros
Audible
20 Hz to 20,000 Hz (human)
Speech, music
Ultrasonic
Above 20,000 Hz
Bats, dolphins, dogs (up to about 50,000 Hz), porpoises
Reflection of sound
Echo: sound heard again after reflection. The reflecting surface must be at least 17 m away (for a delay of 0.1 s, since the ear keeps a sound for 0.1 s).
Reverberation: repeated reflections in a big hall, reduced by sound-absorbing materials (curtains, carpets, acoustic tiles).
Uses of reflection:megaphone, stethoscope, soundboards, curved ceilings in halls.
SONAR (Sound Navigation and Ranging): uses ultrasonic waves to measure depth, locate submarines, shoals of fish. Distance = (speed x time) / 2.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.