Sound

Production of sound

Sound is produced by vibrating objects. A vibration is a rapid to-and-fro motion. When you pluck a stretched rubber band or the string of a guitar, you can see it shake; when you strike a tuning fork and touch it to water, the water splashes — proof that the fork is vibrating. In humans, sound is produced by the vocal cords in the voice box (larynx); in a drum, by the stretched membrane; in a flute, by a vibrating column of air. The moment the vibration stops, the sound stops too. So the simple rule is: no vibration, no sound.

Propagation of sound

The vibrating object pushes the particles of the surrounding medium (usually air). These particles bump into their neighbours, which bump into the next, and so the disturbance travels outward as a wave. It is very important to understand that the particles of the medium do not travel with the sound — they only vibrate about their fixed positions and pass the energy along, like a row of people passing buckets. The matter that carries sound from one place to another is called the medium. Because sound needs a medium, it is called a mechanical wave.

Sound needs a medium — the bell-jar experiment

An electric bell is hung inside a sealed glass jar connected to a vacuum pump. At first the ringing bell is heard clearly. As the pump removes the air, the sound becomes fainter and fainter, and when the jar is almost empty the sound can no longer be heard, even though we can still see the hammer striking. This proves that sound cannot travel through a vacuum. This is why astronauts on the airless Moon cannot talk directly and must use radios.

Longitudinal and transverse waves

Sound travels as a longitudinal wave: the particles of the medium move back and forth parallel to the direction in which the wave travels. As they crowd together they form regions of high pressure and density called compressions (C), and as they spread apart they form regions of low pressure and density called rarefactions (R). In a transverse wave (like a wave on a string or water surface), particles move perpendicular to the direction of travel, forming crests and troughs. Sound in air is always longitudinal.

Characteristics of a sound wave

A sound wave is described by a few key quantities. Wavelength (λ) is the distance between two consecutive compressions or two consecutive rarefactions; its unit is the metre (m). Frequency (f or ν) is the number of complete vibrations (oscillations) made per second; its unit is the hertz (Hz). Time period (T) is the time taken for one complete vibration, and it is the reciprocal of frequency: T = 1 ÷ f. Amplitude (A) is the maximum displacement of a particle from its rest position. The speed of the wave is v = λ ÷ T = f × λ.

Pitch, loudness and quality

Three things let us tell sounds apart. Pitch tells how shrill or flat a sound is and depends on frequency — higher frequency means higher pitch (a whistle is high-pitched, a lion’s roar is low-pitched). Loudness depends on the amplitude of the wave — a larger amplitude means a louder sound; loudness is measured in decibels (dB). Quality or timbre is what lets us recognise different instruments playing the same note — a flute and a violin sound different even at the same pitch and loudness.

Speed of sound in different media

The speed of sound depends on the medium and its temperature. Sound travels fastest in solids, slower in liquids, and slowest in gases, because particles in solids are packed closely and pass on the vibration quickly. In air at 22°C the speed of sound is about 344 m/s, in water about 1500 m/s, and in steel about 5960 m/s. The speed of sound in a gas increases with temperature — that is why sound travels a little faster on a hot day.

Reflection of sound and echo

Sound bounces off hard surfaces just as light bounces off a mirror; this is the reflection of sound. It obeys the same laws: the angle of incidence equals the angle of reflection, and both rays and the normal lie in the same plane. An echo is the sound we hear when a sound is reflected back from a distant surface. Our brain holds a sound for about 0.1 second (the persistence of hearing), so to hear a clear, separate echo the reflected sound must reach us at least 0.1 s after the original. Using speed 344 m/s, the minimum distance to the reflecting surface must be about 17.2 m (because the sound travels there and back, 2 × 17.2 = 34.4 m in 0.1 s). Repeated reflections cause reverberation, the lingering of sound in a hall, which is reduced by covering walls and ceilings with sound-absorbing materials.

Uses of multiple reflection and the range of hearing

Multiple reflection of sound is used in megaphones, horns, stethoscopes, soundboards behind stages, and curved ceilings of concert halls. The human ear can hear sounds in the audible range of 20 Hz to 20,000 Hz. Sound below 20 Hz is called infrasound (made by elephants, whales and during earthquakes) and sound above 20,000 Hz is called ultrasound. Ultrasound has many uses: cleaning hidden parts, detecting cracks in metals, ultrasound scans of the body, and breaking small kidney stones. Bats and dolphins navigate using ultrasonic echoes (echolocation).

SONAR

SONAR stands for SOund NAvigation And Ranging. It sends ultrasonic waves into water and detects the reflected waves from underwater objects such as the sea-bed, submarines, shoals of fish, or sunken ships. By measuring the time the pulse takes to return and using the speed of sound in water, the distance is found by d = (v × t) ÷ 2 (dividing by 2 because the sound travels to the object and back).