Sound

Sound is a form of energy that produces a sensation of hearing in our ears. Every sound we hear, from a plucked guitar string to a clapping hand, is produced by a vibrating object. Vibration means a rapid to-and-fro (back and forth) motion of an object about its mean position.

You can feel this directly. Touch your throat gently while speaking and you will feel your vocal cords vibrating. Stretch a rubber band between two fingers and pluck it — you both hear a sound and see it vibrate. Strike a tuning fork on a rubber pad and touch it to the surface of water in a beaker; the water splashes because the vibrating prongs disturb it. If the same tuning fork, still vibrating, is touched to a suspended table-tennis ball hung by a thread, the ball is repeatedly pushed away, showing that the fork's prongs are moving rapidly.

When a body vibrates, it sets the particles of the medium around it (usually air) into vibration too, and this disturbance travels outward as sound. So the source of every sound is a vibrating object, and the amount of vibration is what eventually decides how loud or soft the sound is.

A vibrating object creates a disturbance in the medium around it, and it is this disturbance, not the matter of the medium itself, that travels as sound. The particles of the medium do not move all the way from the source to your ear; each particle vibrates about its own mean position and passes the disturbance on to its neighbour, like a row of collapsing dominoes.

Sound therefore needs a material medium to travel — solid, liquid or gas. It cannot travel through vacuum, because there are no particles there to vibrate and carry the disturbance forward. This is shown by the classic bell-in-a-jar experiment: an electric bell is suspended inside an airtight bell jar connected to a vacuum pump. As the bell rings, its sound is heard clearly. As air is pumped out of the jar, the sound becomes fainter and fainter, and once nearly all the air is removed the sound becomes inaudible, even though the hammer of the bell can still be seen striking the gong. This proves that air (a medium) is essential for sound to travel; it also proves sound cannot travel through vacuum, which is why astronauts on the Moon, which has no atmosphere, cannot talk to each other directly through the air and instead use radio waves.

Sound generally travels fastest in solids, slower in liquids and slowest in gases, because particles are packed most tightly in solids, allowing the disturbance to be passed on to the next particle more quickly.

When a tuning fork or any vibrating source moves forward into the air, it pushes the air particles close to it, crowding them together into a region of higher density and pressure. This region is called a compression (C). When the source then moves backward, it creates a region where the particles are spread further apart, of lower density and pressure, called a rarefaction (R). As the source keeps vibrating, it produces a continuous series of alternating compressions and rarefactions that move outward through the medium, and this moving pattern is the sound wave.

In this wave, the particles of the medium vibrate back and forth along the same direction in which the wave itself travels (parallel to it), not perpendicular to it. A wave in which the particles of the medium vibrate parallel to the direction of propagation of the wave is called a longitudinal wave. Sound is always a longitudinal wave when it travels through air or any fluid. (Contrast this with a transverse wave, such as a ripple on water or the wave on a plucked string, where particles vibrate perpendicular to the direction the wave travels — sound in a stretched string or the surface of a solid can show transverse components, but sound in air and other fluids is purely longitudinal.)

A sound wave can be represented on a graph as pressure or density plotted against distance: compressions appear as crests (regions of high pressure) and rarefactions as troughs (regions of low pressure) even though no actual up-down motion occurs in the medium.

A sound wave, like any wave, is described by four quantities.

Wavelength (λ): the distance between two consecutive compressions or two consecutive rarefactions, measured in metres (m). It equals the distance covered by the wave during one complete vibration of a particle of the medium.

Frequency (ν or f): the number of complete oscillations (compressions and rarefactions) produced per unit time. Its SI unit is the hertz (Hz); 1 Hz means one vibration per second. Frequency is a fixed property of the source and does not change as the wave moves into a new medium.

Time period (T): the time taken by one complete oscillation of density (or pressure) of the medium, measured in seconds. It is the reciprocal of frequency, T = 1/ν.

Amplitude (A): the magnitude of the maximum disturbance of the particles of the medium from their mean (undisturbed) position, on either side, when the wave passes. It is measured in the same unit as displacement of the medium particles.

These combine to give the speed of the wave: distance travelled by the wave in one time period is one wavelength, so speed = wavelength ÷ time period, i.e. v = λ/T = λν, since ν = 1/T. This is the general wave equation and applies to every kind of wave, not only sound.

Loudness of a sound depends on the amplitude of the wave — larger amplitude means louder sound; it is measured in decibel (dB). Pitch (how shrill or how flat a sound is) depends on frequency — higher frequency means a higher pitch (shriller sound). The quality or timbre of sound helps us distinguish two sounds of the same pitch and loudness, e.g. a violin and a flute playing the same note — richer sound is said to have better quality.

The speed of sound is the distance travelled by a sound wave in the medium per unit time. It depends on the nature and the temperature of the medium the wave is passing through, and, unlike frequency, it changes when the wave moves from one medium to another.

In general, the speed of sound is greatest in solids, less in liquids, and least in gases. This is because the particles of a solid are the most closely packed and most strongly bound to their neighbours, so a disturbance is transferred from particle to particle very quickly; particles in gases are the most loosely packed and farthest apart, so the transfer is slowest. For example, the approximate speed of sound at 25 °C is about 346 m/s in air, about 1,498 m/s in water, and about 5,960 m/s in steel — roughly seventeen times faster in steel than in air.

The speed of sound also increases as the temperature of the medium rises, because warmer particles vibrate faster and pass the disturbance on more quickly. This is why thunder from a distant cloud takes a measurable time to reach us, and also why divers or people with an ear to a railway track can hear an approaching train through the rail (a solid) before they hear it through the air.

Sound, like light, gets reflected from a surface. When sound waves strike a large, hard, and smooth surface — a wall, a hill, or a cliff — they bounce back. This obeys the same laws of reflection that apply to light: the angle of incidence equals the angle of reflection, and the incident sound wave, the reflected sound wave, and the normal at the point of incidence all lie in the same plane.

Sound requires a surface, solid or liquid, for reflection; large smooth surfaces of big buildings, cliffs, or mountains work well because they are much larger than the wavelength of sound.

If we hear our own sound after it has been reflected off a large, distant surface, we call it an echo. For an echo to be heard as a separate, distinct sound from the original one, the reflected sound must reach our ear at least 0.1 second after the original sound is heard — this is because a sound sensation persists in our brain for about 0.1 s (this is called the persistence of hearing); if the gap is smaller than this, the original sound and its echo blend into one another and no separate echo is perceived.

Using the speed of sound in air as 344 m/s, the sound must travel out to the reflecting surface and back within at least 0.1 s to be perceived as an echo, so the minimum distance of the reflecting surface from the source (and listener) works out as follows: distance travelled by sound in 0.1 s = v × t = 344 × 0.1 = 34.4 m; since sound covers this distance going out and coming back, the reflecting obstacle must be at least half of this away, i.e. 34.4/2 = 17.2 m. So for an echo to be heard, the minimum distance between the source of sound (and the listener) and the reflecting surface must be about 17.2 m, and this distance would change if the temperature of the air (and hence the speed of sound) changes.

Echoes can be reinforced multiple times if there are several reflecting surfaces at suitable distances, producing multiple, progressively fainter echoes — this is what happens in the rolling sound of thunder, which is due to the successive reflection of sound from a number of clouds.

In a large hall or auditorium, sound reflects repeatedly off the walls, ceiling and floor before it dies out. This persistence of sound, caused by multiple reflections, is called reverberation. A certain amount of reverberation is desirable to make a sound rich, but if it is excessive, successive sounds (or syllables of speech) overlap and blend into one another, making the sound distorted and hard to follow — this is a problem in large empty halls with hard, bare walls.

To reduce excessive reverberation, the roof and walls of the hall are generally covered with sound-absorbent materials, such as compressed fibreboard, rough plaster, or draperies (heavy curtains); the seat materials are also selected on the basis of their sound-absorbing properties. This is why an auditorium looks and feels 'soft' — carpets, cushioned seats, and fabric-covered walls all serve an acoustic purpose, not just a decorative one.

Multiple reflection of sound is deliberately put to use in several devices and structures.

Megaphones and horns (loudhailers), musical instruments such as trumpets and shehnais, and even the human ear's own external structure, are all designed with a shape that guides sound in a particular direction using multiple reflections, so that sound does not spread out in all directions and can be heard clearly over a longer range.

Stethoscopes, used by doctors to listen to sounds produced inside the body (chiefly the heart and lungs), work on the principle of multiple reflection of sound: the sound of the patient's heartbeat reaches the doctor's ears by multiple reflections along the tube of the instrument.

Soundboards — curved, concave surfaces placed behind the stage in concert halls and conference halls — are used to reflect sound evenly towards the audience across the hall, so that people sitting even at the back can hear clearly. The ceilings of large halls and auditoriums are often designed in a curved shape for the same reason, so that sound, after reflection, reaches all corners of the hall.

The human ear is sensitive to only a limited range of frequencies. An average healthy young human ear can detect sound waves with frequencies from about 20 Hz to 20,000 Hz (20 kHz); this is called the audible range. Sensitivity to the upper limit generally decreases with age, so older people may be less able to hear very high-frequency sounds.

Sounds with frequencies below 20 Hz are called infrasonic sound or infrasound. Rhinoceroses can produce and detect infrasound at frequencies as low as approximately 5 Hz; whales, elephants and giraffes also communicate using infrasound over long distances; and infrasound waves are generated during events such as earthquakes even before the shocks arrive, which some animals appear to sense in advance.

Sounds with frequencies above 20,000 Hz are called ultrasonic sound or ultrasound. Bats, dolphins and porpoises can produce and detect ultrasonic waves; bats use it to hunt for prey and navigate in the dark by emitting ultrasonic squeaks and listening for the echo bouncing off obstacles or insects, a technique called echolocation. Dogs can hear ultrasound to a somewhat higher frequency than humans, which is why 'silent' dog whistles work.

Because ultrasonic waves have a high frequency (short wavelength), they can be produced as a narrow, directed beam carrying a lot of energy without spreading out, and are widely used in industry and medicine.

Cleaning: objects with hard-to-reach parts — spiral tubes, odd-shaped machine parts, intricate jewellery — are placed in a cleaning solution and ultrasonic waves are passed through it. The high-frequency vibrations loosen and detach dust, grease and dirt clinging to the object, giving a thorough clean without dismantling it.

Detecting flaws in metal components: ultrasonic waves are passed through a metal casting or machine part. A small internal defect reflects the waves back, revealing its presence and location — a crack that would otherwise remain hidden and could later weaken the structure.

Medical use: ultrasound generates images of internal organs (ultrasonography) to detect abnormalities such as kidney or gallbladder stones, and to observe a foetus during pregnancy; the waves reflect differently off tissues of different densities, and the echoes build an image on screen. High-intensity ultrasound also breaks kidney stones into fine grains that are flushed out in urine, without surgery.

SONAR (covered next) is another major application, used to detect underwater objects and measure sea depth.

SONAR stands for SOund NAvigation Ranging — a device using ultrasonic waves to find the distance, direction and speed of underwater objects such as submarines, shipwrecks and icebergs, and to measure the depth of the sea.

A SONAR system has a transmitter and a detector, installed on a boat or ship. The transmitter sends out ultrasonic waves; these travel through water, strike an object, and reflect back; the detector receives this echo and converts it into an electrical signal, which is interpreted to give the distance and location of the object.

The time between transmitting and receiving the pulse is recorded, and knowing the speed of sound in water, the distance is calculated: the wave covers the source-to-object-and-back path twice, so the one-way distance is half of speed × total time.

Worked example: a SONAR sends a signal and receives its echo from an object 5 s later, the object being 3625 m away. Total distance travelled = 2 × 3625 = 7250 m. Speed = 7250/5 = 1450 m/s.

The human ear converts pressure variations (sound waves) in air into electrical signals the brain interprets as sound. It has three parts: the outer ear, the middle ear and the inner ear.

The outer ear is the pinna, which collects sound and funnels it down the auditory canal to a thin, elastic membrane, the eardrum (tympanic membrane). A compression pushes the eardrum inward; a rarefaction pushes it outward — so the eardrum vibrates in tune with the incoming wave.

The middle ear has three small bones — the malleus (hammer), incus (anvil) and stapes (stirrup) — connected to each other and to the eardrum, which amplify its vibrations before passing them on. The Eustachian tube connects the middle ear to the throat, equalising air pressure on both sides of the eardrum so it can vibrate freely.

The amplified vibrations reach the fluid-filled inner ear through the coiled cochlea, where receptor cells convert the pressure variations into electrical signals, carried to the brain by the auditory nerve, which interprets them as sound.