Sound

Sound is a form of mechanical energy produced by a vibrating body. When a body vibrates, it pushes and pulls the surrounding medium (usually air), creating disturbances that travel outward as a wave.

When a tabla is struck in a classical music performance, the stretched skin vibrates rapidly. This vibration disturbs the air molecules next to the skin, which in turn disturb the next layer of molecules, and the disturbance travels outward as a sound wave.

Sound requires a material medium — solid, liquid, or gas — to travel. It cannot travel through vacuum. This is why the ticking of a clock placed inside a glass jar from which air has been removed cannot be heard, even though you can see the clock's hands moving.

Sound travels as a longitudinal wave. In a longitudinal wave, the particles of the medium vibrate back and forth in the same direction as the wave travels.

This creates alternating regions of higher pressure (compressions) and lower pressure (rarefactions). In a compression, air molecules are closer together. In a rarefaction, they are spread apart.

These pressure variations travel outward from the source at the speed of sound and eventually reach our ears, causing the eardrum to vibrate and producing the sensation of sound.

Frequency is the number of complete vibrations (cycles) per second. The SI unit is hertz (Hz). High frequency gives a high-pitched sound; low frequency gives a deep, low-pitched sound. The high pitch of a flute and the deep beat of a dhol differ mainly because of their different frequencies.

Wavelength is the distance between two consecutive compressions or between two consecutive rarefactions. It is related to frequency and wave speed.

Amplitude is the maximum displacement of a medium particle from its mean position during one vibration. A loud sound has higher amplitude than a soft sound of the same frequency. The loud beat of a dhol has higher amplitude than a gentle knock on a door.

The wave equation connects the three fundamental quantities of a wave: speed, frequency, and wavelength.

Speed of sound tells us how fast the wave pattern of compressions and rarefactions travels through the medium. For a fixed medium and temperature, sound speed is constant.

Sound travels at different speeds in different media. In general, it is fastest in solids, slower in liquids, and slowest in gases. In solids, particles are tightly packed and can transfer vibrations very quickly.

In air at 25 °C, the speed of sound is about 346 m/s. In water it is about 1500 m/s, and in steel it is about 5100 m/s.

You can observe this near a railway track. If you press your ear against an iron rail, you hear an approaching train long before you can hear it through the air. The iron rail transmits the vibrations of the train much faster than air does.

The speed of sound in air also increases with temperature. Sound travels slightly faster on a hot summer afternoon than on a cold winter morning.

Loudness depends on the amplitude of the sound wave. Larger amplitude means more energy and louder sound. Loudness is measured in decibels (dB). Noise pollution in Indian cities is a public health concern — the permissible limit near residences is 55 dB during the day.

Pitch is the quality of sound that depends on its frequency. A high frequency gives high pitch (like a woman's voice or a flute). A low frequency gives low pitch (like a man's voice or a bass drum).

Quality (timbre) allows us to distinguish between two sounds of the same loudness and pitch produced by different instruments. The sound of a sitar and a veena playing the same note at the same loudness still sound different because of their different overtones.

When sound strikes a hard surface, it bounces back. This is called reflection of sound. Like light, sound obeys the law of reflection: the angle of incidence equals the angle of reflection.

Hard, smooth, and large surfaces reflect sound well. Walls, rocky hillsides, cliff faces, and buildings all reflect sound. Soft and porous surfaces (like curtains, carpets, and foam panels) absorb sound rather than reflecting it.

Reflection of sound is used in many devices and designs. Concert halls and sabha mandaps in India are designed so that reflected sound reaches every corner of the hall clearly.

An echo is a reflected sound that reaches the listener at least 0.1 seconds after the original sound. The human ear can distinguish two sounds as separate only if there is at least a 0.1-second gap between them.

For this 0.1-second gap, the sound must travel to the reflecting surface and back in 0.1 s. At 344 m/s, this means the reflecting surface must be at least about 17.2 m away from the source.

Travellers who visit the Ajanta and Ellora caves in Maharashtra, the Gol Gumbaz in Bijapur, or rocky valleys in the Himalayas often notice clear echoes. The stone walls and valley sides act as efficient reflectors of sound.

In a closed hall or room, sound can reflect multiple times from walls, ceiling, and floor. Each reflection produces another, and the sound persists for some time after the original source stops. This prolonged persistence of sound is called reverberation.

Too much reverberation makes speech unclear. A speaker's words overlap with earlier reflections, creating confusion. In large marriage halls, temple mandaps, and school auditoriums in India, excessive reverberation is a common problem.

To reduce reverberation, sound-absorbing materials are used: curtains, carpets, padded seats, perforated tiles, and false ceilings. Rough walls scatter and absorb more sound than smooth walls.

Many devices and structures use the principle of reflection of sound to direct or amplify it.

A stethoscope uses multiple internal reflections to guide the sound of the patient's heartbeat and breathing through the tube to the doctor's ears. Doctors examining patients in clinics and hospitals all over India rely on this device.

Megaphones and loudspeakers are shaped to direct sound forward by reflection, making the voice travel farther without spreading in all directions. A curved reflector behind the speaker on a stage in an outdoor event amplifies and directs the sound toward the audience.

In cinema halls and auditoriums, the ceiling and walls are curved to ensure reflected sound reaches every seat. This is why the sound is clear even in the last row.

The normal human ear can hear sounds in the frequency range of about 20 Hz to 20,000 Hz (20 kHz). This is called the audible range or range of hearing.

Sounds below 20 Hz are called infrasound. Earthquakes produce infrasound, which animals like elephants and dogs can sense before we do. Sounds above 20,000 Hz are called ultrasound.

The ability to hear high frequencies decreases with age. Young children often hear up to 25 kHz, while older adults may only hear up to 15 kHz. This is why older people often find it hard to hear the high-pitched beeps of electronic devices.

Ultrasound is sound with frequency greater than 20,000 Hz, above the upper limit of human hearing. Bats and dolphins use ultrasound for echolocation — they emit high-frequency pulses, which reflect off objects, and by detecting the reflected signal they can navigate and hunt in total darkness or murky water.

In India, ultrasound scanners (sonography machines) are widely used in hospitals and diagnostic centres to examine internal organs, check on a developing foetus, and detect tumours without surgery. The sound waves reflect differently off tissues of different densities, creating an image.

Industries use ultrasound to detect cracks inside large metal parts such as railway tracks, aircraft components, and large pipes — without cutting them open. Ultrasound is also used to clean intricate machinery parts by sending high-frequency vibrations through a cleaning liquid.

SONAR stands for Sound Navigation and Ranging. It is a technique that uses ultrasound to detect and measure the position of underwater objects.

A SONAR device emits a pulse of ultrasound toward the sea bed or a target. The pulse reflects off the target and the reflected pulse is detected. By measuring the time taken for the pulse to travel to the target and return, and knowing the speed of sound in water, the distance can be calculated.

The Indian Navy uses SONAR systems to detect submarines and underwater obstacles in the waters surrounding the Indian subcontinent. SONAR is also used to map the ocean floor, locate shoals of fish, and study underwater geography.

The human ear converts sound waves into electrical nerve signals that the brain interprets as sound. It has three parts: the outer ear, the middle ear, and the inner ear.

The outer ear (pinna) collects sound waves and directs them into the ear canal toward the eardrum (tympanic membrane). The eardrum is a thin membrane that vibrates when sound waves reach it.

In the middle ear, three tiny bones — the malleus (hammer), incus (anvil), and stapes (stirrup) — amplify the vibrations and transmit them to the inner ear through the oval window.

The inner ear contains the cochlea, a fluid-filled spiral structure. Vibrations cause waves in the cochlear fluid, which stimulate tiny hair cells. These cells convert mechanical vibrations into electrical signals, which are carried by the auditory nerve to the brain.

The Eustachian tube connects the middle ear to the back of the throat, equalising air pressure on both sides of the eardrum. You feel this when your ears "pop" as an aeroplane gains altitude or as you drive through the mountain passes of Himachal Pradesh.

Sound: mechanical longitudinal wave; needs medium; cannot travel in vacuum.

Wave equation: . Time period: .

Sound fastest in solids > liquids > gases. In air ≈ 344 m/s at 25 °C.

Loudness ← amplitude. Pitch ← frequency. Quality ← waveform.

Echo: reflected sound heard separately; reflector must be ≥ 17 m away.

Reverberation: multiple overlapping reflections in enclosed space.

Ultrasound: frequency > 20,000 Hz. Used in sonography, flaw detection, SONAR.

SONAR distance: . Human ear: pinna → cochlea → auditory nerve.