Sound

Sound is a form of energy that produces the sensation of hearing in our ears. Every sound, without exception, is produced by a vibrating object. A vibration is a rapid to-and-fro (back-and-forth) motion of an object about its rest position. When you pluck a stretched rubber band, strike a drum, or speak, something is vibrating — the rubber band, the drum skin, or the vocal cords in your throat. If you touch your throat gently while humming, you can feel these vibrations. When the vibration stops, the sound stops too.

Once produced, sound must travel from its source to our ears. This travelling of sound is called the propagation of sound. Sound travels as a wave, and sound waves are longitudinal waves, meaning the particles of the medium vibrate back and forth in the same direction in which the sound travels. The vibrating object pushes the particles next to it together (a compression) and then pulls back, leaving them spread apart (a rarefaction). This pattern of compressions and rarefactions passes from particle to particle, carrying the sound forward, while the particles themselves only vibrate about their fixed positions.

A crucial fact about sound is that it needs a material medium — a solid, liquid, or gas — to travel through. Sound cannot travel through a vacuum (a space with no particles), because there are no particles to pass the vibration along. This is famously shown by the bell-jar experiment: an electric bell ringing inside a sealed glass jar grows fainter and finally becomes silent as the air is pumped out, even though the hammer is still seen striking. This proves that without a medium, sound cannot reach us — which is why space is completely silent.

Sounds differ from one another in several ways — some are loud, some soft; some shrill, some deep; and we can tell a flute from a guitar even when they play the same note. These differences are described by the characteristics of sound, which depend on the properties of the sound wave.

The first characteristic is loudness, which is how loud or soft a sound seems. Loudness depends on the amplitude of the vibration — the maximum distance a particle moves from its rest position. A large amplitude produces a loud sound, while a small amplitude produces a soft sound. Hitting a drum harder gives it a larger amplitude and a louder sound. Loudness is measured in units called decibels (dB).

The second characteristic is pitch (or shrillness), which is how high or low a sound seems. Pitch depends on the frequency of the vibration — the number of vibrations made in one second, measured in hertz (Hz). A high frequency gives a high-pitched (shrill) sound, like a whistle or a child's voice, while a low frequency gives a low-pitched (deep) sound, like a drum or a man's voice. The time period is the time taken for one complete vibration, and it is the reciprocal of frequency.

The third characteristic is quality (or timbre), which is the feature that lets us distinguish between two sounds of the same loudness and pitch produced by different sources. Quality is why a note played on a violin sounds different from the same note played on a piano, and why we can recognise a friend's voice on the phone without seeing them. So, in summary: amplitude decides loudness, frequency decides pitch, and quality decides the character of the sound.

Sound takes time to travel from its source to our ears, which means it has a definite speed. The speed of sound depends strongly on the medium it travels through. Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because the particles in a solid are tightly packed and can pass on vibrations very quickly, while the particles in a gas are far apart and pass them on slowly. In air at room temperature, the speed of sound is about 340 metres per second — far slower than the speed of light. This difference is why, in a thunderstorm, we see the lightning flash before we hear the thunder, even though both happen at the same time: light reaches us almost instantly, but sound takes longer to arrive.

Human ears can only hear sounds within a certain range of frequencies, called the audible range, which is from about 20 Hz to 20,000 Hz (20 kHz). Sounds outside this range exist but cannot be heard by a normal human ear. Sounds with frequencies below 20 Hz are called infrasound (infrasonic); some animals such as elephants, whales, and rhinoceroses produce and detect infrasound, and it is also produced before earthquakes. Sounds with frequencies above 20,000 Hz are called ultrasound (ultrasonic); animals such as bats, dolphins, and dogs can hear ultrasound, and bats use it to navigate and catch insects in the dark.

Ultrasound has many important uses for humans. It is used in medical scans (ultrasonography) to see inside the body, including images of an unborn baby; to clean delicate machine parts and jewellery; to detect cracks and flaws in metal blocks; and in SONAR, which ships use to measure the depth of the sea and locate underwater objects by sending out ultrasound and timing its echo.

Just as light bounces off surfaces, sound also bounces back when it strikes a hard obstacle such as a wall, a cliff, or a tall building. This bouncing back of sound is called the reflection of sound. Like light, reflected sound obeys the laws of reflection: the angle at which the sound strikes the surface equals the angle at which it bounces off. Sound reflects best from hard, smooth surfaces (like a brick wall) and is mostly absorbed by soft, rough surfaces (like curtains, carpets, and cushions).

When a reflected sound is heard as a distinct, separate repetition of the original sound, it is called an echo. You may have heard an echo when shouting in a large empty hall, near a hill, or in a deep well. For an echo to be heard clearly, the reflecting surface must be far enough away. This is because our brain can distinguish two sounds as separate only if they reach our ears at least 0.1 second apart. Since sound travels at about 340 m/s in air, the sound must travel to the obstacle and back in at least 0.1 second, which means the obstacle must be at least about 17 metres away (the sound covers about 34 m in total). If the wall is closer than this, the echo merges with the original sound and is not heard separately.

A related effect is reverberation, which is the persistence of sound caused by repeated, overlapping reflections inside a closed space such as a large hall. Instead of a single clear echo, the sound bounces many times off the walls, ceiling, and floor, so it seems to linger and blur. Too much reverberation makes speech and music in a hall sound muddled. To reduce it, the walls and ceilings of auditoriums, cinema halls, and recording studios are covered with sound-absorbing materials such as soft boards, curtains, foam, and carpets, and the ceilings are often curved to spread sound evenly.

Reflection of sound also has useful applications, such as the megaphone (loudhailer), which channels sound in one direction, the stethoscope, which carries the sounds of the heart and lungs to the doctor's ears by multiple reflections, and the sound boards placed behind a stage to reflect sound towards the audience.

Not all sounds are pleasant. Sounds can be broadly divided into two kinds. A musical sound is pleasant to hear and is produced by regular, smooth vibrations — for example, the sound of a flute, a sitar, or a song. A noise is unpleasant and harsh to the ear and is produced by irregular vibrations — for example, the blaring of horns, machines in a factory, or a slamming door. The presence of excessive or unwanted sound in the environment is called noise pollution.

Noise pollution comes from many sources: vehicle horns and engines, aeroplanes, loudspeakers at functions and during festivals, construction work and machinery, factory equipment, household appliances such as mixers and washing machines, and bursting crackers. In crowded cities, these sources combine to create constant high levels of noise.

Continuous exposure to loud noise has serious harmful effects on health. It can cause hearing-related problems and even permanent loss of hearing if the sound is very loud or prolonged. It also causes a range of other problems: lack of sleep, headache, high blood pressure, anxiety, irritability, stress, and difficulty in concentrating. Sound levels above about 80 decibels become uncomfortable, and exposure to very high levels can damage the ears. Children, the elderly, and patients are especially affected by noisy surroundings.

Fortunately, noise pollution can be reduced through several control measures. Loud activities should be kept away from homes, schools, and hospitals; silent zones are marked around such places. Vehicle horns should be used sparingly, and machinery should be fitted with silencers. Planting trees along roadsides and around buildings helps, because plants absorb sound. Using sound-absorbing materials in buildings, keeping appliances well maintained, and following rules that limit the use of loudspeakers and crackers all help to keep the environment peaceful. Reducing noise pollution protects both our hearing and our overall well-being.