Sound

Sound is a form of energy that produces the sensation of hearing in our ears. Every sound we hear — speech, music, a ringing bell, a barking dog — has one thing in common at its source: vibration. Sound is produced by vibrating objects. A vibration is a rapid to-and-fro (back-and-forth) motion. When an object vibrates, it pushes and pulls the surrounding particles of the medium, sending the disturbance outward as sound. If the vibration stops, the sound stops too.

We can see and feel these vibrations in many ways. When a stretched rubber band or a guitar string is plucked, it vibrates rapidly and produces sound. When a drum or a tabla is struck, its stretched membrane vibrates. A tuning fork vibrates when struck, and if its tip is touched to water, it splashes the water, showing the vibration. In humans, sound is produced by the vocal cords in the throat — when air from the lungs passes over them, they vibrate to make our voice. In all these cases, a vibrating object is the source of sound.

Once produced, sound must travel (propagate) from the source to our ears. Sound travels in the form of waves through a medium (a material such as a solid, liquid, or gas). As the source vibrates, it makes the particles of the medium next to it vibrate; these pass the vibration to the next particles, and so on, so the disturbance travels outward as a sound wave. Importantly, the particles themselves do not travel along with the sound — they only vibrate back and forth about their positions, passing the energy along.

A crucial fact is that sound needs a medium to travel and cannot travel through a vacuum (empty space with no particles). This is because, without particles, there is nothing to pass the vibration along. This can be shown by the bell-in-a-jar experiment: an electric bell ringing inside a glass jar is heard clearly, but as the air is pumped out of the jar, the sound becomes fainter and finally cannot be heard, even though the bell is still seen vibrating. This proves that sound requires a material medium to propagate — which is why there is silence in the vacuum of outer space.

Not all sounds are alike — some are loud and some soft, some high and some low, and we can recognise different instruments or voices. These differences are described by the characteristics of sound: its loudness, its pitch, and its quality (timbre). Each of these depends on a particular feature of the vibration or sound wave producing it.

Loudness is how loud or soft a sound seems to our ears, and it depends on the amplitude of the vibration. Amplitude is the maximum extent of the to-and-fro movement of the vibrating particles — in other words, how big the vibration is. A larger amplitude (a bigger vibration) produces a louder sound, while a smaller amplitude produces a softer sound. This is why striking a drum harder (making it vibrate more) produces a louder sound. Loudness is measured in units called decibels (dB).

Pitch is how high or low a sound seems, and it depends on the frequency of the vibration. Frequency is the number of vibrations (or waves) produced in one second, measured in hertz (Hz). A higher frequency (more vibrations per second) gives a higher-pitched sound, such as a whistle or a small bell, while a lower frequency gives a lower-pitched sound, such as a drum or a man's deep voice. So pitch and frequency go together: faster vibrations mean higher pitch.

Quality (or timbre) is the characteristic of a sound that lets us tell apart two sounds of the same loudness and pitch coming from different sources. It is the reason we can recognise a flute from a violin, or one person's voice from another, even when they play or sing the same note at the same loudness. Quality depends on the particular mixture of vibrations a source produces. Together, loudness (from amplitude), pitch (from frequency), and quality (timbre) describe every sound we hear and explain the rich variety of sounds in the world around us.

Sound takes time to travel from its source to a listener, because it moves at a definite speed. The speed of sound is the distance the sound travels in one second. Unlike light, which travels almost instantly, sound is much slower — which is why, in a thunderstorm, we see the lightning first and hear the thunder a little later, even though both happen together. The speed of sound depends mainly on the medium through which it travels.

Sound travels at different speeds in different media. It travels fastest in solids, slower in liquids, and slowest in gases — that is, speed in solids > speed in liquids > speed in gases. This is because the particles in a solid are tightly packed and can pass on vibrations quickly, while in a gas the particles are far apart, so the vibration travels more slowly. In air, the speed of sound is about 340 metres per second (340 m/s) at ordinary temperatures. So sound travels roughly one-third of a kilometre each second through air.

When sound strikes a hard surface, it bounces back, just as a ball bounces off a wall. This reflection of sound can produce an echo — the repetition of a sound heard after it reflects off a distant surface like a wall, cliff, or hill. We hear an echo as a distinct, separate sound only if the reflecting surface is far enough away. For our ears to tell the original sound and the echo apart, the reflecting surface should be at least about 17 metres away, so that the reflected sound returns after a short delay; if it is closer, the echo merges with the original sound.

The reflection of sound has important uses. SONAR (Sound Navigation and Ranging) uses sound waves sent into the sea and their echoes from the seabed or objects to measure the depth of the ocean and to locate ships, submarines, and shoals of fish. Ultrasound (very high-frequency sound) is reflected by structures inside the body and used in medical scans to form images of organs and of a baby in the womb. Bats and dolphins also use reflected sound to navigate and find food. So the speed of sound and the reflection of sound (echoes) are not just interesting facts but the basis of valuable technologies.

Although there are many sounds in the world, the human ear can hear only those within a certain range of frequencies. The range of hearing is the band of sound frequencies that a person can hear. For a healthy human, this audible range is from about 20 Hz to 20,000 Hz (20 kHz). Sounds within this range can be detected by our ears; sounds outside it cannot be heard by humans, even though they may exist and may be heard by other animals.

Sounds are classified according to whether they fall within or outside this human audible range. Audible sound is sound with a frequency between 20 Hz and 20,000 Hz, which humans can hear. Infrasonic sound (or infrasound) is sound with a frequency below 20 Hz — too low for humans to hear. Ultrasonic sound (or ultrasound) is sound with a frequency above 20,000 Hz — too high for humans to hear. So infrasonic and ultrasonic sounds lie just below and just above the limits of human hearing, respectively.

Many animals can hear sounds outside the human range. Dogs can hear frequencies higher than humans can, which is why a "silent" dog whistle (which produces ultrasound) is heard by the dog but not by us. Bats produce and hear ultrasonic sounds and use their reflections to find their way and catch insects in the dark, a process called echolocation. Dolphins and porpoises also use ultrasound. Some animals, like elephants and whales, can produce and detect infrasonic sounds, which travel long distances and help them communicate.

Knowing the range of hearing has practical uses. Ultrasound, being beyond human hearing, is used widely in technology and medicine — for cleaning delicate objects, detecting cracks in metals, and in medical scans. The fact that the human audible range also tends to narrow with age (older people often cannot hear the highest frequencies) is part of normal hearing. Understanding audible, infrasonic, and ultrasonic sound helps us appreciate both the limits of our own hearing and the remarkable abilities of other animals.

Sounds can be pleasant or unpleasant. A musical sound is pleasant to hear, while a noise is an unpleasant, harsh, or unwanted sound. When unwanted or excessively loud sound becomes harmful to people and the environment, it is called noise pollution. Noise pollution is the presence of excessive or unwanted sound in the surroundings, and it has become a serious problem, especially in busy cities, because of growing traffic, industry, and machinery.

Noise pollution comes from many sources. Major sources include the horns and engines of vehicles (cars, buses, trucks, motorcycles), aeroplanes and trains, the machines in factories and at construction sites, loudspeakers at functions and during festivals, and loud household appliances like mixers and generators. Even very loud music played through headphones can harm hearing. In crowded areas, all these combine to create a constantly high level of noise.

Noise pollution has harmful effects on health. Constant or very loud noise can cause loss of hearing (especially with long exposure to loud sounds), and it can lead to stress, irritation, anxiety, and lack of sleep. It makes it hard to concentrate or rest, can raise blood pressure, and can cause headaches and tiredness. Children's learning and people's mental wellbeing can be affected. So noise pollution harms not only our ears but our overall health.

Fortunately, noise pollution can be controlled and reduced in several ways. We can avoid unnecessary use of vehicle horns and keep machines well-maintained to run quietly. Loudspeakers and loud music should be kept at a reasonable volume and not used late at night. Planting trees along roads and around buildings helps absorb sound. Noisy factories and airports should be located away from homes and schools, and soundproofing can be used in buildings. By being considerate about the noise we make and supporting these measures, everyone can help reduce noise pollution and protect health and peace. This completes our study of sound — a form of energy that, like all things, must be used wisely.