Sound: How We Hear the World

Press your fingertips lightly against your throat and hum. You can feel a buzzing, and that buzzing is the source of the sound you hear. Every sound, from a whisper to a thunderclap, begins with something vibrating. The vibration pushes on the surrounding air, the push travels outwards as a wave, and the wave eventually makes your eardrum vibrate too. This article follows that whole journey, from the vibrating source to the signal that your brain recognises as speech or music.

1.What Sound Is

Sound is a wave created by a vibrating object. When a guitar string, a loudspeaker cone or your vocal cords move back and forth, they push and pull on the nearby air particles. Those particles do not travel across the room. Each one only moves a tiny distance back and forth around its resting position, but it bumps into its neighbour and passes the disturbance along, like a row of people passing a message down a line while staying in their places.

Sound is a longitudinal wave. This means the particles vibrate parallel to the direction in which the wave travels. (In a transverse wave, such as a wave on a rope or a ripple on water, the vibration is at right angles to the direction of travel.) A longitudinal wave is made of alternating regions:

• Compressions: places where particles are squeezed close together, so the pressure is slightly higher than normal.
• Rarefactions: places where particles are spread further apart, so the pressure is slightly lower than normal.

Sound is also a mechanical wave: it carries energy from one place to another by passing vibrations through matter, not by moving the matter itself.

2.Why Sound Needs a Medium

Because sound is passed from particle to particle, it needs a medium (a substance to travel through). It can travel through gases, liquids and solids. It cannot travel through a vacuum, which is empty space with no particles. A well-known demonstration places a ringing bell inside a sealed jar and slowly pumps the air out: the sound grows fainter and fainter, although the bell can still be seen vibrating. This is also why, in the near-vacuum of outer space, there is no sound that travels from one spacecraft to another; astronauts communicate by radio, which uses electromagnetic waves, not sound.

Generally, sound travels fastest in solids, more slowly in liquids and slowest in gases, because the particles in a solid are closer together and spring back more quickly.

3.Frequency, Wavelength and Amplitude

Any regular wave can be described with a small set of quantities:

These are linked by the wave equation:

4.Pitch, Loudness and Quality

Three features of a sound are easy to notice:

• Pitch is how high or low a note sounds. It depends mainly on frequency: a high frequency gives a high-pitched sound such as a whistle; a low frequency gives a low-pitched sound such as a drum.
• Loudness depends mainly on amplitude: a larger amplitude carries more energy and sounds louder. Pitch does not change when you turn up the volume.
• Quality (also called timbre) is what makes a piano and a violin sound different even when they play the same note at the same loudness. Real instruments produce a mixture of frequencies, and the mixture gives each instrument a different wave shape.

An oscilloscope or a microphone connected to a computer can display these wave shapes, and this is a good classroom experiment: sing a note, then sing it louder, then sing a higher one, and watch what changes.

5.The Speed of Sound

The speed of sound depends on the medium and its temperature, but not on how loud or how high-pitched the sound is. In dry air at 20 °C it is about 343 m/s, which is roughly 1235 km/h. In air the speed rises with temperature by about 0.6 m/s for each degree Celsius, so the value is close to 331 m/s at 0 °C. The values below are approximate and differ slightly between sources and conditions.

6.Measuring Loudness: The Decibel

The loudness of a sound is measured on the decibel (dB) scale. The scale is logarithmic, which means equal steps on the scale represent equal multiples of sound intensity, not equal additions. An increase of 10 dB is a tenfold increase in sound intensity, and an increase of about 3 dB is a doubling of intensity. Our ears do not judge loudness in a simple way, but a 10 dB rise is usually heard as roughly twice as loud.

These are typical, rounded values that vary with distance and the exact source. Hearing damage depends on both the level and the time of exposure, which is why a very loud sound can harm in minutes while a moderate one takes hours.

7.Reflection, Echoes and Sonar

When a sound wave meets a hard, smooth surface such as a wall or a cliff, most of it is reflected, and the law of reflection applies: the angle of incidence equals the angle of reflection. An echo is a reflected sound that you can hear separately. To hear it as distinct from the original, the delay should be at least about 0.1 s. With sound at 343 m/s, the round trip in 0.1 s is about 34 m, so the reflecting surface must be at least about 17 m away. In smaller rooms, reflections arrive so quickly that they merge with the original sound; this is called reverberation.

Engineers use reflection in sonar (sound navigation and ranging). A ship sends out a pulse of sound (often ultrasound) and measures the time until the echo returns from the sea bed or from a shoal of fish. Since the pulse travels down and back, the depth is half of the total distance:

8.Absorption, Diffraction and Interference

Absorption. Soft, porous materials such as curtains, carpets and foam take in sound energy instead of reflecting it, converting a little of it to heat. Concert halls and recording studios use a carefully chosen mix of reflecting and absorbing surfaces to control echoes.

Diffraction. Waves spread out when they pass through a gap or around an obstacle. You can hear someone talking in the next room even when you cannot see them, because sound bends around the door frame. Sound diffracts easily because its wavelengths in air (from a few centimetres to several metres) are similar in size to doorways and everyday objects.

Interference. When two sound waves meet, they add together. If a compression meets a compression, the sound gets louder; if a compression meets a rarefaction of equal size, they can cancel. Noise-cancelling headphones use this idea: a microphone detects outside noise and the headphones play a matching "opposite" wave that cancels part of it, especially steady low-frequency noise such as engine hum.

9.Musical Instruments and Resonance

Instruments make sound by making something vibrate, and they let you change the frequency in predictable ways.

• Stringed instruments: a shorter, tighter or thinner string vibrates at a higher frequency, giving a higher pitch. This is why guitar strings are tuned by tightening pegs, and why pressing a string against a fret (making the vibrating part shorter) raises the note.
• Wind instruments: the air inside a tube vibrates. A shorter air column gives a higher pitch. Opening finger holes on a flute makes the vibrating column effectively shorter.
• Percussion: drums and xylophones vibrate when struck. Smaller, tighter or lighter parts give higher pitches.

Every object has one or more natural frequencies at which it vibrates most easily. If a regular push or sound matches a natural frequency, the vibrations build up. This is called resonance. A child on a swing shows it well: small pushes timed with the swing's natural rhythm make it go high. Resonance helps instruments sound loud (a guitar's hollow body vibrates in sympathy with the strings), and engineers have to design buildings, bridges and machines so that ordinary vibrations do not match their natural frequencies.

10.The Doppler Effect

When a sound source moves towards you, each new wave crest starts from a point closer to you than the last, so the crests arrive more often and you hear a higher frequency. When the source moves away, the crests arrive less often and you hear a lower frequency. This is the Doppler effect, named after the Austrian physicist Christian Doppler. It explains why an ambulance siren seems to drop in pitch as it passes you. Notice that the siren's own frequency has not changed; only the frequency you receive has. Radar speed guns and some medical blood-flow scanners use related Doppler principles with other types of waves.

11.Ultrasound and Infrasound

Young people with healthy hearing can usually detect frequencies from about 20 Hz to 20 000 Hz. The upper limit falls with age and with hearing damage. Sounds above this range are ultrasound; sounds below it are infrasound.

• Ultrasound uses: medical imaging (for example, examining a developing baby in the womb, because it does not use ionising radiation), cleaning delicate objects in an ultrasonic bath, sonar, and checking metal parts for hidden cracks. Bats and dolphins use ultrasound to locate objects by echolocation.
• Infrasound: produced by earthquakes, some volcanoes and large animals such as elephants, which are known to use very low-frequency sounds to communicate over long distances.

Many animals hear different ranges from humans. Dogs, for example, hear higher frequencies than we do, which is why a dog whistle seems silent to us.

12.How the Human Ear Works

The ear is a remarkably sensitive detector. It has three main parts:

1. Outer ear. The pinna (the visible part) collects sound and funnels it along the ear canal to the eardrum.
2. Middle ear. Sound waves make the eardrum vibrate. Three tiny bones, the malleus, incus and stapes (the ossicles), carry and strengthen the vibrations to the oval window of the inner ear. The Eustachian tube links the middle ear to the throat and helps equalise air pressure, which is why your ears "pop" when you change altitude.
3. Inner ear. The cochlea is a coiled, fluid-filled tube lined with thousands of tiny hair cells. Vibrations in the fluid bend the hair cells, which send electrical signals along the auditory nerve to the brain. Different positions along the cochlea respond best to different frequencies, which is how we tell pitch. The inner ear also contains the semicircular canals, which help with balance.

13.Protecting Your Hearing

The hair cells in the cochlea do not regrow in humans, so hearing damage from loud noise is usually permanent. Health agencies such as the US National Institute for Occupational Safety and Health (NIOSH) recommend limiting exposure to 85 dBA over 8 hours, and every 3 dB above that halves the safe time. The World Health Organization also publishes guidance on safe listening with personal audio devices.

• Keep headphone volume moderate, and follow the "if others can hear it, it is too loud" rule.
• Take listening breaks, and choose headphones that block outside noise so you do not need to turn the volume up.
• Wear ear protection around loud machinery, fireworks and very loud concerts.
• If you have ringing in the ears, muffled hearing or ear pain, see a doctor. This is general education, not medical advice.

14.Common Mistakes to Avoid

15.Check Your Understanding

Try each question before opening the answer.

Summary

• Sound is a longitudinal mechanical wave produced by vibrations; it needs a medium and cannot pass through a vacuum.
• v = fλ. Pitch depends on frequency, loudness on amplitude, quality on wave shape.
• Sound is faster in solids and liquids than in gases; in air it is about 343 m/s at 20 °C.
• Reflection gives echoes and sonar; diffraction lets sound bend around corners; interference allows noise cancelling.
• The ear converts vibrations to nerve signals, and loud sounds can cause permanent damage, so protect your hearing.

Keep Learning on EDUSAMBAM

Sources and Further Reading

This article is original EDUSAMBAM writing. Definitions, typical values and safety guidance were checked against the sources below. Diagrams are original and were not copied from any source.

• OpenStax, College Physics 2e: chapter on sound, the speed of sound, sound intensity and the decibel scale, the Doppler effect, and hearing. OpenStax — College Physics 2e.
• OpenStax, University Physics Volume 1: sound waves, interference and standing waves. OpenStax — University Physics Volume 1.
• Encyclopaedia Britannica: Sound (properties and speed of sound in different media).
• CDC / NIOSH: noise and hearing loss prevention, including the 85 dBA recommended exposure limit. NIOSH — Noise and Hearing Loss Prevention.
• World Health Organization: deafness and hearing loss fact sheet, with safe-listening guidance. WHO — Deafness and hearing loss.
• Further reading kept from the original article: Doppler Effect — Encyclopaedia Britannica.
• Further reading kept from the original article: Learn Science with NASA.

Note on the numbers: speeds and decibel levels are typical, rounded values. Please check them against your own syllabus or exam board specification before relying on them.