Chapter 7 — Sound

What is sound?
Sound is a form of energy produced when something vibrates. When an object vibrates it pushes and pulls the particles of the medium around it. These particles pass the disturbance on to neighbouring particles, so the disturbance travels as a wave. In everyday life, we notice sound when vibrations reach our ear and cause parts of the ear to move. Sound needs a material medium — it cannot travel in empty space where there are no particles to carry the vibration.

Sound has three main perceptible properties: pitch (how high or low a sound seems), loudness (how strong or soft a sound seems) and quality or timbre (why two instruments playing the same note sound different). These properties depend on how fast, wide and complex the vibrations are.

Many objects produce sound: strings, air columns, membranes, vibrating columns of water, and even our vocal cords. Everyday examples include a ringing bell, a plucked rubber band, and a drum skin hit with a stick. Simple experiments such as plucking a stretched rubber band or striking a tuning fork help show that vibration and sound are linked.

It is useful to use correct words when discussing sound: source (the vibrating object), medium (material that carries sound), and receiver or detector (ear or instrument that senses the sound). Being clear about these terms helps when we study how sound travels and how it can be controlled or used in technology.

Longitudinal waves
Sound travels through a medium as a longitudinal wave. In a longitudinal wave, particles of the medium move back and forth along the same direction the wave travels. This produces regions of compression where particles are close together and rarefaction where they are spread apart. A sound wave is therefore a repeating pattern of compressions and rarefactions moving outward from the source.

To visualise this, imagine a spring (a slinky) on a table. If you push one end and pull it back, a pulse travels along the spring as coils move in and out. The coils do not travel with the wave; only the disturbance travels. Similarly in air the air molecules oscillate about fixed positions, and the pressure changes travel as the sound wave.

Wavelength is the distance between two consecutive compressions (or two rarefactions). Frequency is the number of compressions passing a point each second, measured in hertz (Hz). Amplitude corresponds to how large the pressure variation is and relates to loudness. Together these ideas explain why different sounds behave differently when they travel or reflect from surfaces.

Understanding longitudinal waves is important because it contrasts with transverse waves (where particle motion is perpendicular to wave travel). Light is an example of a transverse wave, but sound in ordinary materials is longitudinal. The particle motion, pressure changes, and energy transfer are the key points students should be able to describe and sketch.

Pitch and frequency
Pitch is the auditory sensation that allows us to classify a sound as 'high' or 'low'. The physical quantity that determines pitch is frequency: the number of complete vibrations or cycles per second, measured in hertz (Hz). A sound with more cycles per second has a higher frequency and is perceived as a higher pitch. For example, the note produced by a piccolo has a much higher frequency than the note from a bass drum, so the piccolo sounds higher.

Musical notes are defined by specific frequencies. In music, notes are arranged so that certain frequencies form scales and octaves. Two notes an octave apart have a frequency ratio of 2:1; that means the higher note vibrates twice as fast as the lower. Instruments are tuned so that the set of notes they produce match standard reference frequencies.

How do instruments change frequency? For strings, frequency depends on tension, length and mass per unit length. Tightening a string increases its frequency; shortening a string raises its frequency; a thicker string (higher mass per unit length) vibrates more slowly and gives a lower frequency. For wind instruments, the length of the air column matters: a shorter column produces higher frequencies. For percussion instruments, shape, size and stiffness of the vibrating surface determine the set of frequencies it can produce.

The ear also processes harmonics. A vibrating object usually produces a fundamental frequency (which sets the pitch) together with higher frequency components called harmonics or overtones. These harmonics do not change the fundamental pitch but they change the sound’s timbre, which is why a piano and a flute can play the same note and sound different. Human hearing has a typical range from about 20 Hz to 20,000 Hz; within this range we identify pitch and detect musical structure. Age and exposure to loud sounds reduce this range over time.

Loudness and amplitude
Loudness describes how strong or soft a sound seems to a listener. The main physical factor behind loudness is amplitude: the greater the amplitude of vibration, the larger the pressure variations in the air and the louder the sound appears. When an object vibrates with small amplitude it produces weak pressure changes and a quieter sound; bigger amplitude produces stronger pressure variations and a louder sound.

However, perceived loudness is not determined by amplitude alone. It depends on frequency, the sensitivity of the human ear at that frequency, and the distance between listener and source. The ear is most sensitive to frequencies important for speech (roughly 500–4000 Hz), so sounds in this range may seem louder than others with identical amplitude. Distance reduces loudness because sound energy spreads out; for a point source in free space the intensity falls approximately as the square of the distance (inverse square law).

Sound intensity is a measurable quantity: it is the power transmitted per unit area by the wave. Intensity relates to amplitude but is often expressed using the decibel (dB) scale because the range of intensities the ear can perceive is very large. The decibel scale is logarithmic: an increase of 10 dB represents about ten times more intensity. Typical values: a quiet library ~30 dB, normal conversation ~60 dB, heavy traffic 80–90 dB, and a rock concert 110–120 dB. Prolonged exposure above about 85 dB can harm hearing.

Practical effects of loudness include design of public-address systems, speaker placement in a classroom, and using sound-absorbing materials to reduce disturbance. Students should understand how amplitude, distance and environment combine to affect what a listener hears. They should be able to explain why the same source sounds quieter when you move away, and why a louder sound is more likely to damage hearing if exposure is long.

Speed of sound in different media
Sound travels at different speeds in gases, liquids and solids. In general, sound travels fastest in solids, slower in liquids, and slowest in gases. This ordering exists because particles in solids are closer together and more rigidly bound, so a disturbance passes from particle to particle more quickly. The elastic properties and density of the medium together determine the exact speed.

For air at room temperature (about 20°C) the speed of sound is approximately 343 m/s. In water it is about 1480 m/s and in steel about 5000 m/s (values vary with temperature and material). In gases the speed increases with temperature because particles move faster and transfer disturbances more rapidly. In solids, the speed depends strongly on stiffness: stiffer materials generally transmit sound faster.

Students learn simple methods to measure the speed of sound. One approach uses echoes: know the distance to a reflecting surface, clap and time the echo return; since the sound travels twice the distance, use v = 2d / t. Another method uses resonance tubes: find the length giving maximum loudness for a known frequency and deduce wavelength; then v = f × λ. When making measurements, account for experimental uncertainties and consider environmental factors such as temperature and wind which affect results.

Understanding relative speeds also explains practical observations: a person may hear thunder much later than seeing lightning because light travels effectively instantaneously while sound is much slower; vibrations travel along a railway track faster than by air, so a signal may be heard earlier through the track. These facts are useful in safety applications and in designing systems such as sonar and ultrasound equipment.

Reflection of sound and echoes
When sound meets a surface, part of it bounces back — this is reflection. A clear example is an echo: when sound reflects from a distant surface and returns after a noticeable time, we hear a distinct repeat of the sound. For an echo to be heard separately, the reflected sound must arrive at least about 0.1 s after the original; this means the reflecting surface must be some distance away from the source and listener (rough estimate: more than 17 m for air at 343 m/s).

The law of reflection for sound is similar to light: angle of incidence equals angle of reflection, measured from the normal to a reflecting surface. This is used in designing auditoria and whispering galleries where sound paths are controlled. Smooth hard surfaces reflect sound well, while soft or rough surfaces absorb and scatter it. Multiple reflections in a large room cause reverberation, which makes sounds linger and can affect speech clarity.

Echoes are useful in sonar, measuring distances underwater, and in medical ultrasound imaging where reflected pulses reveal internal structure. In daily life, echoes may cause confusion in speech or create unpleasant reverberation in large rooms; architects use baffles, curtains and irregular surfaces to reduce unwanted reflection. Hard, flat surfaces give clear echoes; porous, soft materials absorb sound and reduce echoes.

Students should be able to perform simple echo experiments, calculate distances using echo time, and sketch ray diagrams showing incidence and reflection. Understanding reflection also prepares students to study reverberation and acoustics in later classes.

Structure and working of the human ear
The human ear changes pressure variations in the air into electrical signals the brain can understand. It has three main parts: the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna and the ear canal; these gather sound and direct it toward the eardrum. The eardrum (tympanic membrane) is a thin membrane that vibrates when air pressure changes arrive from sound waves.

The middle ear contains three tiny bones known collectively as the ossicles: the malleus, incus and stapes. These bones form a lever chain that transmits vibrations from the eardrum to the inner ear and amplify the motion so that it can move the fluid in the cochlea. The Eustachian tube connects the middle ear to the back of the throat and helps equalise pressure on both sides of the eardrum, for example during changes of altitude.

The inner ear contains the cochlea, a spiral-shaped, fluid-filled structure. Inside the cochlea lies the basilar membrane which supports thousands of hair cells. As the stapes pushes on the oval window of the cochlea, waves travel through the cochlear fluid; different parts of the basilar membrane vibrate most strongly for different frequencies. Hair cells at those places bend and convert mechanical motion into electrical nerve impulses which travel along the auditory nerve to the brain. The brain interprets patterns of nerve signals as pitch, loudness and timbre.

Damage to any part of this chain—blocked ear canal, ruptured eardrum, stiffened ossicles, or damaged hair cells—can lead to different types of hearing loss. Conductive loss involves problems transmitting sound to the inner ear; sensorineural loss involves damage to hair cells or the auditory nerve. Protecting ears from loud sounds, seeking timely medical care for infections, and maintaining ear hygiene are practical points students should know. Simple demonstrations, like gently pressing the ear canal to reduce sound or yawning to open the Eustachian tube, help make the working clear.

Noise and sound quality
Sound quality, or timbre, is the characteristic that allows us to tell two instruments apart even when they play the same pitch and loudness. Timbre depends on the mix of the fundamental frequency and its harmonics (overtones), on how the sound starts and stops (attack and decay), and on the resonance characteristics of the instrument body. For example, a violin and a flute playing the same note have different harmonic content and envelope shapes, producing distinct timbres.

Noise is sound that is unwanted, unpleasant or lacking in musical structure. Noise usually contains many frequencies mixed irregularly, without the clear periodic pattern that gives musical notes their pitch. Examples include the rumble of traffic, construction noise and loud crowd sounds. While a musical instrument produces a set of harmonics above a fundamental frequency, noise spreads energy over a wide range of frequencies, and this chaotic mixture often makes it irritating or disturbing.

The study of noise includes its effects on health and comfort. Prolonged exposure to high noise levels can cause stress, sleep disturbance and hearing loss. Noise in classrooms reduces concentration and learning. Controlling noise involves measures at three levels: source control (quieter machines, mufflers), path control (barriers, insulation, absorbers), and receiver protection (ear plugs, ear muffs). Acoustic design in buildings uses carpets, curtains, foam panels and irregular surfaces to reduce reflections and lower reverberation time, improving speech clarity and comfort.

Students should be able to listen critically, distinguish musical tones from noise, describe timbre using simple terms (bright, mellow, nasal), and suggest practical ways to reduce unwanted sound at school and home. A useful classroom activity is to compare a tuning fork (a nearly pure tone) with a recording of city traffic to illustrate the difference between a clean harmonic spectrum and broad-spectrum noise.

Musical instruments and resonance
Musical instruments rely on vibration and resonance to produce loud, sustained sound. Resonance occurs when a system is driven at one of its natural frequencies so that the response builds up to a large amplitude. Every musical instrument has natural frequencies determined by its physical properties: length and tension for strings, air column length for wind instruments, and size and stiffness for drums and plates.

For string instruments like violin or guitar, the string vibrates with a fundamental frequency and higher harmonics. The body of the instrument acts as a resonator, amplifying the sound by efficiently coupling the string vibrations into the surrounding air. Changing the effective length of the string (by pressing a finger on a fret) or changing tension (by tuning pegs) alters the natural frequencies and so the pitch.

Wind instruments use standing waves in air columns. An open pipe supports antinodes at both ends while a closed pipe has a node at the closed end and an antinode at the open end; this changes which harmonics are present and thus the tone. Changing the length by opening or closing holes changes the resonant frequencies, producing different notes. Percussion instruments produce more complex spectra because their vibrating surfaces have many modes; their timbre depends on which modes are strong.

Resonance also has non-musical consequences. If a driving frequency matches the natural frequency of a structure, vibrations can become dangerously large; engineers must design buildings and bridges to avoid resonance with wind or traffic. Musical instrument makers tune sizes and materials to get desirable resonance and timbre. Classroom demonstrations — plucking strings, blowing across tubes, or showing a tuning fork exciting a resonance box — help students connect the idea of resonance to sound amplification and musical tone.

Ultrasound and infrasound
Sound frequencies outside the normal human hearing range are useful in many fields. Ultrasound refers to sound above about 20,000 Hz. Because ultrasound has short wavelengths, it can detect small details and reflect from small features, which makes it valuable for imaging and measurement. In medicine, ultrasound scanners send short pulses into the body; boundaries between tissues reflect a portion of the pulse back. By measuring the time delay and intensity of the echoes and knowing the speed of sound in tissue, a two- or three-dimensional image of internal organs or a developing fetus can be constructed.

Ultrasound is also used in industry for non-destructive testing: it can find cracks or flaws inside metal parts by detecting unexpected reflections. Ultrasonic cleaners use high-frequency vibrations in a liquid to remove dirt from small and delicate objects. Bats and some rodents use ultrasonic echoes to navigate and catch prey; engineered sonar systems work on the same principle for locating objects underwater or mapping the sea floor.

Infrasound refers to frequencies below about 20 Hz. Although inaudible to humans, infrasound travels long distances with little attenuation and can be generated by natural events such as earthquakes, volcanic eruptions and severe storms. Scientists use infrasound detectors to monitor such events and sometimes to detect large explosions. Animals like elephants use infrasound for long-range communication because these low frequencies travel farther through the ground and air.

Both ultrasound and infrasound require safety and ethical consideration. Very intense ultrasound can heat tissue and cause damage, so medical devices use controlled intensities. Infrasound at high amplitude can be disturbing or harmful, though ordinary environmental levels are usually safe. Students should know basic principles: high frequency for fine detail, low frequency for long-distance travel, and common applications such as medical scanning, sonar, cleaning and environmental monitoring.

Hearing range and protection
The human ear typically hears frequencies from about 20 Hz up to about 20,000 Hz, though the exact range varies with age and health. The ear is most sensitive in the mid-range (roughly 500–4000 Hz) where human speech lies. As people grow older or are exposed to loud sounds, they commonly lose sensitivity to higher frequencies first, reducing the upper limit of hearing.

Hearing damage is usually caused by high sound levels and long exposure times. The decibel (dB) scale quantifies sound level; exposure above about 85 dB for long periods can lead to permanent hearing loss. Very loud sounds (over 120 dB) can cause immediate pain and damage. Ear protection — ear-plugs or ear-muffs — reduce the sound energy reaching the ear and are essential in noisy workplaces, concerts, and when using loud tools. Keeping volume moderate on personal audio devices and taking regular breaks reduces risk.

There are two broad types of hearing loss. Conductive hearing loss happens when sound is not efficiently transmitted through the outer or middle ear, for example due to earwax blockage or middle-ear infection. Sensorineural hearing loss results from damage to the inner ear hair cells or the auditory nerve and is often permanent. Early signs of hearing trouble include difficulty following conversations, needing higher volume on devices, or ringing in the ears (tinnitus). Seeking medical advice early improves the chance of useful treatment.

Students should learn practical steps for protection: maintain safe distances from loud sources, lower headphone volume and limit listening time, use noise-cancelling headphones in noisy places so volume need not be high, and use protective gear in workshops. School policies on noise and simple classroom design changes such as carpets and curtains help reduce harmful levels and improve comfort and learning.

Measurement experiments with sound
Practical experiments make understanding sound concrete. Common classroom experiments measure speed using echoes, study resonance in pipes, and observe beats and standing waves on strings. A well-planned experiment requires clear aim, a list of apparatus, a diagram of the setup, careful measurement, repeated trials and simple error handling such as taking averages.

Echo method: stand at a known distance from a flat reflecting surface, produce a sharp sound (hand clap or starter pistol) and measure the time between the sound and its echo with a stopwatch or audio recorder. Since the sound travels to the reflector and back, the total distance is twice the separation. Use v = 2d / t to calculate speed. Repeat several times and note temperature, since speed in air depends on temperature. Record uncertainties and comment on possible error sources such as timing reaction or wind.

Resonance tube method: a vertical tube partly filled with water can be used with a tuning fork of known frequency. Adjust the water level until a loud sound indicates resonance — the air column length corresponds to a particular fraction of the wavelength. From the measured length and known frequency find wavelength and speed using v = f × λ. For better accuracy, find several resonant lengths and use average wavelength.

Other experiments include producing beats by sounding two close frequencies and counting beat rate, or setting up standing waves on a stretched string to identify nodes and antinodes and measure wavelength. Students should practise presenting results in tables, calculating mean values, and making brief conclusions. Safety includes keeping volumes moderate, using appropriate protective equipment and handling tuning forks and apparatus gently.

Numerical problems on sound
Sound problems at this level mainly use two simple relations: speed = distance / time and wave relation v = f × λ. For echoes remember that the sound travels to the reflecting surface and back, so the total distance is twice the separation. Always write down values with units, convert units when necessary, and perform calculations step by step to avoid mistakes.

Start by listing known quantities and the unknown. For example, if the echo time is given and the distance to the reflector is required, use distance = v×t/2. If frequency and wavelength are given, calculate speed with v = f×λ. If speed and time are given, multiply to find distance. Students should be comfortable rearranging formulas: t = distance / speed, λ = v / f, and so on. Include realistic values: use about 340–343 m/s for speed of sound in air at room temperature unless the question specifies temperature.

Work systematically: convert centimetres to metres, milliseconds to seconds, and ensure final answers have correct units. Check whether the result is reasonable by comparing it to expected magnitudes; for instance, sound takes about 3 seconds to travel 1 km in air, so a one-way 1 km distance corresponds to ~3 s. Practise a variety of problems: echo times and distances, wavelengths and frequencies for musical notes, and combined problems where you first compute speed from an experiment and then use it in another calculation.

When presenting answers in exams, show key steps and write final answers clearly with appropriate significant figures. If asked, comment on possible sources of error in experimental values, such as reaction time when using a stopwatch, temperature variations, or approximate distances. These remarks demonstrate understanding beyond mere calculation.

Revision and real-life links
This topic ties together the unit’s key ideas and shows how they explain everyday sounds and technologies. Sound is produced by vibrating sources, travels through matter as longitudinal waves made of compressions and rarefactions, and shows properties such as frequency, wavelength and amplitude. Frequency relates to pitch, amplitude to loudness, and harmonic content to timbre. Reflection, transmission and absorption determine how sound behaves in rooms and outdoor spaces.

Apply these ideas to familiar situations. Speech uses vibrations of the vocal cords and resonances in the mouth and throat to produce different vowel and consonant sounds; understanding frequency ranges helps explain why hearing speech is harder in noisy places. Musical instruments rely on resonance: strings and air columns have natural frequencies that give musical notes, while the instrument body amplifies sound. Sonar and ultrasound use timed echoes to measure distances; a ship sends a pulse and times the echo to find sea depth, while doctors use ultrasound echoes to image internal organs.

Noise control is a practical application: to reduce unwanted sound use source reduction (quieter machines), path control (barriers, insulation, absorbers) and receiver protection (ear-plugs). Architectural choices — soft furnishings, diffusing surfaces and room shape — affect reverberation time and speech clarity. Safety advice such as avoiding prolonged exposure to loud noises, using ear protection, and keeping personal audio volumes moderate directly follows from understanding intensity and decibel scales.

For revision, practise drawing labelled diagrams: a longitudinal wave showing compressions and rarefactions, a ray diagram of reflection and echo, a cross-section of the ear, and standing waves in a tube or string. Solve numerical questions using v = distance / time and v = f × λ, and describe simple experiments you carried out. Clear explanations, correct use of units and concise reasoning will prepare students for board-level questions and practical assessments.