Chapter 13 — Sound

What is sound? Sound is a form of energy produced when an object vibrates. These vibrations set the surrounding particles of a medium (solid, liquid or gas) into motion and travel as mechanical waves to our ears. Sound cannot travel in vacuum.

How sound is produced and propagated

• When a body vibrates (for example, a plucked guitar string or a struck tuning fork), it causes successive compressions and rarefactions in the surrounding medium.
• These variations (regions of higher and lower pressure) travel away from the source as longitudinal waves. In a longitudinal wave particles oscillate back and forth parallel to the direction of wave travel.
• Particles of the medium do not travel with the wave permanently; they only vibrate about their mean positions and pass the disturbance on to adjacent particles.

Characteristics of sound

• Amplitude: Maximum displacement of particles from mean position. Larger amplitude → greater loudness.
• Frequency (f): Number of vibrations per second (measured in hertz, Hz). Higher frequency → higher pitch.
• Period (T): Time for one complete vibration. T = 1/f.
• Wavelength (λ): Distance between two successive compressions (or rarefactions).
• Pitch: How high or low a sound is perceived — related to frequency.
• Loudness: A subjective measure related to amplitude and intensity. Intensity ∝ (amplitude)^2.
• Timbre: Quality or colour of sound produced by different sources even at same pitch and loudness (depends on overtones).

Important facts

• Human audible range: about 20 Hz to 20,000 Hz (20 kHz).
• Speed of sound depends on medium: generally fastest in solids, slower in liquids, slowest in gases. For air at 20°C, v ≈ 343 m/s.
• Speed in air changes with temperature approximately as v ≈ 331 + 0.6·T(°C).

Simple demonstrations: Strike a tuning fork and touch it lightly to water — ripples or splashes show vibration. Hold a stretched string and pluck to see/feel vibrations. Bring a vibrating tuning fork near your ear through air vs soaked in water to compare transmission.

What is propagation of sound?
Propagation of sound means the transfer of sound energy from the source to the listener through a medium by means of vibrations of the particles of the medium. Sound travels as a longitudinal wave — particles of the medium vibrate to and fro along the direction of wave travel creating compressions and rarefactions.

Need for a medium
Sound requires a material medium (solid, liquid or gas) to travel because it is transferred by particle-to-particle interactions. It cannot travel through a vacuum. Example: thunder is not heard inside a vacuum chamber.

How particle motion transmits sound
When an object vibrates (e.g., a tuning fork), it pushes neighbouring air molecules. These molecules push their neighbours and so on, passing the disturbance forward. Energy (not the particles themselves) moves through the medium.

Dependence on medium
The speed of sound depends on the medium’s elasticity and density. In general: speed in solids > speed in liquids > speed in gases, because solids are more rigid (higher elastic modulus) and transmit vibrations faster. For example (approximate values at room temperature): air ~343 m/s, water ~1480 m/s, steel ~5000 m/s.

Effect of temperature
In gases (air) the speed of sound increases with temperature because molecules move faster and transmit pressure changes more quickly. A useful approximate formula for air is v ≈ 331 + 0.6T (m/s), where T is temperature in °C.

Reflection and transmission at boundaries
When sound meets a boundary between two media part of it is reflected (echo) and part is transmitted. The amount depends on the impedance (related to density and elasticity) of the two media. This explains why you can sometimes hear people on the other side of a wall (transmitted sound) and why echoes occur from large hard surfaces.

Attenuation
Sound intensity decreases with distance and due to absorption by the medium (energy converted to heat). High-frequency sounds are absorbed faster than low-frequency sounds.

What is speed of sound?
Speed of sound is how fast a sound wave travels through a medium. It is the distance travelled by the sound wave per unit time. The SI unit is metre per second (m/s).

Basic definition and formula
Speed (v) = distance / time. For waves, the relation between speed, frequency and wavelength is:
v = f × λ (where f is frequency, λ is wavelength)

Typical values
In air at 20 °C, speed of sound ≈ 343 m/s. In general: solids > liquids > gases (sound travels fastest in rigid, elastic media).

Dependence on temperature (for air)
The speed of sound in air increases with temperature. A simple practical formula for ordinary temperatures is:
v_air (m/s) ≈ 331 + 0.6 × T (°C)

Measuring speed of sound using echo
If you produce a sharp sound at distance d from a reflecting surface (cliff or wall) and measure the time t between the sound and its echo, the sound travels to the wall and back (distance 2d). So:
v = 2d / t

Example quantitative relations to remember
- v = distance / time (general)
- v = f × λ (wave relation)
- v_air ≈ 331 + 0.6 T (°C) (temperature dependence)
- Echo method: v = 2d / t

Factors affecting speed of sound
- Medium: solids (highest), liquids, gases (lowest).
- Elasticity: greater elasticity of the medium increases speed.
- Density: higher density tends to reduce speed if elasticity does not increase proportionally.
- Temperature: in gases, higher temperature increases speed.

Connection to hearing: pitch and loudness
Frequency determines pitch (high f → high pitch). Amplitude relates to loudness (larger amplitude → louder sound). For a given speed, higher frequency means smaller wavelength (λ = v/f).

Practical uses
Echo and sonar to measure distances, ultrasound imaging (medical), navigation and communication in air and underwater, estimating distance of thunderstorm by time delay between lightning and thunder.

Sound is a form of energy produced by vibrating objects and transmitted through a medium (solid, liquid or gas) as longitudinal waves. Several observable properties — called characteristics of sound — describe how sound behaves and how we perceive it. The main characteristics are pitch, loudness, quality (timbre), and speed. Below is a simple description of each and how they depend on measurable quantities.

1. Pitch

• Definition: Pitch is how high or low a sound seems to a listener.
• Physical cause: Pitch is determined by the frequency (f) of the sound wave. Higher frequency → higher pitch; lower frequency → lower pitch.
• Perception: Humans typically hear frequencies from about 20 Hz to 20,000 Hz; musical notes have specific frequencies.

2. Loudness

• Definition: Loudness is how strong or soft a sound seems.
• Physical cause: Loudness is related to the amplitude of the sound wave and to the wave's intensity (power per unit area). Larger amplitude (and higher intensity) → louder sound.
• Perception and units: Loudness is perceived subjectively and is often measured objectively as sound level in decibels (dB).

3. Quality (Timbre)

• Definition: Quality or timbre is what makes two sounds with the same pitch and loudness sound different (for example, a flute and a violin playing the same note).
• Physical cause: Timbre depends on the waveform shape and the presence and relative strengths of harmonics (overtones) in the sound.

4. Speed of Sound

• Definition: Speed is how fast the sound wave travels through a medium.
• Dependence: Speed depends on the medium and its properties (temperature for gases, and density/elasticity for solids and liquids). In air at 20°C, speed ≈ 343 m/s.

Other related ideas

• Wavelength (λ): Distance between two successive compressions or rarefactions. Related to speed and frequency by v = fλ.
• Frequency (f): Number of vibrations per second (Hz). f = 1/T where T is the time period.
• Intensity and amplitude: Intensity is proportional to amplitude squared (I ∝ A²), so small changes in amplitude give larger changes in intensity.

Understanding these characteristics helps explain many everyday phenomena (why a whistle is high-pitched, why a thunderclap is loud but low in pitch, why two instruments sound different even when playing the same note, and how echoes allow distance measurement).

Definition

Sound is a mechanical wave produced by vibrating bodies. The audible range is the range of sound frequencies that can be heard by the average human ear. Frequencies below the audible range are called infrasound and frequencies above it are called ultrasound.

Ranges (typical)

• Infrasound: frequencies < 20 Hz
• Audible sound (human hearing): about 20 Hz to 20 000 Hz (20 kHz). This range narrows with age and exposure to loud sounds.
• Ultrasound: frequencies > 20 000 Hz (20 kHz)

Key physical relation

The fundamental relation between wave speed, frequency and wavelength is:

v = f × λ

where v is the speed of sound (≈ 343 m/s in air at 20 °C), f is frequency in hertz (Hz), and λ is wavelength in metres (m).

Characteristic properties

• Infrasound has very long wavelengths (many metres) and can travel long distances with little attenuation. It diffracts around obstacles easily.
• Audible sound wavelengths range from metres (low Hz) to millimetres/centimetres (high kHz), and are used for communication, music and hearing-based sensing.
• Ultrasound has short wavelengths (millimetre or smaller in air/liquid) and can resolve small details; it is strongly attenuated in air but transmits well in liquids and solids.

Detection and biological examples

• Many animals hear beyond the human audible range: dogs and bats hear higher frequencies (ultrasound used by bats for echolocation), while elephants and some whales produce and detect infrasound for long-distance communication.

Uses

• Infrasound: monitoring earthquakes and volcanoes, studying atmospheric phenomena, long-range animal communication detection.
• Audible sound: speech, music, alarms and everyday hearing.
• Ultrasound: medical imaging (sonography), industrial non-destructive testing, SONAR, cleaning small parts, ultrasonic welding.

Human hearing sensitivity & intensity

The threshold of hearing (typical) corresponds to an intensity I0 = 1×10⁻¹² W/m². Sound level in decibels is given by:

β = 10 log10(I / I0) (in dB)

Threshold of pain corresponds to about 1 W/m² → β ≈ 120 dB.

Simple examples using v = f × λ

• For f = 20 Hz, λ = v/f ≈ 343 / 20 ≈ 17.15 m (infrasound wavelength in air).
• For f = 20 000 Hz, λ = 343 / 20000 ≈ 0.01715 m ≈ 1.7 cm (upper audible/low ultrasound wavelength in air).

Summary

Infrasound, audible sound and ultrasound are simply classifications by frequency. Their wavelengths and propagation behavior differ, which determines how they are used in nature and technology.

Reflection of sound is the bouncing back of sound waves when they strike a surface that does not absorb all the sound. Like light, sound waves obey the law of reflection: the angle of incidence equals the angle of reflection (useful when treating sound as rays for large smooth surfaces).

Echo: An echo is a distinct repetition of a sound heard when the reflected sound arrives at the listener's ear after a short delay. For the reflected sound to be heard as a separate sound (an echo) the time gap between the original sound and the reflected sound must be large enough (usually >= 0.1 s for human hearing). Echoes are produced by large hard surfaces such as cliffs, tall buildings, or empty halls.

Reverberation: Reverberation is the persistence of sound in an enclosed space due to multiple reflections from walls, floor and ceiling. If reflections arrive in quick succession (time gaps < 0.1 s) they overlap the original sound and are heard as a prolonged or 'echoey' effect rather than separate echoes. Reverberation increases loudness and fullness of sound but too much reverberation reduces speech clarity.

Why echo vs reverberation depends on distance and time:

• If a reflecting surface is far enough that the round-trip travel time of sound is ≥ 0.1 s → you hear a distinct echo.
• If the surface is nearer so reflections return sooner (< 0.1 s) → reflections overlap → reverberation.

Practical control: Reverberation can be reduced by using sound-absorbing materials (curtains, carpets, foam panels) or by changing room geometry. Concert halls are designed to provide a suitable reverberation time for music, while classrooms aim for low reverberation for speech intelligibility.

Connections & applications: Echo principles are used in sonar and echo-sounding to measure distance (ships, fish-finders), and animals such as bats and dolphins use reflected sound for echolocation.

Introduction
The human ear is the organ of hearing and also helps maintain balance. It converts sound waves (mechanical vibrations in air) into electrical signals that the brain interprets as sound.

Structure of the Ear

• Outer ear (External ear): Consists of the pinna (auricle) and the ear canal (external auditory meatus). The pinna collects and funnels sound waves into the ear canal toward the eardrum.
• Middle ear: An air-filled cavity containing the tympanic membrane (eardrum) and three tiny bones called ossicles — malleus (hammer), incus (anvil) and stapes (stirrup). The Eustachian tube connects the middle ear to the throat to equalize pressure.
• Inner ear: Contains the cochlea (a fluid-filled, spiral structure for hearing) and the vestibular apparatus (semicircular canals and vestibule) for balance. The cochlea contains the basilar membrane and hair cells (sensory receptors) connected to the auditory (cochlear) nerve.

How the Ear Works (Step-by-step)

1. Collection: Sound waves in air are collected by the pinna and travel down the ear canal.
2. Vibration of eardrum: Sound waves strike the tympanic membrane and cause it to vibrate with the same frequency as the incoming sound.
3. Transmission and amplification: Vibrations are transferred from the eardrum to the ossicles. Because the tympanic membrane has a larger area than the oval window and due to the lever action of the ossicles, pressure at the oval window is increased (middle ear amplification). Typical overall pressure gain ≈ 20–25× (area ratio × lever effect), helping efficiently transmit sound into the fluid of the inner ear.
4. Fluid waves in cochlea: The stapes pushes on the oval window, producing pressure waves in the cochlear fluid. These waves create a travelling wave along the basilar membrane.
5. Frequency mapping and receptor activation: Different frequencies cause maximum displacement of the basilar membrane at different places (high frequencies near the base, low frequencies near the apex). Hair cells at the location of maximum displacement bend and convert mechanical motion into nerve impulses (electrochemical signals).
6. Signal to brain: The auditory nerve carries these impulses to auditory centres in the brain where they are interpreted as pitch, loudness and quality of sound.

Key functional points

• Pitch depends on frequency (Hz): higher frequency → higher pitch.
• Loudness depends on amplitude (greater amplitude → louder sound). Loudness is related to sound intensity and is measured on a logarithmic decibel (dB) scale.
• Protection: Muscles (stapedius and tensor tympani) and reflexes reduce transmission of very loud sounds; the Eustachian tube equalizes pressure; earwax (cerumen) traps dust and microbes.

Common issues

• Conduction hearing loss: when outer or middle ear cannot transmit sound well (e.g., earwax blockage, otitis media).
• Sensorineural hearing loss: damage to hair cells or auditory nerve (e.g., prolonged exposure to loud noise, certain drugs, aging).
• Ear popping: pressure changes (airplane) — Eustachian tube opening equalizes pressure.

Summary
The ear is a mechanical-to-electrical transducer. Its three-part structure (outer, middle, inner) collects, amplifies and converts sound waves into nerve signals, which the brain interprets as sound.

What produces sound in musical instruments? Musical instruments produce sound when some part of the instrument vibrates. These vibrations set the surrounding air into periodic motion (sound waves) which reach our ears. The audible properties of the sound are:

• Pitch — related to the frequency of vibration (higher frequency = higher pitch).
• Loudness — related to the amplitude of vibration (larger amplitude = louder sound).
• Timbre (quality) — determined by the waveform and presence of overtones/harmonics (gives instruments their distinct sound).

Types of instruments and how they produce sound

• String instruments (guitar, sitar, violin): Sound is produced by vibrating strings. The string vibrates in standing-wave patterns with nodes and antinodes. The body or soundboard of the instrument amplifies the sound by resonance.
• Wind instruments (flute, organ pipe, clarinet): Sound arises from vibrating columns of air inside tubes. The tube supports standing waves; whether the tube is open at both ends or closed at one end affects allowed frequencies (harmonics).
• Percussion instruments (tabla, drums, xylophone): Sound is produced by vibrating membranes or bars. The shape and material determine the set of modes and resulting timbre.
• Tuning forks and bells: Metal parts vibrate and excite air; tuning forks are useful for demonstrating pure tones (dominant single frequency).

Standing waves and resonance

Musical notes correspond to standing waves. For a stretched string fixed at both ends, only certain wavelengths (and hence frequencies) fit as standing waves: those with nodes at the ends. Similarly, air columns in pipes form standing waves with nodes and antinodes depending on boundary conditions. Resonance occurs when a vibrating source drives another object or air column at one of its natural frequencies, greatly increasing amplitude (e.g., soundbox of a guitar, sympathetic vibration of another tuning fork).

How changing instrument parameters changes sound

• For strings: increasing tension raises the pitch; increasing length lowers the pitch; thicker strings (greater mass per unit length) lower the pitch.
• For pipes: a longer air column gives lower pitch; shortening the effective length (e.g., pressing holes on a flute) raises pitch. An open-open pipe and an open-closed pipe have different series of harmonics.
• Loudness can be changed by striking/ plucking/ bowing harder (larger amplitude) or by changing resonance/efficiency of the soundboard/body.

Important observable features for Class 8

• Sound needs a medium to travel (air, water, solids).
• Pitch & frequency relationship (higher frequency corresponds to higher pitch).
• Quality or timbre arises from presence of harmonics besides the fundamental frequency.

Summary: Musical instruments produce sound by vibrating parts (strings, air columns, membranes, bars). The pitch depends mainly on frequency which in turn depends on physical parameters (length, tension, effective length of air column, mass per unit length). Resonance and standing waves decide the allowed notes and the richness (timbre) of the sound.

What is noise? Noise is an unwanted, unpleasant or disturbing sound. Unlike a musical tone (which is regular and has a definite pitch), noise is generally irregular, random and contains many frequencies. Examples: honking, engine roar, construction hammering.

What is noise pollution? Noise pollution is the presence of excessive or disturbing sound that harms human health, wildlife, or the environment. It occurs when sound levels are too high or persist long enough to cause annoyance, hearing damage, stress or other negative effects.

Characteristics that make sound a noise

• Unpleasant or unwanted.
• Irregular waveform (broad frequency content).
• Loudness and duration often high.
• Occurs at times or places when silence is expected (night, hospitals, schools).

Major sources: road traffic, rail and air transport, factories and construction sites, loudspeakers and public events, household appliances, firecrackers.

Effects on health and environment: short-term—annoyance, difficulty in concentrating, sleep disturbance; long-term—permanent hearing loss, stress, high blood pressure, reduced work/school performance, disturbance to wildlife.

How we measure and compare sound: Sound energy received per unit area is called sound intensity (I, in W/m²). Because human hearing spans a very wide range of intensities, sound is commonly measured in decibels (dB), a logarithmic scale. Typical reference values: threshold of hearing ≈ 0 dB (I0 = 1×10⁻¹² W/m²), normal conversation ≈ 60 dB, heavy traffic ≈ 80–90 dB, threshold of pain ≈ 120–130 dB.

Control and prevention: use silencers and mufflers, maintain vehicles and machines, create noise barriers, plant trees, enforce time limits for loud activities, use ear protection (ear-plugs), move noisy industries away from residential areas, follow local noise regulations.

Sound is a mechanical wave that travels through a medium (air, water, solids). Different ranges of sound frequency — infrasonic (below 20 Hz), audible (20 Hz–20 kHz) and ultrasonic (above 20 kHz) — have many practical uses. Applications of sound exploit properties such as speed, reflection (echo), transmission, frequency (pitch) and intensity (loudness).

Main categories of applications

• Communication: Speech, telephones, radios, microphones and loudspeakers convert between sound and electrical signals to send information.
• Navigation & detection (echolocation and SONAR): Bats and dolphins use echolocation. SONAR (Sound Navigation and Ranging) sends a pulse and uses the echo to detect objects and measure distance under water.
• Medical uses (ultrasound): Ultrasonic waves are used in imaging (sonography), blood-flow measurement (Doppler ultrasound) and physiotherapy.
• Industrial & cleaning: Ultrasonic cleaning removes dirt from jewellery and precision parts; nondestructive testing uses sound to find cracks in materials.
• Music & entertainment: Musical instruments, concert-hall acoustics and audio systems rely on controlled sound production and reflection (reverberation).
• Measurement & surveying: Echo-sounding for ocean depth, range-finders and certain surveying instruments use sound wave travel-time measurements.
• Everyday devices: Alarms, ultrasonic pest repellents, door sensors and parking sensors use ultrasonic pulses.
• Safety and standards: Sound level meters measure noise; noise control and insulation reduce harmful effects of excessive sound (noise pollution).

Key physical ideas used in applications

• Reflection & echo: Time delay between emission and reception gives distance (used in SONAR and echo-sounding).
• Frequency & pitch: High frequencies (ultrasound) provide better resolution in imaging and cleaning but do not travel as far in air.
• Intensity & loudness: Intensity decreases with distance (approximately inverse-square law in free field), relevant for speaker placement and noise control.
• Doppler effect: Change of frequency with relative motion is used in Doppler ultrasound to measure blood flow speed and in some speed-detection systems.

Practical considerations and limits

• Air absorption increases with frequency — ultrasonic waves are good for short-range/high-resolution tasks (cleaning, imaging) but not long-distance air communication.
• In water, sound travels faster and with lower attenuation than in air, so SONAR is highly effective underwater.
• Excessive sound intensity damages hearing — noise control, safe exposure limits and sound-insulating materials are important in design.

Summary: Sound is used for communication, sensing, medical diagnosis and treatment, industrial cleaning and inspection, entertainment, and safety. Choosing the right frequency, intensity and medium is essential for each application.

Measurement, Intensity and Attenuation

What is sound intensity? Sound intensity measures the amount of sound energy passing per second through a unit area placed normal (at right angles) to the direction of sound propagation. It tells us how strong or loud a sound is at a point. Intensity depends on the power of the sound source and the distance from the source.

How do we measure loudness? Loudness is related to intensity but is a subjective sensation measured in decibels (dB). A small change in intensity can produce a noticeable change in loudness because the ear responds roughly logarithmically.

Key ideas — intensity and distance: For a point-like source that radiates sound uniformly in all directions, the energy spreads over the surface of a sphere. The spherical surface area grows as 4πr², so intensity falls with the square of the distance. This is called the inverse-square law: intensity decreases rapidly as you move away from the source.

Attenuation (reduction) of sound: Attenuation means loss of sound strength as it travels. Attenuation is caused by three main processes: absorption (medium converts sound energy to heat), scattering (sound redirected by obstacles), and reflection/refraction (energy redirected away from original path). In many media and practical situations, the intensity decreases approximately exponentially with distance because of absorption:

I(x) = I₀ e^−αx, where α is the attenuation coefficient and x is distance traveled.

Decibel scale: Because the range of sound intensities the ear can detect is huge, we use a logarithmic scale. The sound level β in decibels is defined by β = 10 log₁₀(I / I₀), where I₀ is a reference intensity (commonly 10⁻¹² W/m², approx. hearing threshold).

Summary of relations:

• Intensity = Power / Area (I = P / A).
• Inverse-square law for point sources: I ∝ 1 / r².
• Exponential attenuation in absorbing media: I(x) = I₀ e^−αx.
• Sound level in decibels: β = 10 log₁₀(I / I₀).

Why this matters in daily life: - At concerts you feel loud sound close to speakers because intensity is high; moving back reduces intensity quickly. - In a hall sound may get weaker because of absorption by curtains and people. - Higher frequency sounds (treble) are often absorbed more quickly than low frequency (bass), which is why bass travels farther.

Practical tips: - To protect hearing, avoid prolonged exposure above 85 dB. - Use sound-absorbing materials (carpets, curtains) to reduce echoes and intensity in rooms. - Microphones and speakers must be positioned considering the inverse-square law to get desired loudness.