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Complete Physics Revision: Sound (Dhwani) for Teaching Exams

This lecture covers the physics of sound for teaching exams, detailing wave characteristics, speeds across different media, and human audible ranges. It explains acoustic phenomena like echo, resonance, Doppler effect, and the human ear anatomy, accompanied by worked numerical problems.

Teaching
Physics
English · 11 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video UP SUPERTET SCIENCE Class-12 | ध्वनि (SOUND) HANDWRITTEN NOTES | उ.प्र. प्राथमिक शिक्षक भर्ती 2026 (opens YouTube in a new tab) by Balaji Study Adda, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Complete Physics Revision: Sound (Dhwani) for Teaching Exams

English11 topics · 7 MCQs · 14 flashcardsWatch the video (opens YouTube in a new tab)

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In short

This lecture covers the physics of sound for teaching exams, detailing wave characteristics, speeds across different media, and human audible ranges. It explains acoustic phenomena like echo, resonance, Doppler effect, and the human ear anatomy, accompanied by worked numerical problems.

Nature of Sound Waves and Propagation

Sound is a mechanical wave produced by vibrations in an object that travels through a medium to reach our ears.
Must knowSound is a form of energy that requires a material medium to propagate and cannot travel through a vacuum.
Vibrations in objects like a tuning fork, tabla, or guitar strings create sound waves.
Sound can propagate through solid, liquid, and gas media.
Sound travels fastest in solids, slower in liquids, and slowest in gases.

Medium	Example	Speed of Sound (approx.)
Solid	Steel	~5000 m/s
Liquid	Water	~1500 m/s
Gas	Air (at 20°C)	~343 m/s

Wave Parameters and Wave Types

Amplitude is the maximum displacement of a particle from its mean position in a vibrating wave.
Frequency is defined as the number of vibrations produced per second, denoted by f, and measured in Hertz (Hz).
Time period is the time taken to complete one full vibration, denoted by T and measured in seconds.
Wavelength is the distance between two consecutive compressions or two consecutive rarefactions in a wave.
Must knowIn longitudinal waves, particles oscillate parallel to the direction of wave propagation; sound waves are generally longitudinal.
In transverse waves, particles oscillate perpendicular (90°) to the direction of wave travel, such as ripples on water or waves in a string.

Sound Phenomena: Echo, Reverberation, and Resonance

An echo is the reflected sound heard when original sound strikes a distant, hard surface and returns.
Must knowTo clearly hear an echo, the time gap between the original and reflected sound must be at least 0.1 second (1/10 second), and the reflecting surface must be at least 17.2 metres (or around 17 metres) away.
Reverberation is the persistence of sound caused by repeated reflections in an enclosed hall or room, making the audio unclear.
Resonance occurs when an object is forced to vibrate at its natural frequency by an external sound wave, causing a drastic increase in amplitude.
Soldiers break step while crossing a bridge to prevent resonance that could match the natural frequency of the structure and cause it to collapse.

Intensity, Sound Levels, and Hearing Range

Sound intensity is the sound energy transmitted per second per unit area perpendicular to the direction of propagation, measured in W/m².
The threshold of hearing for a normal human ear has an intensity of 10⁻¹² W/m².
Sound level is expressed on a logarithmic scale measured in decibels (dB).
Audible sound ranges from 20 Hz to 20,000 Hz, which is the range that the human ear can detect.
Must knowInfrasonic sound has a frequency below 20 Hz (e.g., earthquake waves, volcanic eruptions, elephant rumbles), while ultrasonic sound has a frequency above 20,000 Hz (e.g., bats, dolphins).

Sound Source	Sound Level (dB)
Whispering of leaves	10 - 20 dB
Normal conversation	60 dB
Busy traffic	80 dB
Traffic horn	90 dB
Loudspeaker (nearby)	100 dB
Jet aircraft	120 dB
Pain threshold / threshold of discomfort	130 dB
Explosions	140 dB

Characteristics of Sound and Voice Production

Pitch indicates the shrillness or gravity of a sound and depends directly on frequency; higher frequency gives a higher, shriller pitch, whereas lower frequency produces a deeper, heavier voice.
Loudness measures the strength or volume of a sound and depends directly on amplitude; higher amplitude creates louder sounds.
Timbre (quality) allows the ear to distinguish between two sounds possessing the identical pitch and loudness, depending on overtones.
Human speech is generated in the larynx (voice box), where exhaled air from the lungs passes through vocal cords, causing them to vibrate.
Tension in the vocal cords alters the pitch: higher tension yields higher frequency and a shriller voice.

Applications of Reflection and Ultrasound

Megaphones and horns use reflection through funnel-like expanding shapes to direct and focus sound in a single forward direction.
A stethoscope relies on multiple reflections of sound waves through a tube to transmit internal body sounds (heart, lungs) to a physician's ears.
SONAR (Sound Navigation and Ranging) uses ultrasonic waves to detect the depth of the sea floor and locate submerged objects like submarines or shipwrecks.
Ultrasonic testing is used in medicine for fetal imaging (sonography) and breaking kidney stones (lithotripsy).
In industries, ultrasonic Non-Destructive Testing (NDT) detects internal flaws, cracks, and cavities in metallic structures without damaging them.

Structure of the Human Ear

The human ear is structurally divided into three regions: outer ear, middle ear, and inner ear.
The outer ear consists of the pinna and auditory canal, which collect sound waves and channel them to the eardrum (tympanic membrane).
The middle ear transfers vibrations from the eardrum via three auditory ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
Must knowThe stapes is the smallest bone in the human body.
The inner ear houses the cochlea, which converts mechanical vibrations into electrical signals that travel via the auditory nerve to the brain.
Semicircular canals located within the inner ear maintain dynamic body equilibrium and posture.

Factors Affecting Speed of Sound

The speed of sound in air at 0°C is 331 m/s (or 332 m/s).
Speed of sound increases with temperature at a rate of approximately 0.61 m/s for every 1°C rise in temperature.
Increasing humidity increases the speed of sound because moist air has lower density than dry air.
Pressure variations have no effect on the speed of sound under normal conditions.
Must knowNewton assumed sound propagation in gases was isothermal, giving v = sqrt(P / ρ); Laplace corrected this to an adiabatic process, giving v = sqrt(γ × P / ρ).

Superposition, Beats, and Doppler Effect

The principle of superposition states that when two waves overlap, the resultant displacement equals the algebraic sum of their individual displacements.
Constructive interference occurs when waves meet in phase (crest to crest), resulting in an increased amplitude (AR = A1 + A2).
Destructive interference occurs when waves meet out of phase (crest to trough), decreasing the resultant amplitude (AR = |A1 - A2|).
Beats are periodic waxing and waning of sound intensity produced when two sound waves of slightly different frequencies superpose; beat frequency equals |f1 - f2|.
Must knowThe Doppler effect is the apparent change in frequency (pitch) of a sound observed when there is relative motion between the source and the listener.
When a sound source moves towards a stationary listener, the perceived frequency increases (pitch becomes shriller); when moving away, the perceived frequency decreases.

Noise Pollution Standards

Continuous sound levels exceeding 85 dB are harmful to human health according to WHO guidelines.
Industrial zones have permissible noise limits of 75 dB during the day and 70 dB at night.
Commercial zones have permissible noise limits of 65 dB during the day and 55 dB at night.
Residential zones have permissible limits of 55 dB during the day and 45 dB at night.
Silence zones (schools, hospitals) have permissible limits of 50 dB during the day and 40 dB at night.

Worked Numerical Problems

Numerical 1: A person claps and hears an echo after 3 s. If the speed of sound in air is 340 m/s, the distance d = (v × t) / 2 = (340 × 3) / 2 = 1020 / 2 = 510 m.
Numerical 2: A SONAR signal returns after 4 s through water where sound speed is 1500 m/s. Depth h = (v × t) / 2 = (1500 × 4) / 2 = 6000 / 2 = 3000 m.
Numerical 3: For a wave with frequency 500 Hz and wavelength 0.68 m, speed v = f × λ = 500 × 0.68 = 340 m/s, and time period T = 1 / f = 1 / 500 = 0.002 s.
Numerical 4: A person standing 850 m from a cliff claps; speed of sound is 340 m/s. Total time taken for echo t = (2 × d) / v = (2 × 850) / 340 = 1700 / 340 = 5 s.
Numerical 5: A sound wave has speed 343 m/s and wavelength 0.7 m. Frequency f = v / λ = 343 / 0.7 = 490 Hz.
Numerical 6: To find the speed of sound at 25°C with base speed 331 m/s at 0°C: v25 = 331 + (0.61 × 25) = 331 + 15.25 = 346.25 m/s.

Key terms

Mechanical wave: A wave that requires a material medium to propagate
Amplitude: Maximum displacement of a particle from its rest position
Frequency: Number of complete wave vibrations per second
Wavelength: Linear distance between two successive compressions or rarefactions
Infrasonic: Sound with a frequency below 20 Hz
Ultrasonic: Sound with a frequency exceeding 20,000 Hz
Pitch: Characteristic of sound dictated by frequency, determining shrillness
Timbre: Quality enabling differentiation between two sounds of same pitch and loudness
Echo: Distinct sound formed by reflection off an obstacle at a distance
Reverberation: Prolonged persistence of sound in a closed space due to multiple reflections
Beats: Periodic variations in sound intensity caused by superposition of slightly different frequencies
Doppler Effect: Apparent frequency shift caused by relative motion between source and listener

Formulas

v = f × λ: Calculates wave speed from frequency and wavelength
T = 1 / f: Calculates wave time period from frequency
d = (v × t) / 2: Calculates distance to reflecting surface in echo/SONAR
vt = v0 + 0.61 × t: Calculates speed of sound in air at temperature t (°C)
v = sqrt(γ × P / ρ): Laplace formula for speed of sound in gas under adiabatic conditions
Beat frequency =: f1 - f2| | Calculates number of beats heard per second

Exam tips and traps

Normal pressure variations do not change the speed of sound in air, but temperature and humidity do.
For echo questions, always divide total round-trip distance or time by 2 to get the one-way distance to the reflector.

Quick revision

Read these the night before the exam.

Sound cannot propagate through a vacuum.
The human audible frequency range is 20 Hz to 20,000 Hz.
Minimum distance to hear a distinct echo in air is approximately 17.2 metres.
Minimum time interval required between original and reflected sound to detect an echo is 0.1 second.
The speed of sound in air at 0°C is 331 m/s.
Pitch of sound depends on frequency, while loudness depends on amplitude.
Stapes is the smallest bone in the human body, located in the middle ear.
Prolonged exposure to noise levels above 85 dB is considered hazardous by WHO.
Sound propagates as an adiabatic process in gases as corrected by Laplace.
Semicircular canals in the inner ear help maintain body balance.

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