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Physics: Sound (Dhwani) MCQs and Key Concepts

A practice class on Sound (Dhwani) for teacher recruitment exams covering key physics principles, formulas, and wave properties. It discusses factors affecting the speed of sound, wave characteristics like pitch and amplitude, Doppler effect, resonance, and the differences between infrasonic, audible, and ultrasonic frequencies.

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Physics
English · 8 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video UP SUPERTET SCIENCE Class-13 | ध्वनि (SOUND) | 61MCQ | | 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.

Physics: Sound (Dhwani) MCQs and Key Concepts

English8 topics · 5 MCQs · 11 flashcardsWatch the video (opens YouTube in a new tab)

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In short

A practice class on Sound (Dhwani) for teacher recruitment exams covering key physics principles, formulas, and wave properties. It discusses factors affecting the speed of sound, wave characteristics like pitch and amplitude, Doppler effect, resonance, and the differences between infrasonic, audible, and ultrasonic frequencies.

Speed of Sound in Gases, Time Period, and Medical Ultrasound

Must knowAt a constant temperature, the speed of sound in a gas is inversely proportional to the square root of its density, making it highest in hydrogen gas (H2) because H2 has the lowest density.
The time period (T) of a vibrating body is the reciprocal of its frequency (f), represented as T = 1 / f.
A body vibrating at a frequency of 5000 Hz has a time period of 1 / 5000 = 0.0002 seconds.
Must knowUltrasonic waves used in medical diagnostic devices such as sonography have an operating frequency greater than 20,000 Hz.

Pitch, Loudness, and Wave Classification

Pitch depends directly on frequency, while loudness depends directly on the amplitude of the vibrating wave.
If the frequency of vibration decreases and the amplitude increases, the pitch decreases while the loudness increases.
Sound waves are mechanical waves and require a material medium for transmission, unlike electromagnetic waves such as UV rays, radio waves, and X-rays.
Must knowSound waves cannot exhibit polarization because polarization is an exclusive property of transverse waves, whereas sound waves in air are longitudinal.

Decibel Limits, Supersonic Flight, and Sound Speed in Media

The pitch of a sound determines how shrill or deep it sounds and depends entirely on frequency.
Must knowAccording to the World Health Organization (WHO), sound levels exceeding 80 to 85 dB are categorized as noise and are harmful to human ears.
Supersonic aircraft fly at speeds greater than Mach 1, which means faster than the speed of sound in air.
Must knowMach number is the ratio of the speed of an object to the speed of sound in the surrounding medium.
The distance between two adjacent compressions or two adjacent rarefactions in a longitudinal wave defines its wavelength (lambda).
The sound level of normal conversation between two people is approximately 60 dB, while whisper levels range around 15 to 20 dB.
Radio waves travel at the speed of light in vacuum, which is 3 × 10⁸ m/s.
Frequency is defined as the number of vibrations per second, and its SI unit is Hertz (Hz).

Echo, Human Hearing Range, and Infrasound

Must knowTo hear a clear echo in air, the minimum required distance between the source and the reflecting obstacle is 17.2 meters, with a minimum time interval of 0.1 seconds.
Women typically have shriller voices than men due to shorter vocal cords vibrating at a higher frequency, resulting in a higher pitch.
Telecommunication, radar, and satellite systems primarily utilize microwaves and radio waves.
Stethoscopes work on the principle of multiple reflection of sound waves inside the tube.
The difference between notes of identical pitch and loudness played on different instruments (like sitar and flute) is distinguished by their timbre (quality), which depends on overtones.
The speed of sound depends on elasticity and density, being fastest in solids, intermediate in liquids, and slowest in gases; it cannot travel in a vacuum.
At 0 degrees Celsius, the speed of sound in air is approximately 331 m/s, rising to about 343 m/s at 20 degrees Celsius.
The speed of sound in water (liquid) is roughly 1500 m/s.

Wave Equation, Reverberation, and Ultrasound Applications

The time taken to complete one full oscillation or vibration is called the time period (T), with the SI unit of second.
The relationship between wave speed (v), frequency (f), and wavelength (lambda) is given by the formula v = f × lambda.
The persistent reflection and prolongation of sound within an enclosed auditorium or hall is known as reverberation.
Must knowThe normal audible frequency range for human hearing is 20 Hz to 20,000 Hz (20 Hz to 20 kHz).
Frequencies below 20 Hz are termed infrasonic; these are produced during earthquakes and perceived by animals such as elephants and whales.
Frequencies above 20,000 Hz are ultrasonic, utilized and heard by dogs, bats, and dolphins.
Bats emit ultrasonic waves during nocturnal flight to detect obstacles and prey through reflected echoes.

Vocal Mechanism, SONAR, and Environmental Factors on Sound Speed

Sound in the human body is generated in the larynx (voice box) via vibrations of the vocal cords driven by air from the lungs.
Must knowSONAR stands for Sound Navigation and Ranging, which employs ultrasonic waves to determine water depth and locate submerged objects.
The formula to measure depth (h) using SONAR is h = (v × t) / 2, where v is sound velocity in water and t is total round-trip travel time.
Increasing temperature increases the kinetic energy of air molecules, increasing the speed of sound by 0.61 m/s for every 1 degree Celsius rise.
Increased humidity reduces the density of air compared to dry air, thereby increasing the speed of sound.
Under constant temperature, changes in atmospheric pressure do not affect the speed of sound in air because density changes proportionally.

Newton-Laplace Formula and Wave Interference

Newton initially assumed that sound propagation in gases is an isothermal process where temperature remains constant.
Must knowLaplace corrected Newton's formula by establishing that compression and rarefaction happen too rapidly for heat exchange, making sound propagation an adiabatic process.
When two sound waves meet in phase (crest on crest), constructive interference occurs, resulting in increased resultant amplitude and louder sound.
When two sound waves of slightly different frequencies combine, periodic rising and falling of sound intensity occurs, known as beats.
Must knowThe apparent change in frequency or pitch due to relative motion between a sound source and an observer is called the Doppler effect.

Sound Intensity, Resonance, and Noise Reduction

The SI unit of sound intensity is watt per square meter (W/m²), whereas sound level is measured on a logarithmic scale in decibels (dB).
The threshold of hearing for humans corresponds to an intensity of 10⁻¹² W/m² (or 0 dB).
Megaphones and horns guide sound along a specific direction using multiple reflections to prevent sound from dispersing widely.
Must knowResonance occurs when an external periodic force matches the natural frequency of an object, causing a dramatic increase in amplitude.
Marching soldiers are ordered to break step when crossing suspension bridges to avoid destructive resonance that could collapse the bridge.
A wave with a frequency of 500 Hz and a wavelength of 0.68 m travels at a speed of 340 m/s (v = 500 × 0.68).
Planting rows of dense trees along highways to absorb noise pollution is referred to as the "Green Muffler" technique.
A compression in a longitudinal wave represents a region of maximum pressure and maximum density where medium particles are crowded together.

Gas / Medium	Property / Factor	Speed Relation
Hydrogen (H2)	Lowest density	Highest speed among gases
Solids	High elasticity	Maximum speed of sound
Liquids (Water)	Moderate elasticity	~1500 m/s
Gases (Air at 20°C)	Low elasticity	~343 m/s
Vacuum	No medium particles	Zero (cannot travel)

Key terms

Mach Number: The ratio of the speed of an object to the speed of sound in the surrounding medium.
Pitch: The auditory characteristic of sound determined solely by frequency that makes a sound shrill or deep.
Reverberation: The persistence and multiple reflections of sound in an enclosed hall after the source has stopped.
Beats: The periodic fluctuation in sound intensity caused by the superposition of two waves of slightly different frequencies.
Doppler Effect: The apparent shift in observed frequency when there is relative motion between a sound source and the listener.
Green Muffler: Planting belts of trees along roadways and populated areas to absorb noise pollution.

Formulas

T = 1 / f: Calculating the time period from wave frequency
v = f × lambda: Calculating wave velocity from frequency and wavelength
Mach Number = Speed of body / Speed of sound: Determining the Mach speed of an aircraft
d = (v × t) / 2: Calculating echo distance or SONAR depth given total transit time
Speed increase with temperature = 0.61 m/s per 1°C: Estimating change in sound speed in air due to temperature

Quick revision

Read these the night before the exam.

Sound cannot propagate through a vacuum because it requires a material medium.
Polarization is exhibited only by transverse waves and not by longitudinal sound waves.
Normal conversational speech produces an average sound level of roughly 60 dB.
The minimum distance required to hear an echo clearly in air is 17.2 meters.
Normal human hearing covers the frequency spectrum from 20 Hz to 20,000 Hz.
Laplace corrected Newton's formula by treating gas sound propagation as an adiabatic process.
The threshold of human hearing intensity is 10⁻¹² W/m², which equals 0 dB.
Atmospheric pressure changes do not alter the speed of sound if temperature stays constant.
High humidity decreases air density, thereby increasing the speed of sound.
Compression regions in longitudinal sound waves feature maximum pressure and maximum density.

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