Sound is part of Physics. Below are notes on 12 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Acoustics of Buildings: Reverberation Control and Noise Pollution

  • Reverberation: Persistence of sound in an enclosed space after the source stops, due to repeated reflections.
  • Reverberation Time (RT): Time taken for the sound intensity to fall by 60 dB (or to onemillionth of its initial value) after the sound source stops.
  • To decrease reverberation time (and improve sound clarity):
    • Increase sound absorption within the hall.
    • Methods include:
      • Using sound-absorbing materials (e.g., carpets, curtains, draperies, soft furnishings, acoustic tiles, false ceilings).
      • Opening windows and doors (allows sound to escape/be absorbed by the external environment).
      • Presence of an audience (people, their clothing, and seats absorb sound).
  • For music concert halls, walls and surfaces should primarily absorb sound to control reverberation and prevent echoes, ensuring clear audibility.
  • Noise Pollution: Unwanted or excessive sound that can negatively impact health.
  • Typical Noise Levels (approximate dB):
    • Normal conversation: ~60 dB
    • Heavy traffic: ~80-90 dB
    • Jet plane take-off (at close range): ~120-150 dB.
    • Threshold of pain: ~120 dB.

Characteristics of Sound: Pitch, Loudness (Intensity, Decibel Scale), and Quality (Timbre)

  • Sound Characteristics:
    • Sound waves in gases are longitudinal.
    • Human audible frequency range: 20 Hz to 20,000 Hz.
    • Infrasonic waves: Frequencies below 20 Hz.
    • Ultrasonic waves: Frequencies above 20,000 Hz.
  • 1. Pitch:
    • Determines the shrillness or graveness of a sound.
    • Depends directly on the frequency of the sound wave.
    • Higher frequency = Higher pitch (more shrill, e.g., female voice).
    • Lower frequency = Lower pitch (more grave, e.g., male voice).
    • An octave higher signifies double the frequency (e.g., 500 Hz to 1000 Hz).
  • 2. Loudness / Intensity:
    • Determines how loud or faint a sound is.
    • Depends directly on the amplitude of the sound wave.
    • Higher amplitude = Louder sound.
    • Intensity: The amount of sound energy passing per unit time per unit area.
    • Unit of sound intensity: Decibel (dB).
    • Decibel Scale (Approximate levels):
      • Rustling leaves: ~20 dB.
      • Whisper: ~10-20 dB.
      • Normal conversation: ~35-60 dB (optimum ~60 dB).
      • Optimum sound level for human ear: ~60 dB.
      • Noisy street / Machine shop: ~90-100 dB.
      • Pain threshold: ~120 dB.
  • 3. Quality / Timbre:
    • Allows us to distinguish between sounds of the same pitch and loudness produced by different sources or instruments (e.g., a violin vs. a flute playing the same note).
    • Depends on the waveform (shape) of the sound wave.
    • Related to the presence and relative intensity of overtones (harmonics) superimposed on the fundamental frequency.
    • Helps in recognizing individual voices or musical instruments.

Doppler Effect in Sound and its Applications

  • Doppler Effect: Apparent change in frequency or pitch of sound perceived by an observer due to relative motion between the sound source and the observer.
  • Mechanism: Apparent frequency increases when source and observer move towards each other; decreases when they move away from each other.
  • Applications of Doppler Effect (Sound):
    • Medical: Doppler Ultrasound (e.g., measuring blood flow velocity, fetal heart monitoring, diagnosing cardiac conditions).
    • Meteorology: Doppler weather radar (detects precipitation, wind speed, and direction).
    • Speed Measurement: Used in some speed guns (though many use electromagnetic waves, the principle is analogous).
  • Sonic Boom: A loud, explosive noise caused by the shock wave from an object moving faster than the speed of sound through the air.
  • Cause of Sonic Boom: Occurs when an object travels at supersonic speed (speed greater than the speed of sound), causing pressure waves to coalesce and form a conical shock wave (Mach cone) that trails behind the object.
  • Supersonic Speed: Refers to a speed greater than the speed of sound (Mach number > 1).
  • Mach Number: Ratio of the speed of an object to the speed of sound in the surrounding medium. Mach 1 indicates the speed of sound.

Energy and Intensity of Sound Waves

  • Sound waves are mechanical waves that require a medium for propagation.
  • Sound waves transmit energy and momentum through a medium, but do not transmit matter itself.
  • The speed of sound (v) is related to its frequency (f) and wavelength (λ) by the formula: `v = f × λ`.
  • The distance (d) traveled by a sound wave is given by: `d = v × t`, where `t` is the time taken.
  • The time (t) taken by a sound wave to travel a distance (d) is given by: `t = d / v`.
  • Common unit conversions for calculations: 1 kHz = 1000 Hz, 1 m = 100 cm, 1 km = 1000 m.

Musical Instruments: Principles of Sound Production

  • Adjusting string tension/length in musical instruments (e.g., sitar) modifies its frequency (pitch) to match other instruments for harmonious tuning.
  • A microphone converts sound energy (mechanical vibrations) into electrical signals.
  • A speaker (or loudspeaker) converts electrical signals into sound energy (mechanical vibrations).
  • Metals are sonorous, meaning they produce a clear, ringing sound when struck (e.g., for school bells).
  • Sound in a flute (a wind instrument) is produced by a vibrating column of air inside the instrument.
  • The pitch of a note produced by a flute is determined by the frequency of the vibrating air column (related to its effective length).
  • The loudness of a note produced by a flute is determined by the amplitude of the vibrating air column, influenced by the momentum of the blowing jet.
  • An electric motor converts electrical energy into mechanical energy.
  • A solar cooker typically uses a concave mirror to concentrate solar energy.

Nature and Propagation of Sound Waves

Sound waves are mechanical waves, requir1. ing a material medium (solid, liquid, or gas) for propagation. Sound waves are longitudinal waves, mean2. ing particles of the medium oscillate parallel to the direction of wave propagation, creating compressions and rarefactions. They are not transverse waves. Sound waves cannot travel through a vacu3. um. Sound can propagate through all states of 4. matter: solids (e.g., copper wire, silver slab, steel), liquids (e.g., water), and gases (e.g., air). The speed of sound is generally highest in 5. solids, followed by liquids, and slowest in gases (Speed: Solids > Liquids > Gases). Sound travels much slower than light. This is 6. evident in phenomena like thunderstorms where lightning (light) is seen before thunder (sound) is heard. Sound travels faster in moist air than in dry 7. air due to the lower density of water vapor compared to dry air at the same temperature and pressure. Wavelength (λ) is the distance between two 8. consecutive compressions (or two consecutive rarefactions) in a sound wave. The relationship between speed (v), fre9. quency (f), and wavelength (λ) is: v = f × λ. Time taken (t) to travel a distance (d) at a 10. constant speed (v) is: t = d / v. A microphone converts sound waves into 11. electrical signals.

Reflection of Sound: Echo, Reverberation, and their Applications

  • Reflection of Sound: Phenomenon where sound waves bounce off a surface.
  • Echo:
    • Definition: A distinct sound heard after the original sound, caused by its reflection from a surface.
    • Cause: Single reflection of sound.
    • Condition for distinct echo: Minimum time interval between original and reflected sound must be at least 0.1 seconds (persistence of hearing).
    • Minimum distance for distinct echo:
      • If speed of sound (v) is 340 m/s, minimum distance (d) = (v × t) / 2 = (340 m/s × 0.1 s) / 2 = 17 meters.
      • Value varies slightly with assumed speed (e.g., 16.5m for 330m/s).
    • Calculation: Distance to reflecting surface (d) = (Speed of sound (v) × Time taken for echo (t)) / 2.
    • Reflection from a rigid wall: A compression (crest) of sound wave reflects as a compression with a phase change of π (180°).
    • Applications: SONAR (Sound Navigation and Ranging) for measuring ocean depth, detecting underwater objects; bats and dolphins for navigation and hunting.
  • Reverberation:
    • Definition: The persistence of sound in a large enclosed space due to repeated (multiple) reflections from walls, ceiling, and floor.
    • Cause: Multiple reflections of sound.
    • Can be undesirable if too prolonged, leading to indistinct sound; controlled by sound-absorbing materials.
  • Applications of Multiple Reflection of Sound (General):
    • Stethoscope: Sound of heartbeat reaches doctor's ears through multiple reflections within the tube.
    • Megaphones, horns, trumpets, shehnais: Designed to send sound in a particular direction by multiple reflections, concentrating sound energy.
    • Soundboards: Curved ceilings/boards behind the stage in large halls use multiple reflections to uniformly distribute sound.

Refraction and Diffraction of Sound Waves

  • Refraction of Sound: Bending of sound waves as they pass from one medium to another or through a medium with varying properties (e.g., temperature, density).
  • Refraction occurs due to a change in the speed of sound.
  • Example: During the day, air near the ground is warmer (higher sound speed), causing sound to bend upwards; at night, air near the ground is cooler (lower sound speed), causing sound to bend downwards and travel further.
  • Diffraction of Sound: Bending of sound waves around obstacles or through openings.
  • Diffraction is more noticeable when the wavelength of sound is comparable to or larger than the size of the obstacle or opening.
  • Sound waves have relatively long wavelengths (e.g., audible range ~1.7 cm to 17 m), making diffraction a common phenomenon (e.g., hearing sound around corners).
  • Nature of Sound Waves: Sound waves are longitudinal waves.
  • Phenomena Exhibited by Sound Waves: Reflection, Refraction, Interference, and Diffraction.
  • Phenomena NOT Exhibited by Sound Waves: Polarisation.
  • Reason for No Polarisation: Polarisation is a characteristic property of transverse waves, where oscillations occur perpendicular to the direction of wave propagation and can be restricted to a single plane. Longitudinal waves oscillate parallel to their direction of propagation, hence they cannot be polarised.

Speed of Sound in Different Media and Factors Affecting It

  • Sound is a mechanical wave, requiring a medium to propagate; it cannot travel in a vacuum.
  • Speed of sound is determined by the medium's elastic properties (resistance to deformation) and inertial properties (density). Generally, `v = sqrt(Elasticity/Density)`.
  • General Order of Speed: Solids > Liquids > Gases.
  • Speed in Solids: Highest due to high elasticity and tightly packed particles (e.g., Steel/ Iron: ~5100-6100 m/s; Glass: ~4500-6000 m/s; Aluminium: ~6420 m/s).
  • Speed in Liquids: Intermediate (e.g., Water at 20°C: ~1500 m/s).
  • Speed in Gases: Lowest (e.g., Air at 20°C: ~343 m/s).
  • Relative Speeds (Example): Iron > Water > Air.
  • Effect of Temperature:
    • Speed of sound increases with increasing temperature in most media, especially gases (`vair ~ 331.4 + 0.6T` m/s, where T is in °C).
    • Decreasing temperature decreases sound speed.
  • Effect of Pressure:
    • In gases, the speed of sound is independent of pressure, provided temperature remains constant. (Pressure and density change proportionally, cancelling out the effect).
  • Effect of Humidity:
    • Speed of sound increases with increasing humidity in air. Humid air is less dense than dry air at the same temperature and pressure (water vapor is lighter than N2/O2), leading to higher speed.
    • A misty/humid night can lead to sound traveling further/being heard clearer due to higher humidity and potential temperature inversions.
  • Effect of Frequency/Amplitude:
    • The speed of sound in a given medium is independent of its frequency, wavelength, and amplitude.
  • Relative Speeds in Specific Liquids: Denser liquids (e.g., Sea water due to dissolved salts) generally have a slightly higher speed of sound than less dense liquids (e.g., Distilled water).
  • Relative Speeds in Specific Gases: Lighter gases allow sound to travel faster due to lower molar mass.
    • Order of speed (fastest to slowest): Hydrogen (H2) > Helium (He) > Oxygen (O2) > Sulphur Dioxide (SO2).
  • Mach Number: A speed with a Mach number greater than 1 is considered supersonic.

Stationary Waves (Standing Waves) in Strings and Organ Pipes

  • Stationary Waves (Standing Waves): Formed by superposition of two identical progressive waves (same amplitude, frequency, speed) traveling in opposite directions. They do not transfer energy. Characterized by fixed positions of zero displacement (nodes) and maximum displacement (antinodes).
  • Stationary Waves in Strings (e.g., Sitar, Guitar, Violin):
    • Nature: Transverse waves.
    • Boundary Conditions: Fixed ends are always nodes.
    • Harmonics/Overtones: All integral multiples of the fundamental frequency are possible (1st, 2nd, 3rd, ... harmonics).
    • Fundamental Frequency (1st Harmonic): \(f1 = \frac{v}{2L}\), where \(v\) is wave speed in the string and \(L\) is string length.
  • Stationary Waves in Organ Pipes (Air Columns): Longitudinal waves.
  • Open Organ Pipe (Open at both ends, e.g., Flute):
    • Boundary Conditions: Open ends are always antinodes.
    • Harmonics/Overtones: All integral multiples of the fundamental frequency are possible (1st, 2nd, 3rd, ... harmonics).
    • Fundamental Frequency (1st Harmonic): \(f1 = \frac{v}{2L}\), where \(v\) is speed of sound in air and \(L\) is pipe length.
  • Closed Organ Pipe (Closed at one end, Open at other, e.g., Clarinet):
    • Boundary Conditions: Closed end is a node, open end is an antinode.
    • Harmonics/Overtones: Only odd integral multiples of the fundamental frequency are possible (1st, 3rd, 5th, ... harmonics).
    • Fundamental Frequency (1st Harmonic): \(f1 = \frac{v}{4L}\), where \(v\) is speed of sound in air and \(L\) is pipe length.

The Human Ear: Structure and Mechanism of Hearing

  • The human ear is sensitive to sound frequencies ranging from 20 Hz to 20,000 Hz (or 20 kHz).
  • Frequencies below 20 Hz are infrasound and are inaudible to humans.
  • Frequencies above 20,000 Hz are ultrasound and are inaudible to humans.
  • The human ear is most sensitive to sound frequencies typically between 1000 Hz and 5000 Hz.
  • Peak sensitivity for the human ear often occurs around 1000 Hz.

Ultrasound and Infrasound: Production, Properties, and Applications (SONAR, Medical, Industrial)

  • Sound Frequencies:
  • Infrasound: Frequencies below 20 Hz.
  • Audible Sound: Frequencies between 20 Hz and 20,000 Hz (20 kHz).
  • Ultrasound: Frequencies above 20,000 Hz (20 kHz).
  • Infrasound:
  • Produced naturally by events like earthquakes (before main shock waves), volcanoes, and large animals (e.g., whales, elephants).
  • Ultrasound Production:
  • Primarily generated by the piezoelectric effect, utilizing piezoelectric materials (e.g., quartz crystals) under an alternating electric field.
  • Ultrasound Properties:
  • Are longitudinal waves.
  • Require a material medium for propagation; cannot travel through a vacuum.
  • Have the same speed as audible sound waves in the same medium. Speed is dependent on the medium, not the frequency.
  • Possess higher frequency and consequently shorter wavelength compared to audible sound waves.
  • Can be reflected, refracted, and absorbed.
  • Exhibit high directionality due to their shorter wavelength.
  • Ultrasound Applications:
  • SONAR (Sound Navigation And Ranging): Uses ultrasonic waves to measure distance, direction, and speed of underwater objects (e.g., submarines, icebergs, shipwrecks, seabed topography). Primarily used by navigators.
  • Medical Imaging (Ultrasonography): Noninvasive technique to visualize internal organs (e.g., fetus, heart, liver, kidneys) and detect abnormalities like gallstones, kidney stones, and tumors.
  • Medical Therapy: Used for breaking kidney stones (lithotripsy) and other therapeutic applications.
  • Industrial Defect Detection: Detects cracks, flaws, porosity, and other internal defects in metal blocks, railway tracks, and structural components without damage.
  • Industrial Cleaning: Cleans intricate parts of machinery, watches, and electronic components using cavitation.
  • Drilling/Cutting: Used for making holes or cutting very hard materials like diamond and glass.
  • Exploration: Used in exploring underwater petroleum deposits.
  • Wavelength Calculation: Wavelength (λ) = Speed (v) / Frequency (f). E.g., for v = 3000 m/s and f = 3 × 10⁵ Hz, λ = 10⁻² m = 1 cm.

10. Electric Current

Practice MCQs: Sound

Acoustics of Buildings: Reverberation Control and Noise Pollution

  1. The time of reverberation of a hall can be decreased by

    Asked in: TNPSC CS Prelims 2019

    1. having a number of loud speakers
    2. opening all windows and doors
    3. closing all windows and doors
    4. speaking loudly
    Show answer

    Answer: B. opening all windows and doors

    Reverberation time is the time it takes for sound intensity to decay by 60 decibels after the sound source stops. It is affected by the volume of the space and the amount of sound absorption.

  2. The walls of the hall built for music concerts should

    Asked in: UPPSC Upper Mains 2007

    1. amplify sound
    2. transmit sound
    3. reflect sound
    4. absorb sound
    Show answer

    Answer: D. absorb sound

    In a music concert hall, it is desirable to have clear sound without excessive echo or reverberation, which can muddy the music. Sound-absorbing materials on the walls and ceiling help to reduce reflections, thus controlling the reverberation time and improving the clarity and definition of the sound.

  3. The noise level during plane take off is around

    Asked in: WBCS Main Paper 4 2019

    1. 100 dB
    2. 150 dB
    3. 200 dB
    4. 250 dB
    Show answer

    Answer: B. 150 dB

    Decibels (dB) measure sound intensity. Levels above 120 dB are generally considered painful and can cause immediate hearing damage.

  4. In concert halls, soft materials and carpets are used to

    Asked in: Civil Engg GA 24 2023 CO 1505 2024

    1. increase sound waves
    2. absorb sound waves
    3. refract sound waves
    4. retract sound waves
    Show answer

    Answer: B. absorb sound waves

    Sound absorption is a crucial aspect of acoustic design in enclosed spaces like auditoriums and concert halls. Soft, porous materials are effective in converting sound energy into heat energy.

Characteristics of Sound: Pitch, Loudness (Intensity, Decibel Scale), and Quality (Timbre)

  1. The property of the sound waves that determines the pitch of the sound is its

    Asked in: UPSC CDS 1 2020

    1. frequency
    2. amplitude
    3. wavelength
    4. intensity
    Show answer

    Answer: A. frequency

    - Pitch is the subjective perception of the frequency of sound. - Higher frequency means higher pitch. - Amplitude relates to loudness or intensity.

  2. The frequency (in Hz) of a note that is one octave higher than 500 Hz is

    Asked in: UPSC CDS 2 2019

    1. 375
    2. 750
    3. 1000
    4. 2000
    Show answer

    Answer: C. 1000

    - In music and acoustics, an octave is an interval between two notes where the higher note has a frequency exactly double that of the lower note. - Conversely, a note one octave lower than a given note has half its frequency.

  3. The pitch of a sound wave depends upon which one of its following characteristics ?

    Asked in: UPSC CDS 2 2022

    1. Speed
    2. Loudness
    3. Amplitude
    4. Frequency
    Show answer

    Answer: D. Frequency

    - Pitch is the quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone. - Frequency is the number of vibrations per unit of time.

  4. The intensity of a normal conversation in decibel is between :

    Asked in: HPPSC HPAS Prelims 2018

    1. 30-35
    2. 60-80
    3. 35-60
    4. 70-80
    Show answer

    Answer: C. 35-60

    Decibels measure the intensity or loudness of sound. Normal human conversation occurs within a specific range of decibel levels, distinct from whispers, shouts, or environmental noise.

  5. एक सिामान्™ बातचीत की तीव्रता डेसिीबल में इनमें सिे ͩकसिके बीच रहती है ?

    Asked in: HPPSC HPAS Prelims 2018

    1. 30-35
    2. 60-80
    3. 35-60
    4. 70-80
    Show answer

    Answer: C. 35-60

    Sound intensity is measured in decibels (dB). A normal conversation typically falls within the range of approximately 50 to 65 dB. Quiet conversation or background noise might be around 30-40 dB. Option (C) 35-60 dB encompasses this range well, covering variations from softer speech to standard speaking volume. Options (A) 30-35 dB is too low for a normal conversation, while options (B) 60-80 dB and (D) 70-80 dB represent noise levels higher than normal conversation (e.g., busy traffic, vacuum cleaner).

  6. What is the audible range (hearing range) of humans?

    Asked in: MPPSC CS Prelims 2018

    1. 20 Hz - 20000 Hz
    2. 80 Hz - 100 Hz
    3. 2 lac Hz - 4 lac Hz
    4. 0 Hz - 20 Hz
    Show answer

    Answer: A. 20 Hz - 20000 Hz

    Frequencies below 20 Hz are called infrasound, and frequencies above 20,000 Hz are called ultrasound; neither are typically audible to humans.

  7. The loudness of sound is related to:

    Asked in: UPSC NDA 2 2015

    1. its frequency
    2. its amplitude
    3. its speed
    4. its pitch
    Show answer

    Answer: B. its amplitude

    Loudness is the perceptual attribute of sound that is most closely related to the intensity of the sound wave, which depends on its amplitude.

  8. The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Statement I : The pitch of the sound wave depends upon its frequency. Statement II : The loudness of the sound wave depends upon its amplitude.

    Asked in: UPSC NDA 2 2018

    1. Both the statements are individually true and Statement II is the correct explanation of Statement I
    2. Both the statements are individually true but Statement II is not the correct explanation of Statement I
    3. Statement I is true but Statement II is false
    4. Statement I is false but Statement II is true
    Show answer

    Answer: B. Both the statements are individually true but Statement II is not the correct explanation of Statement I

    - Pitch is the perceptual property that allows sounds to be ordered on a frequency-related scale, or more commonly, pitch is the quality that makes it possible to judge sounds as "higher" and "lower". - Loudness is the subjective perception of sound pressure. It is related to the intensity of the sound wave, which in turn is proportional to the square of the amplitude. - Frequency and amplitude are independent physical characteristics of a wave.

Doppler Effect in Sound and its Applications

  1. Sonic boom is produced when a source of sound travels at a speed :

    Asked in: UPSC CDS 2 2023

    1. greater than the speed of sound.
    2. greater than the speed of light.
    3. lesser than the speed of sound.
    4. equal to the speed of sound.
    Show answer

    Answer: A. greater than the speed of sound.

    A sonic boom is a loud sound produced when an object, such as an aircraft, travels through the air faster than the speed of sound. When an object moves at supersonic speed (Mach number greater than 1), it creates shock waves as it compresses the air ahead of it. These shock waves propagate outwards and reach the listener's ears as a 'boom'.

Energy and Intensity of Sound Waves

  1. A sound wave has a frequency of 4 kHz and wavelength 30 cm. How long will it take to travel 2.4 km ?

    Asked in: UPSC CDS 2 2021

    1. 2.0 s
    2. 0.6 s
    3. 1.0 s
    4. 8.0 s
    Show answer

    Answer: A. 2.0 s

    The speed of a wave is the product of its frequency and wavelength (v = fλ). Time taken to travel a distance is distance divided by speed (t = d/v). Ensure units are consistent (e.g., meters and seconds).

  2. A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :

    Asked in: UPSC CDS 2 2024

    1. 0.5 s
    2. 1 s
    3. 1.5 s
    4. 2 s
    Show answer

    Answer: A. 0.5 s

    - The speed of a wave (v) is given by the product of its frequency (f) and wavelength (λ): v = f λ. - The distance travelled (d) is given as 150 times the wavelength: d = 150λ. - The relationship between speed, distance, and time (t) is v = d / t. - Combining these equations, we get: f λ = (150λ) / t. - We can cancel out λ from both sides (assuming λ is non-zero): f = 150 / t. - We are given the frequency f = 300 Hz. - Rearranging the equation to find t: t = 150 / f. - Substituting the value of f: t = 150 / 300 = 0.5 s.

  3. Calculate the distance travelled by a sound wave having frequency 1000 Hz and wavelength 0.25 m, if it travels for 5 seconds in a certain medium.

    Asked in: MPSC CS Prelims 2022

    1. 50 km
    2. 1250 m
    3. 800 m
    4. 80 km
    Show answer

    Answer: B. 1250 m

    The relationship between wave speed, frequency, and wavelength is a fundamental concept in wave physics: Speed = Frequency × Wavelength. Distance travelled is calculated using the basic formula: Distance = Speed × Time.

  4. When sound waves are propagated through a medium, the physical quantity/ quantities transmitted is/are

    Asked in: UPSC NDA 2 2016

    1. matter only
    2. energy only
    3. energy and matter only
    4. energy, momentum and matter
    Show answer

    Answer: B. energy only

    Waves, in general, are mechanisms for transferring energy and momentum without transferring matter over a macroscopic distance. In sound waves, the particles of the medium oscillate but do not travel along with the wave.

  5. A sound wave of frequency of 2 kHz has a wavelength of 35 cm in a given medium. How long will it take to travel a distance of 2⋅1 km through the medium ?

    Asked in: UPSC CAPF 2023

    1. 30 s
    2. 2⋅1 s
    3. 3⋅0 s
    4. 4⋅1 s
    Show answer

    Answer: C. 3⋅0 s

    - The speed of a wave ($v$) is given by the product of its frequency ($f$) and wavelength ($ \lambda$): $v = f \times \lambda$. - Given $f = 2 \text{ kHz} = 2000 \text{ Hz}$ and $\lambda = 35 \text{ cm} = 0.35 \text{ m}$. - Speed $v = 2000 \text{ Hz} \times 0.35 \text{ m} = 700 \text{ m/s}$. - The time ($t$) taken to travel a distance ($d$) is given by $t = d / v$. - Given $d = 2.1 \text{ km} = 2100 \text{ m}$. - Time $t = 2100 \text{ m} / 700 \text{ m/s} = 3 \text{ seconds}$.

  6. A sound wave has frequency of 2 kilo Hertz and wave length 35 cm. How much time it will take to travel distance of 1.5 kilometers ?

    Asked in: Combined (Group C) Exam 2023

    1. 70 sec.
    2. 3000 sec.
    3. 2.1 sec.
    4. 13.5 sec.
    Show answer

    Answer: C. 2.1 sec.

    The speed of a wave is the product of its frequency and wavelength. The time taken to cover a distance is the distance divided by the speed. Unit conversions (kHz to Hz, cm to m, km to m) are crucial for accurate calculation.

Musical Instruments: Principles of Sound Production

  1. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he/she is adjusting

    Asked in: BPSC Prelims Exam 68th

    1. amplitude of sound
    2. intensity of sound
    3. frequency of the sitar string with the fre‐ quency of other musical instruments
    4. More than one of the above
    Show answer

    Answer: C. frequency of the sitar string with the fre‐ quency of other musical instruments

    Adjusting the tension of a string directly alters its frequency of vibration, which in turn changes the pitch of the sound produced. Tuning involves matching frequencies for harmonious playing.

  2. Consider the following statements about the microphone and the speaker of a mobile phone : 1. The microphone converts sound to a mechanical signal. 2. The microphone converts sound to an electrical signal. 3. The speaker converts a mechanical signal to sound. 4. The speaker converts an electrical signal to sound. Which of the statements given above are correct?

    Asked in: UPSC CDS 1 2023

    1. 1 and 3
    2. 1 and 4
    3. 2 and 3
    4. 2 and 4
    Show answer

    Answer: D. 2 and 4

    Microphones convert sound energy into electrical energy. Speakers convert electrical energy into sound energy.

  3. School bells are made of metals because metals are

    Asked in: UPSC CDS 2 2022

    1. Malleable
    2. Sonorous
    3. Ductile
    4. Lustrous
    Show answer

    Answer: B. Sonorous

    Metals are used for making bells because they are sonorous.

  4. Which one of the following statements is not true for a flute, a musical instrument ?

    Asked in: UPSC NDA 1 2023

    1. Momentum of waves on the blowing jet de‐ termines the loudness of the produced note.
    2. Arrival time of the waves on the blowing jet determines the pitch of the produced note.
    3. Sound comes from a vibrating column of air inside the flute.
    4. Sound comes from a vibrating column of air inside as well as outside the flute.
    Show answer

    Answer: D. Sound comes from a vibrating column of air inside as well as outside the flute.

    While there is complex airflow and some turbulence outside the flute, the characteristic musical tone and pitch are produced by the standing waves formed within the air column confined by the tube. The tube length (modified by finger holes) determines the resonant frequencies (pitches).

  5. Consider the following statements and select correct answer from the codes given below: 1) In solar cooker concave mirror is used. 2) In loud speaker electrical energy is converted into sound energy. 3) In motor electrical energy is converted into mechanical energy. Codes:

    Asked in: Civil Judge 2018

    1. only 1 and 2
    2. only 2 and 3
    3. only 3 and 1
    4. all 1, 2 and 3
    Show answer

    Answer: D. all 1, 2 and 3

    This question covers basic physics principles related to energy conversion and applications of optical and electromagnetic phenomena.

Nature and Propagation of Sound Waves

  1. Sound waves cannot travel through a

    Asked in: UPSC CDS 1 2018

    1. copper wire placed in air
    2. silver slab placed in air
    3. glass prism placed in water
    4. wooden hollow pipe placed in vacuum
    Show answer

    Answer: D. wooden hollow pipe placed in vacuum

    - Sound propagation requires a material medium (solid, liquid, or gas). - Sound travels at different speeds in different media (typically fastest in solids, slower in liquids, and slowest in gases). - Vacuum has negligible matter, so it cannot support the propagation of mechanical waves like sound.

  2. Which one of the following phenomena verifies the fact that light travels much faster than sound?

    Asked in: UPSC CDS 2 2020

    1. Twinkling of stars in night sky
    2. Lighting of a matchstick
    3. Thunderstorm
    4. Mirage
    Show answer

    Answer: C. Thunderstorm

    The time difference between seeing lightning and hearing thunder during a thunderstorm proves that light travels significantly faster than sound.

  3. A sound wave has frequency of 4 kHz and its wavelength is 35 cm, then how much time will it take to travel a distance of 3 km?

    Asked in: CGPSC Prelims Exam 2021

    1. 21.4 sec
    2. 2.14 sec
    3. 12.4 sec
    4. 1.24 sec
    Show answer

    Answer: B. 2.14 sec

    The speed of a wave (v) is given by the product of its frequency (f) and wavelength (λ): v = f λ. Given f = 4 kHz = 4000 Hz and λ = 35 cm = 0.35 m. Speed v = 4000 Hz 0.35 m = 1400 m/s. The time taken to travel a distance (d) is given by time = distance / speed. Given d = 3 km = 3000 m. Time t = 3000 m / 1400 m/s = 300 / 14 s = 150 / 7 s. Calculating the value: 150 / 7 ≈ 21.428 seconds. Rounding to one decimal place, the time is approximately 21.4 seconds.

  4. Which one of the following statements is NOT true about the sound waves?

    Asked in: UPSC Geoscientist 2020

    1. Sound waves are longitudinal in nature
    2. Sound waves can propagate through water
    3. Speed of sound waves in steel is more than its speed in air
    4. Sound waves can propagate through vacu‐ um
    Show answer

    Answer: D. Sound waves can propagate through vacu‐ um

    Sound waves are mechanical waves and require a medium (solid, liquid, or gas) to propagate.

  5. What do we call the distance between two consecutive compressions of a sound wave?

    Asked in: JPSC CS Prelims 2024

    1. Wave number
    2. Frequency
    3. Wavelength
    4. Amplitude
    Show answer

    Answer: C. Wavelength

    Wavelength (λ) is a fundamental property of a wave, representing the spatial period of the wave. It is related to the speed of the wave (v) and its frequency (f) by the equation v = fλ.

  6. Which of the following cannot travel in vacuum?

    Asked in: JPSC CS Prelims 2024

    1. Light
    2. Heat
    3. Sound
    4. Electromagnetic waves
    Show answer

    Answer: C. Sound

    Light, heat (in the form of radiation), and other electromagnetic waves (like radio waves, X-rays, etc.) are electromagnetic waves. Electromagnetic waves do not require a medium and can travel through the vacuum of space. This is why we receive light and heat from the Sun across the vacuum between the Sun and Earth.

  7. Which one of the following does not apply to sound waves in fluids ?

    Asked in: UPSC NDA 1 2024

    1. They transport energy
    2. They need a medium to travel
    3. They are transverse
    4. They travel faster in liquids than in gases
    Show answer

    Answer: C. They are transverse

    Sound waves are mechanical waves that require a medium to propagate. In fluids (liquids and gases), sound waves are longitudinal waves, meaning the particles of the medium vibrate parallel to the direction of wave propagation, creating areas of compression and rarefaction.

  8. The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Statement I : Sound wave cannot propagate in vacuum. Statement II : Sound waves are elastic waves and require a medium to propagate.

    Asked in: UPSC NDA 2 2018

    1. Both the statements are individually true and Statement II is the correct explanation of Statement I
    2. Both the statements are individually true but Statement II is not the correct explanation of Statement I
    3. Statement I is true but Statement II is false
    4. Statement I is false but Statement II is true
    Show answer

    Answer: A. Both the statements are individually true and Statement II is the correct explanation of Statement I

    - Sound is a form of energy that travels as waves. - Sound waves are longitudinal mechanical waves, causing compressions and rarefactions in the medium. - The presence of a medium is essential for the vibrations to be transmitted.

Reflection of Sound: Echo, Reverberation, and their Applications

  1. A person rings a metallic bell near a strong concrete wall. He hears the echo after 0.3 s. If the sound moves with a speed of 340 m/s, how far is the wall from him?

    Asked in: UPSC CDS 1 2016

    1. 102 m
    2. 11 m
    3. 51 m
    4. 30 m
    Show answer

    Answer: C. 51 m

    An echo involves sound travelling to a surface and back, so the distance to the surface is half the total distance covered by the sound.

  2. A person burned a firecracker in front of a cliff and heard its echo 5 s after it burst. The distance of the cliff from the person, if the speed of the sound is 340 m/s, is close to

    Asked in: UPSC CDS 1 2023

    1. 1700 m
    2. 170 m
    3. 85 m
    4. 850 m
    Show answer

    Answer: D. 850 m

    The total distance covered by an echo is twice the distance to the reflecting surface. Distance = Speed × Time.

  3. An echo is heard after 5 seconds of the production of sound which moves with a speed of 340 m/s. What is the distance of the mountain from the source of sound which produced the echo ?

    Asked in: UPSC CDS 2 2019

    1. 0.085 km
    2. 0.85 km
    3. 0.17 km
    4. 1.7 km
    Show answer

    Answer: B. 0.85 km

    In an echo scenario, the time measured is for the sound's round trip (source to obstacle and back), so the distance to the obstacle is half the total distance traveled by the sound.

  4. Reverberation of sound ensures

    Asked in: UPSC Geoscientist 2023

    1. a single refraction
    2. a single reflection
    3. multiple reflections
    4. multiple refractions
    Show answer

    Answer: C. multiple reflections

    - Reverberation is the persistence of sound. - It occurs in enclosed spaces. - It is caused by multiple reflections of sound waves. - Contrast with echo, which is a single, delayed reflection.

  5. Which of the following is a cause of echo?

    Asked in: JPSC CS Prelims 2024

    1. Absorption of sound
    2. Transmission of sound
    3. Reflection of sound
    4. Refraction of sound
    Show answer

    Answer: C. Reflection of sound

    When sound waves encounter a hard surface, they bounce back, similar to how light reflects off a mirror. If the surface is far enough away (typically more than 17 meters for a distinct echo in air), the reflected sound is perceived as a separate sound from the original, which is called an echo.

  6. When a compression of sound waves is incident on rigid wall, it is reflected as :

    Asked in: MPSC CS Prelims 2020

    1. Compression with a phase change of π (by 180°)
    2. Compression with no phase change
    3. Rarefaction with a phase change of π/2 (by 90°)
    4. Rarefaction with no phase change
    Show answer

    Answer: A. Compression with a phase change of π (by 180°)

    - Reflection of a wave from a rigid boundary results in a phase change. - For sound waves, reflection of a compression from a rigid wall results in a reflected compression. - The phase change of π (180°) refers to the displacement wave, not the pressure wave.

  7. The sound created in a big hall persists because of the repeated reflections. The phenomenon is called

    Asked in: UPSC NDA 1 2021

    1. Reverberation.
    2. Dispersion.
    3. Refraction.
    4. Diffraction.
    Show answer

    Answer: A. Reverberation.

    - Reflection of sound waves occurs when they bounce off surfaces. - In an enclosed space, sound waves reflect repeatedly from walls, ceilings, and other surfaces. - Reverberation is the persistence of sound after the source has stopped, caused by these multiple reflections delaying the sound's decay.

  8. Reverberation is a phenomenon associated with a

    Asked in: UPSC NDA 2 2021

    1. multiple refraction of sound.
    2. multiple reflection of sound.
    3. single refraction of sound.
    4. single reflection of sound.
    Show answer

    Answer: B. multiple reflection of sound.

    Reverberation is distinct from a single echo, which is a clearly discernible reflection. Reverberation occurs when reflected sound waves arrive at the listener in rapid succession, blending together and prolonging the original sound. It is a phenomenon resulting from numerous reflections.

Refraction and Diffraction of Sound Waves

  1. Which one of the following phenomena CANNOT be exhibited by sound waves ?

    Asked in: UPSC CAPF 2019

    1. Reflection
    2. Refraction
    3. Interference
    4. Polarisation
    Show answer

    Answer: D. Polarisation

    The key principle here is understanding the difference between longitudinal and transverse waves and which wave phenomena apply to each. Polarisation is a characteristic property of transverse waves.

Speed of Sound in Different Media and Factors Affecting It

  1. Which one of the following statements is correct ? The velocity of sound :

    Asked in: UPSC CDS 1 2016

    1. does not depend upon the nature of media
    2. is maximum in gases and minimum in li‐ quids
    3. is maximum in solids and minimum in li‐ quids
    4. is maximum in solids and minimum in gases
    Show answer

    Answer: D. is maximum in solids and minimum in gases

    Sound travels fastest in solids, slower in liquids, and slowest in gases.

  2. In which of the following media is the speed of sound the maximum?

    Asked in: UPSC CDS 2 2023

    1. Glass
    2. Stainless steel
    3. Water
    4. Oxygen
    Show answer

    Answer: B. Stainless steel

    The speed of sound is determined by the medium's properties, specifically its bulk modulus (a measure of stiffness) and density. The formula for the speed of sound is approximately $v = \sqrt{B/\rho}$ for liquids and solids, where B is the bulk modulus and $\rho$ is the density. Solids generally have much higher bulk moduli than liquids and gases, leading to higher sound speeds, even though they might also have higher densities. Among solids, stiffer materials tend to have higher sound speeds.

  3. In which one among the following mediums would the speed of sound be maximum ?

    Asked in: UPSC CDS 2 2024

    1. Distilled water
    2. Sea water
    3. Drinking water
    4. Methanol
    Show answer

    Answer: B. Sea water

    - Speed of sound increases with the stiffness (bulk modulus) of the medium and decreases with density, though the effect of stiffness is usually more significant in liquids and solids. - Dissolved salts in sea water increase its density and bulk modulus compared to fresh water.

  4. Which of the following statements is/ are true in relation to sound waves? Statement I : Speed of sound decreases when we go from solid to gaseous state Statement II : In any medium as we decrease the temperature, the speed of sound increases Statement III : Speed of sound is less in aluminium medium than in glass medium

    Asked in: CGPSC Prelims Exam 2020

    1. Statement I, II and III all are true
    2. only Statement I and II are true
    3. only Statement I is true
    4. only Statement II and III are true
    Show answer

    Answer: C. only Statement I is true

    Statement I: The speed of sound is generally highest in solids, lower in liquids, and lowest in gases. This is because the particles are closer together and interact more strongly in denser media. Thus, speed decreases from solid to gaseous state. (True) Statement II: The speed of sound in any medium generally decreases with decreasing temperature. For gases, speed is proportional to the square root of absolute temperature. (False) Statement III: The speed of sound in Aluminium (~6420 m/s) is significantly higher than in glass (typically between 3980 m/s and 6000 m/s depending on type). (False)

  5. Arrange the following gases according to speed of sound in them from lowest to highest : (1) Sulphur dioxide (2) Helium (3) Oxygen (4) Hydrogen Choose the correct answer from the options given below :

    Asked in: HPPSC HPAS Prelims 2024

    1. (2), (1), (3), (4)
    2. (1), (3), (2), (4)
    3. (4), (3), (1), (2)
    4. (3), (4), (1), (2)
    Show answer

    Answer: B. (1), (3), (2), (4)

    - The speed of sound in an ideal gas is given by the formula \(v = \sqrt{\frac{\gamma RT}{M}}\), where \(v\) is the speed of sound, \(\gamma\) is the adiabatic index (ratio of specific heats \(C_p/ C_v\)), R is the universal gas constant, T is the absolute temperature, and M is the molar mass of the gas. - Assuming the temperature is constant for all gases, the speed of sound is primarily influenced by the molar mass (inversely proportional to \(\sqrt{M}\)) and the adiabatic index (\(\sqrt{\gamma}\)). - Molar masses: Sulphur dioxide (SO2) ≈ 64 g/mol, Helium (He) ≈ 4 g/mol, Oxygen (O2) ≈ 32 g/mol, Hydrogen (H2) ≈ 2 g/ mol. - Adiabatic index (\(\gamma\)): For monoatomic gases (He) ≈ 1.67, for diatomic gases (O2, H2) ≈ 1.4, for polyatomic gases (SO2) can vary, but is generally lower than diatomic gases (e.g., ≈ 1.29 for SO2 at room temp). - Considering both factors, the order of speed of sound from lowest to highest is generally correlated inversely with molar mass, although gamma plays a role. - Lowest molar mass usually means highest speed. H2 (2 g/mol) has the highest speed. He (4 g/mol) has the second highest speed due to its low mass and high gamma. O2 (32 g/ mol) and SO2 (64 g/mol) are significantly heavier, resulting in lower speeds. SO2 is heavier than O2 and also has a lower gamma, giving it the lowest speed. - Approximate speeds at 0°C: SO2 (~213 m/s), O2 (~317 m/s), He (~965 m/s), H2 (~1284 m/s). - Arranging from lowest to highest speed: Sulphur dioxide

  6. Sound waves travel fastest in

    Asked in: MPPSC CS Prelims 2017

    1. solids
    2. liquids
    3. gases
    4. vacuum
    Show answer

    Answer: A. solids

    BJS -2023 Sound waves are mechanical waves that require a medium to travel. Their speed depends on the properties of the medium, particularly its elasticity (stiffness) and density. Sound travels fastest in solids because their particles are closely packed and interactions between them are strong, allowing vibrations to propagate quickly. Sound travels slower in liquids and slowest in gases due to the weaker interactions and greater distances between particles. Sound cannot travel through a vacuum.

  7. Choose correct statements from the following: (a) Sound waves travel faster in solids than in gases. (b) Sound waves travel fastest through vacuum (c) Sound waves are longitudinal waves (d) Sound waves travel slower in gases than in liquids.

    Asked in: MPSC CS Prelims 2014

    1. (a), (c) and (d) only
    2. (a) and (b) only
    3. (c) and (d) only
    4. (b) and (d) only
    Show answer

    Answer: A. (a), (c) and (d) only

    - (a) Sound waves travel faster in solids than in gases: True. The speed of sound is generally highest in solids, followed by liquids, and then gases, because particles are closer together and more strongly interacting in denser media. - (b) Sound waves travel fastest through vacuum: False. Sound waves are mechanical waves and require a material medium (solid, liquid, or gas) to propagate. They cannot travel through a vacuum. - (c) Sound waves are longitudinal waves: True. In gases and liquids, sound waves are longitudinal, meaning the particles oscillate parallel to the direction of wave propagation. In solids, sound waves can be both longitudinal and transverse. The primary form of sound propagation in fluids is longitudinal. - (d) Sound waves travel slower in gases than in liquids: True. Consistent with point (a), sound travels slower in gases than in liquids.

  8. Consider following statements : a. The whistle of a passing train is clearly heard on a quiet misty night. b. Velocity of sound in air is proportional to humidity in air. Which of the above statements are true ?

    Asked in: MPSC CS Prelims 2016

    1. a and b both are correct
    2. Neither a nor b is correct
    3. a is correct but b is not
    4. b is correct but a is not
    Show answer

    Answer: C. a is correct but b is not

    - a. The whistle of a passing train is often heard more clearly on a quiet misty night. This phenomenon is typically due to atmospheric conditions like temperature inversions that often occur on clear, still nights, especially with ground-level mist or fog. In an inversion, air temperature increases with altitude for a layer, causing sound waves to refract (bend) downwards towards the ground, enhancing propagation over distance. - b. Velocity of sound in air is not directly proportional to humidity in air. The speed of sound in air increases slightly with humidity, but the relationship is not a simple linear proportionality. The speed primarily depends on temperature (proportional to the square root of the absolute temperature) and is also affected by the composition of the air (humidity changes the average molar mass).

Stationary Waves (Standing Waves) in Strings and Organ Pipes

  1. In a Sitar, which type of sound vibrations are produced?

    Asked in: RO ARO pre 2021

    1. Progressive and Longitudinal
    2. Progressive and Transverse
    3. Stationary and Longitudinal
    4. Stationary and Transverse
    Show answer

    Answer: D. Stationary and Transverse

    Vibrations in string instruments like the sitar produce stationary and transverse waves.

The Human Ear: Structure and Mechanism of Hearing

  1. Which one of the following frequency ranges is sensitive to human ears ?

    Asked in: UPSC NDA 1 2018

    1. 0 – 200 Hz
    2. 20 – 20,000 Hz
    3. 200 – 20,000 Hz only
    4. 2,000 – 20,000 Hz only
    Show answer

    Answer: B. 20 – 20,000 Hz

    The typical range of frequencies that the human ear can perceive is from approximately 20 Hertz (Hz) to 20,000 Hertz (or 20 kilohertz, kHz). Sounds with frequencies below 20 Hz are called infrasound, and those above 20,000 Hz are called ultrasound; neither is audible to humans under normal conditions. While the sensitivity varies significantly with age (higher frequencies are often less audible as people get older), the range 20-20,000 Hz represents the general limits of human hearing.

  2. Audible range for adult human beings is :

    Asked in: OPSC CS Prelims 2019

    1. Less than 20 Hz
    2. 20 to 20,000 Hz
    3. 21,000 to 50,000 Hz
    4. 100 to 120 kHz
    Show answer

    Answer: B. 20 to 20,000 Hz

    - Frequencies below 20 Hz are called infrasound and are generally inaudible to humans, although they can sometimes be felt as vibrations. - Frequencies above 20,000 Hz are called ultrasound and are also inaudible to humans. - The upper limit of hearing tends to decrease with age.

  3. Which one of the following is the audible range of hearing for humans ?

    Asked in: UPSC CAPF 2024

    1. 20 kHz – 200 kHz
    2. 20 Hz – 20 kHz
    3. 20 Hz – 35 kHz
    4. 20 Hz – 40 kHz
    Show answer

    Answer: B. 20 Hz – 20 kHz

    - 'Hz' stands for Hertz, which is a unit of frequency (cycles per second). - 'kHz' stands for kilohertz, which is 1000 Hertz. - The lower limit of human hearing is around 20 Hz, and the upper limit is around 20,000 Hz (20 kHz).

  4. Which of the following frequencies is not in the audible range of sound for human beings?

    Asked in: WBCS Main Paper 4 2021

    1. 5 Hz
    2. 50 Hz
    3. 500 Hz
    4. 5000 Hz
    Show answer

    Answer: A. 5 Hz

    Human hearing is limited to a specific range of sound frequencies, typically 20 Hz to 20,000 Hz. Frequencies outside this range are inaudible.

  5. Human ear is most sensitive to the sound of frequencies in between

    Asked in: WBCS Main Paper 4 2022

    1. 100 – 500 Hz
    2. 500 – 5000 Hz
    3. 5000 – 10000 Hz
    4. 10000 – 20000 Hz
    Show answer

    Answer: B. 500 – 5000 Hz

    - The full range of human hearing is typically from about 20 Hz to 20,000 Hz. - The region of maximum sensitivity corresponds roughly to the frequency range of human speech.

  6. Which of the following statement is not correct?

    Asked in: RO ARO pre 2021

    1. Human ear becomes most sensitive at 1000 Hz.
    2. Human eye becomes most sensitive for red colour.
    3. Persistence of hearing is 1/10 second.
    4. Persistence of vision is 1/16 second.
    Show answer

    Answer: B. Human eye becomes most sensitive for red colour.

    This question relates to human physiology and perception, specifically the sensitivity of the human ear and eye to different frequencies and durations.

Ultrasound and Infrasound: Production, Properties, and Applications (SONAR, Medical, Industrial)

  1. Ultrasonic waves are produced by making use of

    Asked in: UPSC CDS 2 2017

    1. ferromagnetic material
    2. ferrimagnetic material
    3. piezoelectric material
    4. pyroelectric material
    Show answer

    Answer: C. piezoelectric material

    Piezoelectric materials convert electrical energy into mechanical vibrations and vice versa. By applying a high-frequency alternating voltage across a piezoelectric crystal, it vibrates at that frequency, generating ultrasonic waves.

  2. What is/are not true among the following? (i) Audible range of sound for human beings is approximately 20 Hz to 10 kHz. (ii) Sound waves with frequencies higher than 10 kHz are called ultrasound. (iii) In earthquake, ultrasounds are produced before main shock waves.

    Asked in: CGPSC Prelims Exam 2022

    1. (i) and (ii)
    2. (i) and (iii)
    3. (i), (ii) and (iii)
    4. Only (iii)
    Show answer

    Answer: A. (i) and (ii)

    - Statement (i) is not true. The approximate audible range of sound for human beings is typically considered to be from 20 Hz to 20,000 Hz (20 kHz), not 10 kHz as the upper limit. - Statement (ii) is not true. Sound waves with frequencies higher than the upper limit of human hearing (approximately 20 kHz) are called ultrasound. Frequencies between 10 kHz and 20 kHz are within the audible range. - Statement (iii) is not true. Earthquakes can produce various types of waves, including infra-sound (frequencies below 20 Hz). Infra-sound waves travel faster through the ground and air than the main seismic waves (P, S, surface waves) and might be detected by some animals before the main shock. Ultrasound (frequencies above 20 kHz) is not typically produced or detected in this manner before the main seismic event.

  3. Which one of the following statements with regard to ultrasonic waves is NOT correct?

    Asked in: UPSC Geoscientist 2021

    1. Ultrasonic waves have frequency above 20 kHz
    2. Ultrasonic waves are longitudinal waves
    3. Ultrasonic waves can travel through water
    4. Ultrasonic waves can travel through vacu‐ um
    Show answer

    Answer: D. Ultrasonic waves can travel through vacu‐ um

    Mechanical waves transport energy through the vibration of particles in a medium. In a vacuum, there are essentially no particles to vibrate, so mechanical waves cannot propagate.

  4. Infrasonic sounds have frequencies

    Asked in: UPSC Geoscientist 2023

    1. above 25 kHz
    2. between 20 kHz and 25 kHz
    3. below 20 Hz
    4. between 20 Hz and 20 kHz
    Show answer

    Answer: C. below 20 Hz

    Sound waves are classified based on their frequency ranges relative to human hearing. The typical range of hearing for a healthy young human is from approximately 20 Hertz (Hz) to 20,000 Hertz (20 kHz). Sounds with frequencies below this audible range are called infrasonic sounds.

  5. Ultrasonic waves of frequency 3 × 10⁵ Hz are passed through a medium where speed of sound is 10 times that in air (Speed of sound in air is 300 m/s). The wavelength of this wave in that medium will be of the order of

    Asked in: UPSC NDA 1 2015

    1. 1 cm
    2. 10 cm
    3. 100 cm
    4. 0·1 cm
    Show answer

    Answer: A. 1 cm

    The relationship between wave speed (v), frequency (f), and wavelength (λ) is given by v = fλ. Given frequency f = 3 × 10⁵ Hz. Speed of sound in air v_air = 300 m/s. Speed of sound in the medium v_medium = 10 × v_air = 10 × 300 m/s = 3000 m/s. We need to find the wavelength λ in the medium. Using the formula λ = v_medium / f: λ = 3000 m/s / (3 × 10⁵ Hz) = 3000 / 300000 m = 1/100 m = 0.01 m.

  6. Which one of the following statements is not correct ?

    Asked in: UPSC NDA 2 2017

    1. Ultrasonic waves cannot get reflected, refracted or absorbed.
    2. Ultrasonic waves are used to detect the presence of defects like cracks, porosity, etc. in the internal structure of common structure materials.
    3. Ultrasonic waves can be used for making holes in very hard materials like diamond.
    4. Ultrasonic waves cannot travel through va‐ cuum.
    Show answer

    Answer: A. Ultrasonic waves cannot get reflected, refracted or absorbed.

    Statement A is incorrect. Like all mechanical waves, ultrasonic waves exhibit properties such as reflection, refraction, absorption, and diffraction when they interact with different media or obstacles.

  7. The frequency of ultrasound waves is

    Asked in: UPSC NDA 2 2018

    1. less than 20 Hz
    2. between 20 Hz and 2 kHz
    3. between 2 kHz and 20 kHz
    4. greater than 20 kHz
    Show answer

    Answer: D. greater than 20 kHz

    - The audible range of sound frequencies for humans is typically considered to be from 20 Hz to 20,000 Hz (20 kHz). - Sound waves with frequencies below 20 Hz are called infrasound. - Sound waves with frequencies above 20 kHz are called ultrasound.

  8. Compared to audible sound waves, ultrasound waves have

    Asked in: UPSC NDA 2 2019

    1. higher speed.
    2. higher frequency.
    3. longer wavelength.
    4. both higher speed and frequency.
    Show answer

    Answer: B. higher frequency.

    Ultrasound waves are defined as sound waves with frequencies higher than the upper limit of human hearing, which is typically considered to be 20 kilohertz (kHz). Audible sound waves have frequencies between approximately 20 Hz and 20 kHz.