Sound: Nature, Speed, Echo, SONAR & More for Competitive Exam

Sound is the energy that makes the world audible – from the gentle rustle of leaves and the melody of a bird to the roar of a jet engine. At its core, sound is a mechanical wave produced by vibrating objects. Understanding how sound is produced, how it travels, and how we perceive it is a fundamental part of physics.

The topic of Sound is a high-yield area in General Science for SSC, Railways (RRB NTPC, Group D, ALP, JE), WBCS, Police, and State PSC exams. Questions are frequently asked on the nature of sound waves, speed of sound in different media, audible frequency ranges, echo calculations, and applications like SONAR and ultrasound. This guide explains everything in simple language with clear diagrams, solved numerical examples, a formula sheet, quick revision memory tricks, and 20 MCQs with answers.

➤ Nature of Sound: Mechanical longitudinal wave · Requires a material medium (cannot travel in a vacuum)
➤ Wave Equation: v = f × λ (Speed = Frequency × Wavelength)
➤ Speed of Sound: Solids > Liquids > Gases · Increases with temperature and humidity
➤ Human Audible Range: 20 Hz to 20,000 Hz (20 kHz)
➤ Echo Condition: Minimum time gap = 0.1 s · Minimum distance = 17.2 m (at 22°C)

Part A: Nature and Production of Sound

• Sound is a form of mechanical wave energy that produces the sensation of hearing in our ears.
• It is produced by the rapid to-and-fro motion (vibrations) of objects. For example, the vibrating prongs of a tuning fork, the vibrating vocal cords in humans, or the vibrating strings of a guitar.
• Material Medium is Essential: Sound requires a material medium (solid, liquid, or gas) to propagate because it travels through the collision of particles. Sound cannot travel through a vacuum (like outer space), unlike light, which is an electromagnetic wave.

Part B: Propagation of Sound Waves

When an object vibrates, it sets the particles of the surrounding medium into vibration. These particles do not travel forward themselves; they just oscillate about their mean position and pass the energy to adjacent particles.

Longitudinal Waves

In gases and liquids, sound travels as a longitudinal wave. The particles of the medium vibrate parallel to the direction of wave propagation, creating regions of:

• Compressions (C): Regions where particles are pushed close together (high pressure and high density).
• Rarefactions (R): Regions where particles are spread apart (low pressure and low density).

Figure 1: Longitudinal sound wave showing compressions (high pressure) and rarefactions (low pressure). The distance between two consecutive compressions is one wavelength (λ).

Speed of Sound in Different Media

The speed of sound depends on the elasticity and density of the medium. Generally, sound travels fastest in solids, moderately in liquids, and slowest in gases.

Factors Affecting Speed of Sound in Air

• Temperature: Speed increases with an increase in temperature. (At 0°C, speed is ~332 m/s; at 22°C, it is ~344 m/s).
• Humidity: Speed increases with humidity. Moist air is less dense than dry air, and speed is inversely proportional to the square root of density.
• Pressure: Changing the pressure at a constant temperature has no effect on the speed of sound in a gas.

Mathematical Relationship

v = f × λ

• v = Speed of sound (m/s)
• f = Frequency (Hz)
• λ (lambda) = Wavelength (m)

Example: A sound wave has a frequency of 500 Hz and a wavelength of 0.7 m. What is its speed?
Solution: v = f × λ = 500 × 0.7 = 350 m/s.

Part C: Characteristics of Sound Waves

Part D: Audible, Infrasonic, and Ultrasonic Frequencies

Based on frequency, sound waves are classified into three categories:

Part E: Reflection of Sound and Applications

Sound obeys the same laws of reflection as light:

1. The angle of incidence is equal to the angle of reflection.
2. The incident wave, the reflected wave, and the normal at the point of incidence all lie in the same plane.

Echo

An echo is a distinct, repeated sound caused by the reflection of sound waves from a distant obstacle. For the human brain to perceive an echo, the reflected sound must reach the ear at least 0.1 seconds after the original sound.

Minimum Distance for Echo = (Speed of Sound × Time) / 2

At 22°C, speed of sound in air is 344 m/s.
Minimum distance = (344 × 0.1) / 2 = 17.2 meters.

Reverberation

When sound is produced in a large hall or closed space, it undergoes multiple reflections, causing the sound to persist even after the source has stopped. This is called reverberation. Excessive reverberation is undesirable in auditoriums and is reduced by using sound-absorbing materials (e.g., compressed fibreboards, rough plaster, heavy curtains).

Practical Applications of Sound Reflection

• SONAR (Sound Navigation and Ranging): Uses ultrasonic waves to measure the depth of the sea, locate underwater hills, valleys, submarines, and sunken ships. Formula: Depth (d) = (v × t) / 2.
• Medical Ultrasonography: Uses ultrasound to create images of internal organs (e.g., liver, gall bladder, kidneys) and monitor fetal development (echocardiography).
• Industrial Flaw Detection: Ultrasonic waves are passed through metal blocks. If there is a crack or flaw, the waves are reflected back, detecting the defect without damaging the object.
• Stethoscope: Works on the principle of multiple reflections of sound, guiding the patient's heartbeat to the doctor's ears efficiently.

Formula and Unit Sheet

Quick Revision Box – Easy Tricks to Memorise

20 MCQs on Sound

Q1. Sound waves cannot travel through:
(a) Water
(b) Steel
(c) Vacuum
(d) Air

Answer: (c) – Sound is a mechanical wave and requires a material medium. It cannot travel through a vacuum.

Q2. The waves produced by a vibrating tuning fork in the air are:
(a) Transverse
(b) Longitudinal
(c) Stationary
(d) Electromagnetic

Answer: (b) – Sound waves in fluids (gases and liquids) are longitudinal, consisting of compressions and rarefactions.

Q3. The speed of sound is maximum in:
(a) Air at 0°C
(b) Water
(c) Iron (Steel)
(d) Vacuum

Answer: (c) – Sound travels fastest in solids due to high elasticity, slower in liquids, and slowest in gases.

Q4. Which of the following factors does NOT affect the speed of sound in a gas?
(a) Temperature
(b) Humidity
(c) Pressure (at constant temperature)
(d) Nature of the gas

Answer: (c) – At a constant temperature, changing the pressure of a gas does not change its density proportionally, so the speed of sound remains unaffected.

Q5. The relationship between speed (v), frequency (f), and wavelength (λ) of a sound wave is:
(a) v = f / λ
(b) v = f × λ
(c) v = λ / f
(d) f = v × λ

Answer: (b) – Wave speed is the product of frequency and wavelength.

Q6. The pitch of a sound is determined by its:
(a) Amplitude
(b) Frequency
(c) Velocity
(d) Wavelength

Answer: (b) – Higher frequency produces a higher pitch (shrillness), while lower frequency produces a lower pitch.

Q7. If the amplitude of a sound wave is doubled, its loudness becomes:
(a) Doubled
(b) Halved
(c) Four times
(d) Unchanged

Answer: (c) – Loudness is proportional to the square of the amplitude (2² = 4).

Q8. The audible frequency range for a normal human ear is:
(a) 2 Hz to 200 Hz
(b) 20 Hz to 20,000 Hz
(c) 200 Hz to 200,000 Hz
(d) 2,000 Hz to 20,000 Hz

Answer: (b) – 20 Hz to 20 kHz is the standard audible range for humans.

Q9. Sounds with a frequency below 20 Hz are called:
(a) Ultrasonic
(b) Supersonic
(c) Infrasonic
(d) Audible

Answer: (c) – Infrasonic waves are below 20 Hz. Elephants and whales use them to communicate over long distances.

Q10. Which animal can hear ultrasonic sound?
(a) Elephant
(b) Bat
(c) Lion
(d) Cow

Answer: (b) – Bats produce and hear ultrasonic waves (above 20,000 Hz) for echolocation and navigation in the dark.

Q11. The minimum distance required to hear a distinct echo in air (at 22°C) is approximately:
(a) 10 m
(b) 17.2 m
(c) 34.4 m
(d) 50 m

Answer: (b) – Distance = (Speed × Time) / 2 = (344 × 0.1) / 2 = 17.2 m.

Q12. The persistence of sound in an enclosed space due to multiple reflections is called:
(a) Echo
(b) Reverberation
(c) Resonance
(d) Refraction

Answer: (b) – Reverberation is the prolongation of sound due to successive reflections. It is reduced in auditoriums using sound absorbers.

Q13. SONAR technology is primarily used for:
(a) Measuring the speed of an aeroplane
(b) Determining the depth of the sea and locating underwater objects
(c) Medical imaging of bones
(d) Detecting cracks in the atmosphere

Answer: (b) – SONAR (Sound Navigation and Ranging) uses ultrasonic waves to map the ocean floor and detect submarines.

Q14. The SI unit of loudness of sound is:
(a) Hertz (Hz)
(b) Metre (m)
(c) Decibel (dB)
(d) Second (s)

Answer: (c) – Decibel (dB) is the unit used to measure the intensity or loudness of sound. Frequency is measured in Hertz.

Q15. A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. What is its speed?
(a) 340 m/s
(b) 735 m/s
(c) 170 m/s
(d) 500 m/s

Answer: (a) – v = f × λ = 500 × 0.68 = 340 m/s.

Q16. The characteristic of sound that allows you to distinguish between a flute and a sitar playing the same note at the same loudness is:
(a) Pitch
(b) Loudness
(c) Quality (Timbre)
(d) Frequency

Answer: (c) – Quality or timbre depends on the waveform and harmonic content, allowing us to identify the source of the sound.

Q17. Ultrasound is used in medical science for:
(a) Breaking kidney stones
(b) Echocardiography and imaging internal organs
(c) Both (a) and (b)
(d) Neither (a) nor (b)

Answer: (c) – Ultrasound is used for imaging (ultrasonography/echocardiography) and also in lithotripsy to break small stones into fine grains.

Q18. When sound travels from air to water, its:
(a) Frequency increases
(b) Wavelength decreases
(c) Speed increases
(d) Speed decreases

Answer: (c) – The speed of sound increases when moving from a gas (air) to a liquid (water). Frequency remains constant, so wavelength must increase.

Q19. A stethoscope used by doctors works on the principle of:
(a) Multiple reflection of sound
(b) Refraction of sound
(c) Interference of sound
(d) Doppler effect

Answer: (a) – The sound of the heartbeat undergoes multiple reflections inside the stethoscope tube, reaching the doctor's ears clearly.

Q20. Which of the following statements is INCORRECT?
(a) Sound travels faster in humid air than in dry air.
(b) Sound can travel through a vacuum.
(c) The speed of sound in solids is greater than in liquids.
(d) Pitch depends on frequency.

Answer: (b) – Sound is a mechanical wave and absolutely cannot travel through a vacuum. Light can, but sound cannot.

Conclusion

Sound is a fascinating mechanical phenomenon that connects vibrations to human perception. For competitive exams, focus on the core formulas (v = fλ), the conditions for an echo (17.2 m minimum distance), the classification of frequencies (Infrasonic, Audible, Ultrasonic), and the practical applications of sound reflection (SONAR, Ultrasound). Remember the "S-L-G" rule for speed and the distinction between pitch (frequency) and loudness (amplitude). Practise the numericals on echo and wave speed, and use the memory tricks above for quick revision before your exam.

Reference: NCERT Science Class IX, Chapter "Sound"; NCERT Physics Class XI (Waves).