Sound
Propagation, Reflection & Human Ear
In-Text Questions (Page 162)
Q1. How does the sound produced by a vibrating object in a medium reach your ear?
When an object vibrates, it sets the particles of the medium around it vibrating. These particles displace adjacent particles, creating a series of compressions (high pressure) and rarefactions (low pressure). This disturbance travels through the medium to reach our ear.
In-Text Questions (Page 163)
Q1. Explain how sound is produced by your school bell.
When the school bell is struck with a hammer, it starts vibrating. These vibrations force the surrounding air particles to vibrate, creating sound waves that travel through the air.
Q2. Why are sound waves called mechanical waves?
Sound waves need a material medium (solid, liquid, or gas) for their propagation. Since they involve the vibration of medium particles, they are called mechanical waves.
In-Text Questions (Page 166)
Q1. Frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Frequency = 100 Hz = 100 vibrations per second.
In 1 minute (60 seconds):
Total vibrations = 100 × 60 =
6000 times.
Q2. Does sound follow the same laws of reflection as light does? Explain.
Yes. The angle of incidence of sound is equal to the angle of reflection, and the incident wave, reflected wave, and normal all lie in the same plane.
In-Text Questions (Page 170)
Q1. What is the audible range of the average human ear?
The audible range is from 20 Hz to 20,000 Hz (20 kHz).
Q2. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?
- (a) Infrasound: Frequency less than 20 Hz.
- (b) Ultrasound: Frequency greater than 20,000 Hz.
Main Textbook Exercises
Q7. A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?
Given: f = 2 kHz = 2000 Hz, λ = 35 cm = 0.35 m, d = 1.5 km = 1500 m.
Step 1: Calculate Velocity (v).
v = f ×
λ = 2000 × 0.35 = 700 m/s.
Step 2: Calculate Time (t).
t = Distance /
Velocity = 1500 / 700 = 2.14 s.
Q13. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? (Given, g = 10 m/s² and speed of sound = 340 m/s).
Step 1: Time for stone to fall (t₁).
s
= ut + ½gt² => 500 = 0 + 0.5 × 10 × t₁² => 500 = 5t₁² => t₁² = 100 => t₁ = 10 s.
Step 2: Time for sound to travel up
(t₂).
t₂ = Distance / Speed = 500 / 340 = 1.47 s.
Total Time: t₁ + t₂ = 10 + 1.47 = 11.47 s.
Q19. What is SONAR?
SONAR stands for Sound Navigation And Ranging. It is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects (like submarines, shoals of fish).
Sound
Propagation, Echo & SONAR
1. Production and Propagation
Sound propagates as longitudinal waves. Particles oscillate parallel to the direction of propagation. Composed of:
- Compressions (C): Regions of high pressure/density.
- Rarefactions (R): Regions of low pressure/density.
2. Characteristics of Sound Wave
Number of oscillations per second. Determines Pitch.
Maximum displacement. Determines Loudness.
Speed (v) = Wavelength (λ) × Frequency (ν)
3. Echo and Reverberation
Echo: Reflection of sound heard distinctly. Minimum time gap needed is 0.1 s.
Minimum Distance: For echo in air (at
22°C):
d = (v × t) / 2 = (344 × 0.1) / 2 = 17.2 m.
SONAR: Uses Ultrasonic waves.
Formula: 2d = v × t (where d is depth, t is time for echo to return).
4. Numerical Examples
A source produces 500 waves in 2 seconds. If the distance between a compression and adjacent rarefaction is 50 cm, find velocity.
Frequency (ν) = 500 / 2 = 250 Hz.
Wavelength (λ) = 2 × 50 cm = 100 cm = 1 m (Distance between C and R is λ/2).
Velocity (v) = λ × ν = 1 × 250 = 250 m/s.
A SONAR echo returns after 3 s. If speed of sound in water is 1440 m/s, find depth.
2d = v × t
2d = 1440 × 3
d = (1440 × 3) / 2
= 2160 m.
Key Facts & Definitions
50+ Important Points to Remember
Form of energy producing sensation of hearing.
Produced by vibration of objects.
Matter through which sound propagates (Solid, Liquid, Gas). Sound cannot travel in vacuum.
Particles vibrate parallel to direction of propagation.
Particles vibrate perpendicular to direction of propagation (e.g., Light).
Region of high pressure/density in a wave.
Region of low pressure/density in a wave.
Distance between two consecutive compressions or rarefactions (SI Unit: m).
Number of oscillations per unit time (SI Unit: Hertz, Hz).
Time taken for one complete oscillation (T = 1/ν).
Maximum displacement of the particle from mean position.
Characteristic determined by frequency. High frequency = High pitch.
Characteristic determined by amplitude. High amplitude = Loud sound.
Distinguishes sounds of same pitch and loudness.
Amount of sound energy passing each second through unit area.
v = νλ. (Approx 344 m/s in air at 22°C).
Solid > Liquid > Gas.
Shock waves produced when object travels faster than speed of sound.
Follows laws of reflection (Angle i = Angle r).
Repetition of sound due to reflection.
Sensation of sound persists in brain for 0.1 s.
Minimum distance to hear an echo in air.
Persistence of sound due to repeated reflections (e.g., in a hall).
Medical instrument based on multiple reflection of sound.
20 Hz to 20,000 Hz (20 kHz) for humans.
Frequency < 20 Hz (Whales, Elephants, Earthquake).
Frequency > 20 kHz (Bats, Dolphins).
Using ultrasound to image the heart.
Imaging internal organs using ultrasound.
Sound Navigation And Ranging.
Formula for SONAR calculations.
Use ultrasonic squeaks to navigate and catch prey (Echolocation).
Pinna. Collects sound.
Passage leading to the eardrum.
Tympanic membrane. Vibrates when sound hits it.
Hammer, Anvil, Stirrup. Amplify vibrations.
Inner ear part. Converts vibrations to electrical signals.
Transmits electrical signals to the brain.
Affects speed of sound. Higher density usually means higher speed (Solids).
Speed of sound increases with increase in temperature.
Sound of single frequency is Tone. Mixture of several frequencies is Note.
Unpleasant sound.
Pleasant sound.
Directs sound in a particular direction using reflection.
~344 m/s.
~1500 m/s.
~5000-6000 m/s.
Named after Heinrich Hertz.
Disturbance that moves through a medium.
Low compared to light energy.