11.1 Production of Sound
What is Sound?
Sound is a form of energy that produces a sensation of hearing in our ears. Sound is produced when objects vibrate and travel through a medium to reach our ears.
Key Concept: Sound is always produced by vibrating objects. No vibration means no sound production.
Tuning Fork Experiment
When a tuning fork is struck and brought near the ear, we hear sound. When we touch the vibrating prong, the vibration stops and sound ceases. When the vibrating tuning fork touches a suspended ball, the ball moves, showing that vibrating objects can transfer energy.
Examples of Sound Production
Natural Sources:
• Human vocal cords vibrating
• Bird wings flapping
• Bee wings buzzing
• Thunder from lightning
Artificial Sources:
• Musical instruments
• Bells ringing
• Machine operations
• Electronic devices
Vibration means rapid to-and-fro motion of an object. All sound-producing objects undergo vibration.
11.2 Propagation of Sound
How Sound Travels
Sound travels through a medium (solid, liquid, or gas) from the source to the listener. Sound cannot travel through vacuum as it requires a material medium for propagation.
Sound Wave Propagation
Vibrating Object → Compressions & Rarefactions → Medium Particles → Ear
Particles don't travel - only energy and disturbance move forward
Compressions and Rarefactions
Compression (C):
• Region of high pressure
• Particles crowded together
• Higher density
• Represented by wave peaks
Rarefaction (R):
• Region of low pressure
• Particles spread apart
• Lower density
• Represented by wave troughs
Sound propagates as alternating compressions and rarefactions through the medium. This can be visualized as propagation of pressure variations or density variations.
11.3 Characteristics of Sound Waves
Sound Waves are Longitudinal Waves
In longitudinal waves, particles of the medium vibrate parallel to the direction of wave propagation. Sound waves are longitudinal because air particles move back and forth in the same direction as the sound travels.
Comparison with Transverse Waves:
• Longitudinal: Particles vibrate parallel to wave direction (sound waves)
• Transverse: Particles vibrate perpendicular to wave direction (light waves, water surface waves)
Wave Parameters
Wavelength (λ)
Distance between two consecutive compressions or rarefactions
Unit: metre (m)
Frequency (ν)
Number of oscillations per unit time
Unit: hertz (Hz)
Time Period (T)
Time for one complete oscillation
Unit: second (s)
Frequency and Time Period Relationship:
ν = 1/T T = 1/ν
Amplitude and Loudness
The amplitude is the maximum displacement of particles from their mean position. It determines the loudness of sound.
Amplitude Effects:
• Larger amplitude → Louder sound
• Smaller amplitude → Softer sound
• Amplitude depends on the force used to create vibration
11.4 Speed of Sound
Speed of Sound (v) = Wavelength (λ) × Frequency (ν)
v = λν
Factors Affecting Speed of Sound
Medium Type:
Solids > Liquids > Gases
(Speed decreases from solid to gas)
Temperature:
Higher temperature → Higher speed
At 0°C: 331 m/s
At 22°C: 344 m/s
Speed in Different Media (at 25°C)
| State |
Substance |
Speed (m/s) |
| Solids | Aluminium | 6420 |
| Steel | 5960 |
| Glass | 3980 |
| Liquids | Water (Sea) | 1531 |
| Water (Distilled) | 1498 |
| Gases | Hydrogen | 1284 |
| Air | 346 |
| Oxygen | 316 |
11.5 Reflection of Sound
Laws of Reflection
Sound follows the same laws of reflection as light:
Laws of Reflection:
1. Angle of incidence = Angle of reflection
2. Incident ray, reflected ray, and normal lie in the same plane
3. Large obstacles are needed for effective reflection
Echo
An echo is the reflected sound that reaches the listener after reflection from a distant obstacle.
Conditions for Hearing Echo:
• Time gap between original and reflected sound ≥ 0.1 s
• Minimum distance from obstacle = 17.2 m (at 22°C)
Echo Calculation:
If echo is heard after 2 seconds and speed of sound is 346 m/s:
Total distance = v × t = 346 × 2 = 692 m
Distance to obstacle = 692/2 = 346 m
Reverberation
Reverberation is the persistence of sound due to repeated reflections from walls, ceiling, and floor of a closed space.
Reducing Reverberation:
• Sound-absorbent materials on walls and ceiling
• Compressed fiberboard, rough plaster
• Appropriate seat materials
• Curtains and draperies
11.6 Applications of Sound Reflection
Megaphones & Horns:
Use curved surfaces to direct sound in specific direction using multiple reflections
Stethoscope:
Uses multiple reflection to amplify heart and lung sounds for medical diagnosis
Concert Hall Design:
Curved ceilings ensure sound reaches all corners through proper reflection
Sound Boards:
Placed behind stage to spread sound evenly across the hall width
11.7 Range of Hearing
Human Audible Range
Human Hearing Range: 20 Hz to 20,000 Hz (20 kHz)
Children and some animals: up to 25 kHz
Infrasound
• Below 20 Hz
• Produced by earthquakes
• Used by elephants, whales
• Animals detect before earthquakes
Audible Sound
• 20 Hz to 20 kHz
• Normal human hearing
• Music, speech
• Everyday sounds
Ultrasound
• Above 20 kHz
• Produced by bats, dolphins
• Medical applications
• Industrial cleaning
Pitch, Loudness, and Quality
Pitch:
• Determined by frequency
• Higher frequency → Higher pitch
• Brain's interpretation of frequency
Quality (Timbre):
• Distinguishes sounds of same pitch and loudness
• Depends on waveform shape
• Enables voice recognition
11.8 Applications of Ultrasound
Medical Applications
Ultrasound Scanning:
• Images of internal organs
• Pregnancy monitoring
• Detecting abnormalities
• Non-invasive diagnosis
Echocardiography:
• Heart imaging
• Kidney stone treatment
• Breaking stones into fine grains
• Safe medical procedures
Industrial Applications
Cleaning:
• Hard-to-reach places
• Electronic components
• Removes dust, grease, dirt
• High-frequency vibrations
Flaw Detection:
• Cracks in metal blocks
• Building construction
• Bridge safety testing
• Quality control
How Ultrasound Detects Flaws
Ultrasonic waves → Metal Block → If defect present → Reflection back to detector
If no defect → Waves pass through → No reflection detected
11.9 Sound Properties Summary
Loudness vs Intensity:
• Loudness: Physiological response of ear
• Intensity: Sound energy per unit area per second
• Same intensity may have different loudness perception
Quality Factors:
• Tone: Single frequency sound
• Note: Multiple frequencies, pleasant
• Noise: Unpleasant, irregular sound
• Music: Pleasant, organized sound
Important Relationships:
v = λν | ν = 1/T | Distance = Speed × Time
11.10 Hearing Aids and Sound Technology
Hearing Aid Working
Sound Waves → Microphone → Electrical Signals → Amplifier → Speaker → Amplified Sound → Ear
Hearing aids help people with hearing loss by converting sound to electrical signals, amplifying them, and converting back to louder sound for the ear.
Natural Ultrasound Users
• Bats: Echolocation for navigation and hunting
• Dolphins: Communication and object detection
• Moths: Detecting bat ultrasound to escape predation
• Rats: Play and communication using ultrasound
Practice Questions & Answers
Q1. A sound wave has a frequency of 2000 Hz and wavelength 0.35 m. Calculate the speed of sound and time taken to travel 1.5 km.
Answer: Speed = λν = 0.35 × 2000 = 700 m/s
Time = Distance/Speed = 1500/700 = 2.14 s
Q2. An echo is heard after 3 seconds. If the speed of sound is 342 m/s, find the distance of the reflecting surface.
Answer: Total distance = v × t = 342 × 3 = 1026 m
Distance to reflecting surface = 1026/2 = 513 m
(Sound travels twice the distance - to obstacle and back)
Q3. Calculate the wavelength of sound in air for frequencies 20 Hz and 20 kHz. Take speed of sound = 344 m/s.
Answer: For 20 Hz: λ = v/ν = 344/20 = 17.2 m
For 20 kHz: λ = 344/20000 = 0.0172 m = 1.72 cm
Lower frequency has longer wavelength
Q4. Why can't we hear sound on the moon? Explain the reason.
Answer: Sound cannot travel on the moon because there is no atmosphere (no medium). Sound requires a material medium (solid, liquid, or gas) for propagation. The moon has vacuum, so sound waves cannot propagate.
Q5. A sound wave travels at 339 m/s with wavelength 1.5 cm. Find the frequency and determine if it's audible to humans.
Answer: Frequency = v/λ = 339/(0.015) = 22,600 Hz = 22.6 kHz
Since 22.6 kHz > 20 kHz, this is ultrasound and not audible to most humans.
Only children under 5 and some animals can hear this frequency
11.11 Key Points to Remember
Sound Production:
• Sound is always produced by vibrating objects
• No vibration = No sound
• Vibration is rapid to-and-fro motion
Sound Propagation:
• Requires a medium (solid, liquid, gas)
• Cannot travel through vacuum
• Travels as compressions and rarefactions
• Particles don't travel, only energy moves
Wave Properties:
• Longitudinal waves (particles vibrate parallel to wave direction)
• Speed = Wavelength × Frequency (v = λν)
• Frequency = 1/Time Period (ν = 1/T)
Sound Characteristics:
• Pitch depends on frequency
• Loudness depends on amplitude
• Quality helps distinguish different sounds
Applications:
• Echo: minimum 0.1s gap, 17.2m distance
• Ultrasound: medical imaging, industrial cleaning
• Sound reflection: megaphones, stethoscopes, concert halls