Short Answer Questions — Chapter 5: Acoustics
Sound is a form of energy produced by vibrating objects. It is a mechanical wave that requires a material medium to travel.
Production of sound:
- When an object vibrates (like a guitar string, vocal cords, or a speaker cone), it disturbs the particles of the surrounding medium.
- Example: When a tuning fork vibrates, its prongs move back and forth, pushing and pulling air molecules.
Transmission of sound:
- The vibrating source creates alternating compressions (particles pushed close together) and rarefactions (particles spread apart).
- These compressions and rarefactions travel outward as a longitudinal wave.
- Energy is transmitted without permanent displacement of the medium particles.
Sound cannot travel in vacuum because there are no particles to carry the vibration.
Frequency (f):
- Number of complete vibrations (cycles) per second
- Unit: Hertz (Hz); 1 Hz = 1 cycle per second
- Higher frequency = higher pitch (shriller sound)
Wavelength (λ):
- Distance between two consecutive compressions or two consecutive rarefactions
- Also: distance covered in one complete vibration
- Unit: metres (m)
Amplitude (A):
- Maximum displacement of a particle from its mean (rest) position during vibration
- Determines the loudness of sound — larger amplitude = louder sound
- Unit: metres (m)
Time Period (T):
- Time taken to complete one full vibration (one cycle)
- T = 1/f
- Unit: second (s)
Relationship: v = fλ (speed = frequency × wavelength)
Echo: An echo is the repetition of a sound heard after it reflects from a hard, distant surface back to the listener.
Conditions for hearing an echo:
1. Minimum distance: The reflecting surface must be at least 17 metres away from the source/listener.
- Reason: Human ear can distinguish two sounds only if there is a time gap of at least 0.1 second (persistence of hearing).
- In 0.1 s, sound travels 340 × 0.1 = 34 m (to wall and back → wall must be 34/2 = 17 m away).
2. Hard, large surface: The reflecting surface must be smooth, hard, and large enough to reflect sound clearly. Soft surfaces (curtains, grass) absorb sound and do not produce clear echoes.
3. Single, short, sharp sound: A prolonged sound mixes with its echo and is not heard separately.
Applications:
- Calculating depth of ocean (SONAR)
- Measuring height of clouds
- Rangefinders
Reverberation: The persistence of sound in an enclosed space due to multiple reflections from walls, ceiling, floor, and other surfaces. Even after the source stops, the sound seems to continue.
Problem with excessive reverberation:
- Sound becomes "muddy" and unclear
- Words blend together making speech unintelligible
- Music loses clarity
Control of reverberation in auditoriums:
1. Sound-absorbing materials on walls and ceiling:
- Curtains, carpets, foam panels, acoustic tiles
- These absorb sound energy instead of reflecting it
2. Irregular surfaces: Rough or irregular wall designs scatter sound in different directions rather than reflecting it directly.
3. Soft furnishings: Padded seats, cushions in the audience area absorb sound.
4. Optimal shape: Auditoriums are designed with curved ceilings to direct reflected sound evenly to all seats.
5. False ceilings and wall paneling with air gaps also absorb sound.
Reverberation time (Sabine's formula): The ideal reverberation time for speech is about 0.5–1 second; for music, about 1–2 seconds.
Ultrasound is sound with frequency greater than 20,000 Hz (20 kHz).
Medical uses:
1. Sonography (Medical imaging): Ultrasound pulses are sent into the body. Reflected pulses (echoes) from different organs/tissues are used to create images. Used to examine:
- Fetal development during pregnancy
- Abdominal organs (liver, kidneys)
- Tumours, cysts
2. Echocardiography: Ultrasound imaging of the heart to detect defects, measure heart chamber size.
3. Physiotherapy: Ultrasound waves cause gentle vibrations in tissues, providing heat therapy for muscle injuries.
4. Breaking kidney stones: High-intensity focused ultrasound (HIFU) breaks kidney and gallstones into small pieces that can pass naturally.
Industrial uses:
1. Detecting flaws: Ultrasound is used to detect cracks or defects inside metal components (non-destructive testing).
2. Cleaning: Delicate objects (optical lenses, surgical instruments, electronic components) are cleaned using ultrasonic vibrations in liquid.
3. SONAR: Detects submarines, maps ocean floor, locates fish shoals.
4. Welding and cutting: Ultrasonic welding joins plastics without glue.
SONAR stands for Sound Navigation And Ranging.
Principle: It uses ultrasound to detect and locate underwater objects by measuring the time taken for an echo to return.
1. A transmitter on the ship sends out a strong pulse of ultrasound. 2. The ultrasound travels through water and reflects off the ocean floor or any object (submarine, rock, fish shoal). 3. The reflected pulse (echo) is received by a hydrophone (underwater microphone). 4. The time interval between sending and receiving is measured. 5. Distance is calculated: d = v × t / 2 (÷ 2 because sound travels to object and back)
Where:
- d = distance to object (m)
- v = speed of sound in water (~1500 m/s)
- t = time for echo to return (s)
Uses of SONAR:
- Measuring depth of oceans
- Detecting submarines (naval defence)
- Finding fish shoals
- Mapping sea floor (marine geology)
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