Sound is a form of mechanical wave produced by vibrating objects and transmitted through a medium such as air, water, or solids. This chapter explains the production, propagation, and properties of sound, including pitch, loudness, and quality. It also covers the speed of sound in different media, echo formation, reverberation, and the uses of ultrasound in medicine and industry. Understanding sound is essential for both theoretical physics and practical applications like communication and engineering.
Note: In Punjab and Sindh Board textbooks, this topic appears as Chapter 1: Sound. It is presented here as Chapter 2 in the merged Class 10 Physics sequence for your convenience.
1. Sound waves are:
- A. Longitudinal mechanical waves ✅
- B. Transverse mechanical waves
- C. Electromagnetic waves
- D. Standing waves only
Explanation: Sound is a longitudinal mechanical wave where particles vibrate parallel to the direction of wave propagation.
2. Which factor does NOT affect the speed of sound in air?
- A. Temperature
- B. Humidity
- C. Loudness ✅
- D. Pressure (when temperature is constant)
Explanation: Loudness is a perception based on amplitude; it does not influence the wave speed in a medium.
3. The speed of sound in air at 20°C is approximately:
- A. 220 m/s
- B. 280 m/s
- C. 343 m/s ✅
- D. 400 m/s
Explanation: At 20°C, sound travels in air at about 343 m/s, depending slightly on humidity.
4. Which property of a sound wave determines its pitch?
- A. Amplitude
- B. Frequency ✅
- C. Wavelength
- D. Wave speed
Explanation: Pitch depends on the frequency — higher frequency means higher pitch.
5. Which sound frequency range can be heard by a normal human ear?
- A. 0 – 100 Hz
- B. 1 – 10,000 Hz
- C. 20 – 20,000 Hz ✅
- D. 50 – 50,000 Hz
Explanation: Humans can hear sounds roughly between 20 Hz and 20 kHz.
6. Ultrasound refers to sound waves with frequencies:
- A. Less than 20 Hz
- B. Between 20 Hz and 20,000 Hz
- C. Greater than 20,000 Hz ✅
- D. Equal to 343 m/s
Explanation: Ultrasound has frequencies higher than the upper limit of human hearing.
7. The time taken for sound to travel to a surface and back is 2 seconds. If the speed of sound is 340 m/s, the distance to the surface is:
- A. 170 m
- B. 340 m ✅
- C. 680 m
- D. 85 m
Explanation: Distance = (speed × time) / 2 = (340 × 2) / 2 = 340 m.
8. Echoes are more distinct when:
- A. The reflecting surface is rough
- B. The reflecting surface is smooth and large ✅
- C. The sound is very loud
- D. The temperature is low
Explanation: A smooth, large surface reflects sound waves better, producing a clear echo.
9. Reverberation in an auditorium can be reduced by:
- A. Using glass walls
- B. Using carpets and curtains ✅
- C. Increasing the sound frequency
- D. Using metallic walls
Explanation: Soft, porous materials absorb sound, reducing reverberation.
10. Which device is used to measure sound intensity levels?
- A. Oscilloscope
- B. Sound level meter ✅
- C. Microphone
- D. Barometer
Explanation: Sound level meters measure sound pressure levels in decibels (dB).
11. The unit of sound intensity level is:
- A. Watt
- B. Decibel (dB) ✅
- C. Hertz (Hz)
- D. Newton
Explanation: Sound intensity level is measured in decibels, a logarithmic scale unit.
12. Which property of sound changes when it travels from air into water?
- A. Frequency
- B. Wavelength ✅
- C. Pitch
- D. Quality
Explanation: Frequency remains constant, but speed changes, so wavelength changes accordingly.
13. Sonar is based on the principle of:
- A. Refraction
- B. Reflection of sound waves ✅
- C. Diffraction
- D. Interference
Explanation: Sonar measures distances by sending sound pulses and detecting their echoes.
14. Which factor increases the speed of sound in air?
- A. Decrease in temperature
- B. Increase in temperature ✅
- C. Lower humidity
- D. Higher altitude
Explanation: Higher temperatures increase the kinetic energy of air molecules, increasing sound speed.
15. The quality of sound is determined by:
- A. Frequency alone
- B. Amplitude alone
- C. Harmonics present in the sound ✅
- D. Wavelength alone
Explanation: The quality or timbre of a sound depends on the number and relative strength of its harmonics.