Sound Waves: Characteristics and Applications
NCERT Class 9 Science • Chapter 10
Quick Answer
This Class 9 physics chapter explains sound as a longitudinal wave produced by vibrations. It covers how sound travels through media, wave characteristics like frequency, wavelength, amplitude, and speed, and properties such as pitch and loudness. Students also learn reflection of sound, echo, reverberation, and applications including SONAR and ultrasound.
↓ Free PDF, NCERT solutions, summary, key terms, FAQs below.
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Key Terms
- Sound Wave
- A longitudinal mechanical wave produced by a vibrating object that travels through a medium as compressions and rarefactions.
- Frequency
- The number of complete vibrations or wave cycles produced per second, measured in hertz (Hz), which determines the pitch of a sound.
- Amplitude
- The maximum displacement of particles from their rest position in a wave; greater amplitude means louder sound.
- Wavelength
- The distance between two consecutive compressions or rarefactions in a sound wave, or two identical points on the wave.
- Echo
- A repetition of sound caused by the reflection of sound waves from a hard surface, heard distinctly after the original sound.
- Ultrasound
- Sound of frequency higher than 20,000 Hz, beyond the range of human hearing, used in medical imaging and industry.
- SONAR
- A technique that uses reflected ultrasound waves to detect and measure the distance, direction, and speed of underwater objects.
Frequently Asked Questions
What type of wave is sound?▾
Sound is a longitudinal mechanical wave, meaning the particles of the medium vibrate back and forth along the direction the wave travels. It needs a medium such as air, water, or solid to propagate and cannot travel through a vacuum.
What determines the pitch and loudness of a sound?▾
Pitch depends on the frequency of the sound wave, with higher frequencies giving higher pitch. Loudness depends on the amplitude of the wave, with greater amplitude producing a louder sound.
What is an echo and how is it formed?▾
An echo is a repeated sound heard when sound waves reflect off a hard surface and return to the listener. To hear a distinct echo, the reflecting surface must be far enough away that the reflected sound arrives at least about 0.1 seconds after the original.
What is the difference between ultrasound and audible sound?▾
Audible sound has frequencies between about 20 Hz and 20,000 Hz, which humans can hear. Ultrasound has frequencies above 20,000 Hz, too high for the human ear, and is used in applications like medical scans and SONAR.
How does SONAR work?▾
SONAR works by sending out ultrasound pulses into water and detecting the waves reflected back from objects. By measuring the time taken for the echo to return, it calculates the distance and location of underwater objects.
Why can't sound travel through a vacuum?▾
Sound needs a material medium with particles to carry the vibrations from one place to another. A vacuum has no particles to vibrate, so sound waves cannot pass through it.
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