Sound
CBSE Class 9 · Science · Notes, formulas and practice questions
A crisp revision guide to the CBSE Class 9 Science chapter Sound: how it is made, how it travels as a longitudinal wave, and how it reflects. It also explains echoes, the range of human hearing, ultrasound applications and SONAR.
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Sound is a mechanical wave produced by a vibrating source. When a source vibrates, it pushes the neighbouring particles of the medium (air, water or metal) closer together and then draws them apart. These moving regions form compressions and rarefactions that travel outward, carrying the disturbance from one place to another. Because sound cannot travel through a vacuum, an astronaut on the Moon cannot shout to another astronaut without a radio.
A sound wave is described by its wavelength, frequency, time period and amplitude. Wavelength is the distance between successive compressions, while frequency is the number of vibrations completed per second. The relation v = f × λ links the speed of the wave to these two. Amplitude decides how loud a sound feels, and frequency decides its pitch. A wave with higher frequency is perceived as having a higher pitch, and one with larger amplitude is heard as louder.
When a sound wave strikes a hard, smooth obstacle, it bounces back. This is the reflection of sound. If the reflected sound arrives after a gap of more than 0.1 seconds, the human ear can hear it as a separate echo. If the reflections reach sooner, they create a lingering effect known as reverberation, which is common inside empty halls. Curtains and upholstery absorb some sound and reduce reverberation.
The normal audible range of the human ear goes from about 20 Hz to 20,000 Hz. Waves having frequencies higher than 20,000 Hz are called ultrasound. Doctors use ultrasound scanning to observe internal organs and unborn babies, industries use it for cleaning and detecting flaws, and underwater SONAR works on the same principle. A SONAR system sends an ultrasonic pulse downward and times the returning echo to measure depth.
Key terms
- Longitudinal wave
- A longitudinal wave is one in which the particles of the medium vibrate to and fro along the same direction the wave travels. Sound waves in air are longitudinal: they consist of travelling compressions and rarefactions. Because sound is a mechanical wave, it must have a medium such as air.
- Compressions and rarefactions
- In a sound wave, a compression is the region where the medium's particles are crowded together, increasing local pressure. A rarefaction is the region where particles are spread apart, giving lower pressure. A moving sound wave is a chain of alternating compressions and rarefactions.
- Wavelength
- The distance between two consecutive compressions (or between two consecutive rarefactions) of a wave. It is denoted by the Greek letter λ and is measured in metres. Wavelength is the spatial length of one complete cycle.
- Frequency
- The number of complete oscillations or cycles of the wave that pass a fixed point per second. Frequency is measured in hertz (Hz); it is the same as the number of vibrations of the source. Higher frequency produces higher pitch.
- Amplitude
- The greatest distance a particle moves from its rest position during vibration. It is a measure of the energy of the wave. A wave of larger amplitude is heard as louder, while smaller amplitude is softer.
- Echo
- An echo is the distinct repetition of a sound caused by its reflection from a large surface. It is heard only when the gap between the original sound and the reflected sound is at least 0.1 seconds. This usually means the reflecting surface is more than about 17 m away.
- Reverberation
- Reverberation is the persistence of sound after the original source has stopped, caused by several reflections arriving at the ear in quick succession, within 0.1 seconds. Too much reverberation makes speech in a room unclear.
- Ultrasound
- Sound waves with a frequency higher than the top of the human audible range, namely above 20,000 Hz. They cannot be heard by humans, but they can be used for medical imaging and for cleaning objects without scratching them.
- SONAR
- A method for locating objects underwater by sending out pulses of ultrasound and listening for their echoes. Knowing the speed of sound in water and the time gap before the echo returns, the depth or distance to the object can be calculated.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Relation between wave speed, frequency and wavelength | v = f × λ | v is the speed of sound in the medium in m/s, f is the frequency in Hz and λ (lambda) is the wavelength in metres. It holds for every wave. |
| Time period from frequency | T = 1/f | T is the time taken for one complete oscillation, measured in seconds, and f is the frequency in hertz. If the source vibrates faster, the time period becomes shorter. |
| Distance to a reflecting surface from echo time | d = v × t / 2 | v is the speed of sound in m/s, t is the total time between sending a sound and hearing its echo in seconds, and d is the distance to the reflecting object in metres. The factor 2 arises because sound travels to the object and back. |
Practice questions with answers
1. Define a longitudinal wave. Give an example.
In a longitudinal wave, the particles of the medium vibrate back and forth along the same line in which the wave travels. It consists of compressions and rarefactions. Sound travelling through air is the most common example of a longitudinal wave.
2. A sound wave has a frequency of 500 Hz and a wavelength of 0.66 m. Calculate its speed.
Speed can be found using v = f × λ. Substituting f = 500 Hz and λ = 0.66 m gives v = 500 × 0.66 = 330 m/s. Hence the speed of the sound wave is 330 m/s.
3. A boy shouts near a cliff wall and hears his echo after 0.5 seconds. If the speed of sound is 340 m/s, what is the distance to the cliff?
In 0.5 s the sound travels from the boy to the cliff and then back, so the total distance covered is 340 × 0.5 = 170 m. This is twice the distance to the cliff, so the cliff is 170/2 = 85 m away.
4. If the time period of a sound wave is 0.002 seconds, find its frequency.
Frequency is the reciprocal of time period: f = 1/T = 1/0.002 = 500 Hz. Therefore the sound wave completes 500 oscillations every second.
5. What is an echo? When does a distinct echo become audible?
An echo is a reflected sound that repeats after the original sound. A distinct echo is heard only when the time gap between the original sound and the reflected sound is at least 0.1 s. If the reflecting surface is too close, the two sounds overlap and no separate echo is heard.
6. How is pitch different from loudness?
Pitch depends on the frequency of the sound wave: higher frequency gives a higher pitch and a lower frequency produces a lower pitch. Loudness depends on the amplitude of the wave: a larger amplitude is heard as a louder sound. A whisper can have a high pitch but a very small loudness.
7. A ship sends a SONAR pulse down and receives the echo from the sea bed after 4 seconds. If the speed of sound in water is 1500 m/s, what is the depth of the sea?
The pulse travels to the bottom and back, covering twice the depth in 4 seconds. Distance travelled = 1500 × 4 = 6000 m. So the depth is 6000/2 = 3000 m.
8. Give two practical uses of ultrasound.
Ultrasound is used in medicine for scanning inside the body, such as checking an unborn baby in the mother's womb. In industry it is used to detect hidden cracks in metal parts and to clean delicate objects like jewellery and intricate machinery components.
9. Why does sound require a medium to travel? Explain with an everyday example.
Sound is a mechanical wave that travels by the vibration of particles about their mean positions. Without particles to vibrate, there is no way to carry the disturbance from one point to another. This is why two astronauts cannot talk to each other directly on the Moon; there is no air to transmit the sound.
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