Sound: Production of Sound | Class 7 | Science | Chapter 18 | Maharashtra State Board
Understanding Sound: Vibrations and Oscillations Dive into the fascinating world of sound with our educational video! Explore how sound is produced through vibrations, the role of oscillatory motion, and the science behind musical instruments. Learn about pitch, intensity, and the decibel levels of common sounds. Perfect for students and curious minds alike! 🎵🔊
Questions & Answers
1. Fill in the blanks.
(a) Sound is generated by the rhythmic vibrations of any object.
(b) The frequency of sound is measured in Hertz (Hz).
(c) If intensity of sound is decreased its loudness also decreases.
(d) A medium is necessary for generation of sound.
2. Match the pairs.
Column 'A' | Answer. | Column 'B' |
Flute | c. Vibrations in the air | a. Frequency less than 20 Hz |
Frequency | d. Measured in Hz | b. Frequency more than 20,000 Hz |
Sound level | e. Decibel | c. Vibrations in the air |
Ultrasonic sound | b. Frequency more than 20,000 Hz | d. Measured in Hz |
Infrasonic sound | a. Frequency less than 20 Hz | e. Decibel |
3. Give scientific reasons.
(a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rails.
Ans. The practice of listening for the arrival of a distant train by putting one’s ear to the rails is based on the principle of sound conduction through solid materials. When a train approaches, the wheels generate vibrations that travel through the train and into the rails. These vibrations are transmitted along the rail tracks and can be detected by placing one’s ear on the rails.
(b) The sounds generated by a tabla and sitar are different.
Ans. The sounds of a tabla and sitar are different because they are distinct musical instruments with different structures and playing techniques. The tabla is a percussion instrument with a set of hand-played drums, producing rhythmic and percussive sounds. On the other hand, the sitar is a stringed instrument played with a plucking technique, creating melodic and resonant tones.
(c) If you were both on the moon, your friend will not be able to hear you call.
Ans. On the moon, your friend wouldn't be able to hear you call because there is no air or atmosphere to transmit sound waves. Sound requires a medium, like air, to travel through, and in the vacuum of space on the moon, there is no medium for sound propagation. Therefore, your voice or any other sound wouldn't be able to reach your friend's ears without air to carry the vibrations.
(d) We can hear the movement of a mosquito wings but we cannot hear the movement of our hands.
Ans. The sound produced by the movement of a mosquito's wings is audible because the wings create vibrations in the air, which our ears can detect as sound. In contrast, the movement of our hands typically doesn't produce significant air disturbances or vibrations at audible frequencies, so we don't hear the movement of our hands in the same way we can hear the buzzing of a mosquito's wings. Essentially, the sound is produced when an object causes air particles to vibrate, and the frequency and amplitude of these vibrations determine whether we perceive them as sound.
4. Write answers to the following questions.
(a) How is sound produced?
Ans. Sound is produced when an object vibrates, creating pressure waves that travel through a medium, usually air. These waves cause particles in the medium to compress and rarefy, producing variations in air pressure that our ears detect as sound.
(b) What does the intensity of sound depend upon?
Ans. The intensity of sound depends on the amplitude of the sound waves. In other words, it is related to how strongly the particles in the medium vibrate as the sound passes through. Greater amplitude results in higher intensity, which corresponds to louder sounds.
(c) Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.
Ans. The frequency of oscillation of a pendulum is not directly affected by its amplitude. However, the frequency is influenced by the length of the pendulum. According to the formula for simple harmonic motion, the frequency (f) is inversely proportional to the square root of the length (L) of the pendulum.
This means that as the length of the pendulum increases, the frequency decreases, and vice versa. The amplitude, which is the maximum displacement from the equilibrium position, does not affect the frequency but can influence the overall behaviour of the oscillation.
(d) Explain the two ways by which the pitch of the sound generated by a stretched string can be changed?
Ans. The pitch of the sound generated by a stretched string can be changed in two ways:
1. Changing the tension: Increasing the tension in the string increases the pitch, while decreasing the tension lowers the pitch. This is because a tighter string vibrates faster, producing a higher frequency sound, and vice versa.
2. Changing the length: Shortening the length of the string increases the pitch, while lengthening it decreases the pitch. A shorter string vibrates faster, resulting in a higher frequency sound, and vice versa.
These principles are commonly used in musical instruments like guitars and violins to produce different notes.
E X T R A
1. Fill in the blanks:
1. Bats detect the obstacles in their path by receiving the reflected ultrasonic waves.
2. An object that moves back and forth is called an oscillator.
3. If the length of a pendulum is increased the time period of oscillation increases.
4. The amplitude of a wave is the maximum distance moved by the object on either side of the mean position.
5. Sound waves do not travel through vacuum.
6. The frequency above which sound is not audible to the human ear is 50,000 Hz.
7. Number of vibrations in a second is called frequency.
8. Higher the frequency greater will be the Pitch.
9. The loudness of sound depends on amplitude.
10. Loudness of sound is measured in units of decibel dB.
11. 100 hertz is equal to hundred oscillations in one second.
2. Name the following:
1. The sound frequencies above 20,000 Hz.
Ans. Ultrasonic
2. The medium through which sound is most clearly propagated.
Ans. Solids
3. The range of frequency in which humans can hear sound.
Ans. 20 – 20,000 Hz
4. The sound level physically painful to humans.
Ans. Above 100 dB
5. The technique used by bats to find their way.
Ans. Echolocation
6. The unit of frequency of sound.
Ans. Hertz
7. The unit of sound level or intensity.
Ans. Decibel
8. The scientist in whose honour was the unit decibel named for intensity of sound.
Ans. Alexander graham Bell
9. Bats and dolphins can hear these sound frequencies.
Ans. Ultra sonic sound
10. The sound frequencies used in RADAR systems.
Ans. Ultra sonic sound
3. Answer in one or two sentences:
1. How is sound produced?
Ans. Sound is produced by vibration of an object.
2. What produces sound in human beings?
Ans. The vibration of the vocal cords in our larynx or voice box produces sound.
3. What is oscillatory motion?
Ans. To and fro motion of an object (oscillator) on either side of a central position is called oscillatory motion.
4. What is periodic motion?
Ans. The motion of an object repeated again and again at fixed intervals of time is called periodic motion.
5. What is frequency of oscillation?
Ans. The number of oscillations completed by an oscillator in one second is called the frequency of oscillation.
6. How is intensity of sound and amplitude of vibration related?
Ans. The intensity of sound is proportional to the square of the amplitude of vibration. For example, if the amplitude is doubled the intensity becomes four times as much.
7. What happens if the frequency of sound is greater than 1000 Hz or 100 dB?
Ans. If frequency of sound is increased more than 100dB, the hearing can be temporarily affected which can result in temporary deafness.
8. What is infrasonic sound?
Ans. A sound with a frequency less than 20Hz is called infrasonic sound.
9. What is application of ultrasonic sound?
Ans. Ultrasonic sound is used in ships as a system known as ‘SONAR’ to detect the obstacle beneath the ocean as well as the depth of an ocean.
10. What is the difference between pitch and frequency?
Ans. Pitch is directly proportional to the frequency of a sound whereas frequency is the repeating vibrations per unit time.
11. What happens when the rubber band is stretched with a greater force and released?
Ans. When a rubber is stretched with a greater force and released, the amplitude increases which in turn result in a louder sound being generated.
12. What happens to frequency if the amplitude is changed?
Ans. The frequency remains the same even if the amplitude is increased or decreased.
4. Define:
1. Echo:
Ans. It is a repetition of sound due to the reflection of original sound by a large and hard obstacle.
2. Frequency:
Ans. It is defined as the number of vibrations per unit time.
3. Hertz (Hz):
Ans. One hertz is one vibration per second.
4. Ultrasonic vibration:
Ans. Sounds of frequency higher than 20,000 Hz are called the ultrasonic.
5. Time period:
Ans. The time taken by an object to complete one oscillation is called time period.
6. Amplitude:
Ans. The distance between the mean or central position to the extreme position of an oscillator on either side is called amplitude.
5. Say True or False with explanation:
1. Sound cannot travel in vacuum.
Ans. True. Sound needs a medium to travel through. Since vacuum does not provide any medium, sound cannot travel through it.
2. If the amplitude of vibration is large, sound is feeble.
Ans. False. When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble, when the amplitude is small. Loudness of a sound is proportional to the square of the amplitude of its vibration.
3. For human ears, the audible range is 2 Hz to 20,000 Hz.
Ans. True. Human being can hear sounds only within the range of 20-20,000 Hz. We cannot hear the sound having frequency out of this range.
4. The lower the frequency of vibrations the higher is the pitch.
Ans. False. Pitch of sound increases when the frequency of vibration increases. The pitch of a sound is directly proportional to its frequency.
5. Unwanted or unpleasant sound is termed as music.
Ans. False. Unwanted, or unpleasant sounds are termed noise. Sounds which are melodious, and pleasing to ears are called music.
6. Noise pollution may cause partial hearing impairment.
Ans. True. Unwanted, unpleasant sounds are called noise. If one is subjected to noise continuously for a long time, one may suffer from partial hearing impairment.
7. Time taken by an object to complete one oscillation is called time period.
Ans. True. Time taken by an object to complete one oscillation is called time period.
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