General Science Notes Chapter 18 Class 7 Maharashtra Board

Notes For All Chapters – General Science Class 7

Sound : Production of Sound

1. Introduction to Sound

• Sound is a form of energy that is produced by vibrations.
• When an object vibrates, it creates sound waves that travel through a medium (air, water, or solid) and reach our ears.
• Sound cannot travel in a vacuum because there is no medium to carry the vibrations.

2. Production of Sound

✔ How is sound produced?

Sound is produced when an object vibrates.

Examples:

• Drums vibrate when struck.
• Guitar strings vibrate when plucked.
• Vocal cords vibrate when we speak.

✔ Oscillations and Vibrations

• Oscillation: A repeated back-and-forth movement of an object.
• Vibration: A very fast oscillation that produces sound.

✔ Example of Oscillation:

• When a pendulum swings from one side to another, it completes one oscillation.

3. Properties of Sound

✔ 1) Frequency (Pitch of Sound)

• Frequency is the number of vibrations per second.
• It is measured in Hertz (Hz).
• More vibrations = Higher frequency = High-pitched sound (e.g., a whistle).
• Fewer vibrations = Lower frequency = Deep sound (e.g., a drum).

✔ 2) Amplitude (Loudness of Sound)

• Amplitude is the height of the sound wave.
• Greater amplitude = Louder sound.
• Smaller amplitude = Softer sound.

✔ 3) Time Period

• Time period is the time taken for one complete vibration.
• Time period = 1 / Frequency.

✔ 4) Speed of Sound

• Sound travels at different speeds in different mediums:

• Example: Sound travels faster in metal than in air.

4. Types of Sound

✔ 1) Audible Sound

• Humans can hear sounds between 20 Hz and 20,000 Hz.

✔ 2) Infrasonic Sound (< 20 Hz)

• Sounds too low for humans to hear.
• Used by elephants and whales for communication.

✔ 3) Ultrasonic Sound (> 20,000 Hz)

• Sounds too high for humans to hear.
• Used by bats, dolphins, and in medical imaging (ultrasound).

5. Propagation (Travel) of Sound

✔ How does sound travel?

• Sound travels in the form of waves through a medium.
• It requires air, water, or solid to move.
• It cannot travel in space (vacuum).

✔ Experiment:

• If we place an alarm clock inside a vacuum chamber, we cannot hear the sound because there is no air for sound waves to travel.

6. Reflection of Sound

✔ Echo:

• An echo is the reflection of sound when it bounces off a hard surface (like walls or mountains).
• Example: Hearing your voice in an empty hall or cave.
• The minimum distance for an echo to be heard is 17 meters.

✔ SONAR (Sound Navigation And Ranging):

• SONAR is used to detect objects underwater.
• Ships and submarines send ultrasonic waves into water, and when the sound bounces back, they calculate the distance of an object.

7. Musical Instruments and Sound Production

✔ Different musical instruments produce sound in different ways:

8. Uses of Sound in Daily Life

✔ Communication – Talking, radio, and telephone.

✔ Entertainment – Music and television.

✔ Medical Uses – Ultrasound scans for pregnancy.

✔ Navigation – Bats use echolocation to find food.

✔ Security – Sirens and alarm systems.

9. Noise Pollution and Its Effects

✔ What is Noise Pollution?

• Unwanted and loud sounds that disturb humans and animals.
• Examples: Traffic, loudspeakers, construction work.

✔ Effects of Noise Pollution:

• Hearing loss
• Lack of sleep
• Stress and high blood pressure
• Disturbance to animals and birds

✔ Ways to Reduce Noise Pollution:

• Use silencers in vehicles.
• Avoid honking unnecessarily.
• Follow rules for loudspeakers.
• Plant trees to absorb sound.