Sound Class 8 Notes Science Chapter 11

School periods end with a sound, like a knock on a door. In games, we use sounds to find others. Around us, we hear birds, voices, and cars. Instruments in the music room make sounds by vibrating. This chapter explains how sound travels, how we hear it, and why some are louder. 

Musical Instrument

Sound is Produced by a Vibrating Body

Vibration When something moves back and forth, it's called vibration. For instance, when you pluck a tight band, it vibrates and makes sound. Once it stops vibrating, no sound is produced.

Visible and Invisible Vibrations sometimes can be seen easily, like in certain cases. But usually, they're too tiny for us to notice. Still, we might feel them even if we can't see them.

Common Instruments Instruments like manjira (cymbals), ghatam, noot (mudpots), and kartal are widespread in our country. They're typically played by beating or striking them.

Ghatam

Sound Produced by Humans

Feeling Vibrations  Try speaking loudly, singing, or buzzing like a bee for a bit. Can you feel any vibrations?

How Humans Make Sound In people, sound comes from a part called the voice box or larynx. Feel your throat to find a hard bump that moves when you swallow – that's the voice box. It's at the upper end of the windpipe.

• Vocal Cord Vibration: Two vocal cords stretch across the voice box, leaving a narrow slit for air to pass. When air from the lungs moves through this slit, the vocal cords vibrate, making sound.
• Adjusting Sound: Muscles connected to these vocal cords can tighten or loosen them. When cords are tight and thin, the voice sounds different from when they're loose and thick.  

Voice box in humans 

Sound Needs a Medium for Propagation

• Vibrating Source: Sound happens when an object vibrates, like when a guitar string is plucked, creating sound waves.
• Medium for Transmission: Sound vibrations need a substance, like air, to move through and carry energy to our ears. In air, these vibrations make nearby air particles collide, passing the energy along.
• Particle Interaction: When an object vibrates, like a guitar string, it pushes and pulls air particles around it. These particles then pass the energy to nearby particles, forming waves of compression (closer particles) and rarefaction (spread-out particles).
• Transmission in Different Media: Sound travels at various speeds based on the medium. It moves faster in solids (e.g., metal) where particles are tightly packed and slower in air due to farther-apart particles.
• Absence of Medium: Unlike light, sound can't travel in a vacuum because it relies on particles bumping into each other for energy transmission. In a vacuum, there are no particles for sound to move through.
• Importance of Medium: Sound needs a medium to travel, explaining why it doesn't move in space. Astronauts use radios to communicate in space. Underwater, sound is heard differently as water transmits sound more effectively than air.

Sound travelling through water 

We Hear Sound through Our Ears

• Ear's Structure: The outer ear resembles a funnel, guiding sound down a canal to the tightly stretched membrane called the eardrum.
• Eardrum's Role: Similar to a tin can model, the eardrum functions as a vital part. It's like a stretched rubber sheet that vibrates when sound waves reach it.
• Sound Transmission: Sound isn't limited to air; it travels through wood, metal, and other solids. Activities can demonstrate how sound travels through strings.
• Hearing Process: The vibrations from the eardrum travel to the inner ear, and then signals are sent to the brain, enabling us to hear.

Human Ear

Amplitude, Time Period and Frequency of a Vibration

• Vibration and Motion: When something moves back and forth, it's called vibration or oscillatory motion, which you've learned about before.
• Frequency Definition: Frequency is the number of back-and-forth motions per second, measured in hertz (Hz). For example, 1 Hz means one oscillation per second.
• Calculating Frequency: If an object moves 20 times in one second, its frequency would be 20 Hz.
• Amplitude Explanation: Amplitude is the measure of the maximum extent of a vibration or oscillation from the central position. It represents the 'height' or intensity of a sound wave.
• Recognizing Sounds: We can identify sounds even without seeing their source. Different sounds have distinguishing factors, including amplitude and frequency.
• Importance of Factors: Amplitude and frequency determine how sounds differ from one another.

Loudness and Pitch 

Frequency and Pitch: The frequency of a sound wave decides its pitch or how high or low it sounds to us. Higher frequencies create shriller or higher-pitched sounds, while lower frequencies result in lower-pitched sounds.

• Loudness and Amplitude The volume of sound is determined by its amplitude. Larger vibrations create louder sounds, while smaller vibrations result in softer sounds.
• Relationship Between Frequency and Pitch Sounds with higher frequency are higher-pitched, like a whistle's sound. Lower frequency sounds, like a lion's roar, are lower-pitched but can be very loud.
• Differences in Voices Children's voices often have higher frequencies than adults'. Women's voices tend to be higher in frequency and more shrill compared to men's voices.

Audible and Inaudible Sounds

• Range of Audible Sounds Vibrating bodies produce sound, but not all sounds are audible to humans.
• Inaudible Sounds Sounds below about 20 vibrations per second (20 Hz) and above about 20,000 vibrations per second (20 kHz) can't be heard by humans. They're known as inaudible sounds.
• Human Hearing Range For the human ear, the audible range of frequencies falls approximately between 20 to 20,000 Hz.

Noise and Music

• Variety of Sounds: Sounds around us differ in their pleasantness. Some sounds are pleasing, while others can cause discomfort.
• Noise Definition: Unpleasant sounds, like those from construction work or vehicle horns, are called noise.
• Classroom Scenario: When all students speak together in a classroom, the resulting sound is called noise.
• Musical Sounds: Musical sounds, like those from instruments such as the harmonium or the strings of a sitar, are pleasing to the ear.
• Melodiousness and Volume: While musical sounds are pleasant, if they become too loud, they might lose their melodious quality.

Noise Pollution

• Noise Pollution Definition: Unwanted or excessive sounds in the environment are termed as noise pollution, similar to air pollution with unwanted gases and particles.
• Sources of Noise Pollution: Major causes include vehicle sounds, explosions like firecrackers, loud machinery, and loudspeakers.
• Noise Sources at Home: Home sources contributing to noise pollution include high-volume television or transistor radio, certain kitchen appliances, desert coolers, and air conditioners.

What are the Harms of Noise Pollution?

• Impact on Health: Excessive noise in surroundings can lead to health problems such as lack of sleep, hypertension (high blood pressure), anxiety, and various other disorders.
• Hearing Impairment: Continuous exposure to loud sounds can cause temporary or even permanent impairment of hearing.

Measures to Limit Noise Pollution

• Controlling Noise Sources: To reduce noise, silencing devices can be installed in aircraft engines, transport vehicles, industrial machines, and home appliances.
• Controlling Noise in Residential Areas: Strategies for reducing noise pollution in residential areas include conducting noisy operations away from residences, setting up noisy industries away from residential zones, minimizing the use of automobile horns, running TVs and music systems at lower volumes, and planting trees along roads and around buildings to mitigate sound impact.