Sound — NCERT Solutions, AI Tutor & Practice

Understand: Sound is produced by objects that are vibrating (moving back and forth rapidly).

Analyze:

• When an object vibrates, it makes the air particles around it vibrate too.
• These vibrations travel through the air and reach our ears, where we hear them as sound.

Example: When you strike a drum, its stretched membrane vibrates and produces sound. Place your fingers gently on your throat (the voice box or larynx) while speaking — you can feel the vibrations.

Conclude: Sound is produced by a vibrating body. When the vibration stops, the sound also stops.

Main parts of the human ear:

1. Outer ear (pinna) – collects sound and directs it into the ear canal.
2. Ear canal – carries sound to the eardrum.
3. Eardrum (tympanic membrane) – a thin, stretched membrane.
4. Middle ear – contains three tiny bones that amplify vibrations.
5. Inner ear (cochlea) – converts vibrations into signals sent to the brain.

Function of the eardrum:

• The eardrum is a thin, stretched membrane like the skin of a drum.
• When sound reaches it, the eardrum vibrates.
• It passes these vibrations to the inner ear, from where signals go to the brain, allowing us to hear.

Frequency: The number of oscillations (vibrations) made by a vibrating body in one second.

Time period: The time taken to complete one oscillation.

Relation: $\text{Frequency} = \frac{1}{\text{Time period}}$

That is, $f = \dfrac{1}{T}$.

SI unit of frequency: hertz (Hz).

$1\ \text{Hz} = 1$ oscillation per second.

Example: If a pendulum takes $0.5\ \text{s}$ for one oscillation, then $f = \frac{1}{0.5} = 2\ \text{Hz}$

Given:

• Number of oscillations $= 40$
• Total time $= 8\ \text{s}$

Frequency: $f = \frac{\text{number of oscillations}}{\text{total time}} = \frac{40}{8} = 5\ \text{Hz}$

Time period: $T = \frac{1}{f} = \frac{1}{5} = 0.2\ \text{s}$

Answer: Frequency $= 5\ \text{Hz}$ and time period $= 0.2\ \text{s}$.

Loudness:

• Depends on the amplitude of vibration (how large the vibration is).
• Larger amplitude → louder sound.
• Measured in decibels (dB).

Pitch:

• Depends on the frequency of vibration (how fast the vibration is).
• Higher frequency → higher (shriller) pitch.
• Lower frequency → lower pitch.

Example:

• A gentle tap on a drum is soft (low loudness); a hard hit is loud (high loudness).
• A whistle produces a high-pitched sound, while a lion's roar is low-pitched.

Conclude: Loudness is 'how strong' the sound is (amplitude), while pitch is 'how high or low' the sound is (frequency).

Understand: Sound needs a material medium (solid, liquid, or gas) to travel because it moves as vibrations of particles.

Analyze:

• In a vacuum, there are no particles to vibrate and pass on the sound.
• Without particles, the vibrations cannot be transmitted.

Evidence: An electric bell ringing inside a sealed glass jar becomes fainter and finally inaudible as air is pumped out, even though we can still see it striking.

Conclude: No, sound cannot travel through vacuum because it requires a medium to propagate.

Order of speed: Sound travels fastest in solids, slower in liquids, and slowest in gases.

$v_{\text{solid}} > v_{\text{liquid}} > v_{\text{gas}}$

Reason:

• Sound travels by vibrations passing from one particle to the next.
• In solids, particles are packed very closely, so vibrations pass on quickly.
• In liquids, particles are a bit farther apart, so it is slower.
• In gases, particles are far apart, so sound is slowest.

Example: Placing your ear on a railway track lets you hear an approaching train much earlier than through air.

Audible range for humans: From about 20 Hz to 20,000 Hz (20 kHz).

Infrasound (infrasonic sound):

• Sound of frequency below 20 Hz.
• Humans cannot hear it.
• Example: Sounds produced by whales and elephants for communication; rhinoceroses also use infrasound.

Ultrasound (ultrasonic sound):

• Sound of frequency above 20,000 Hz.
• Humans cannot hear it.
• Example: Sounds used by bats and dolphins; also used in medical ultrasound scans.

Noise pollution: The presence of excessive or unwanted (unpleasant) sound in the environment is called noise pollution.

Two harmful effects:

1. It can cause loss of hearing or damage to the ears.
2. It leads to problems like lack of sleep, headache, high blood pressure, anxiety and stress.

Two ways to reduce it:

1. Plant more trees along roads and around buildings, as they absorb sound.
2. Keep noisy machines and factories away from residential areas; avoid unnecessary honking and use silencing devices.

Conclude: Controlling noise pollution keeps the environment healthy and pleasant.

Musical sound:

• A sound that is pleasant to the ear.
• Produced by regular, smooth vibrations.
• Example: Sound of a flute, piano, sitar, or a tuning fork.

Noise:

• A sound that is unpleasant to the ear.
• Produced by irregular vibrations.
• Example: Sound of vehicles honking, machines running, a crowd shouting.

Conclude: Both are sounds, but musical sound is pleasant while noise is unpleasant and can cause discomfort. A pleasant musical sound can even become noise if it is too loud.

Amplitude: The maximum displacement of a vibrating body from its mean (rest) position.

Simple diagram (described):

        crest
         /\           <- amplitude = height from centre line to top
        /  \
-------/----\-------  <- mean (rest) position
      /      \
     \/       
     trough

• The distance from the centre line to the top of the crest is the amplitude.

Relation to loudness:

• Loudness depends directly on amplitude.
• Larger amplitude → louder sound
• Smaller amplitude → softer sound

Example: Hitting a drum harder increases the amplitude of its vibrations, making the sound louder.

Understand: Lightning and thunder are produced at the same moment during a storm.

Analyze:

• Light travels extremely fast (about $3 \times 10^8\ \text{m/s}$), so we see the lightning flash almost instantly.
• Sound travels much slower (about $340\ \text{m/s}$ in air), so the sound of thunder takes time to reach us.

Conclude: Because light reaches us much faster than sound, we see the lightning first and hear the thunder a little later. The greater the distance, the longer the delay.