Sound Waves (Sound): NCERT Solutions (2026)

When an object vibrates, it sets the particles of the medium around it vibrating. These particles do not travel all the way from the vibrating object to the ear. Instead, a particle of the medium in contact with the vibrating object is displaced from its equilibrium position. It exerts a force on the adjacent particle, displacing it, and then returns to its original position. This process continues in the medium creating a series of compressions (high pressure) and rarefactions (low pressure) until the sound wave reaches our ear.

A sound wave is called a mechanical wave because it absolutely requires a material medium (like air, water, or steel) for its propagation. It cannot travel through a vacuum because there are no particles to compress and expand to carry the energy forward.

No, we will not be able to hear any sound on the moon. The moon has no atmosphere (it is a vacuum). Since sound is a mechanical wave that requires a material medium to travel, it cannot propagate in a vacuum.

• (a) Loudness is determined by the Amplitude of the sound wave. Greater the amplitude, louder the sound.
• (b) Pitch is determined by the Frequency of the sound wave. Higher the frequency, higher the pitch.

A guitar generally has a higher pitch than a car horn because the strings of a guitar vibrate at a higher frequency compared to the heavy diaphragm of a car horn.

• Wavelength: The distance between two consecutive compressions (C) or two consecutive rarefactions (R).
• Frequency: The number of complete waves (or oscillations) produced in one second.
• Time Period: The time taken to complete one full oscillation or cycle.
• Amplitude: The maximum displacement of the medium particles from their original undisturbed position.

The speed ($v$) of a sound wave is equal to the product of its wavelength ($\lambda$) and its frequency ($\nu$).

Formula: $$v = \lambda \times \nu$$

Given:
Frequency ($\nu$) = $220 \text{ Hz}$
Speed of sound ($v$) = $440 \text{ m/s}$

Formula: $v = \lambda \times \nu$

So, Wavelength ($\lambda$) = $v / \nu$
$$\lambda = 440 / 220 = 2 \text{ meters.}$$

The time interval between successive compressions is exactly equal to the Time Period ($T$) of the wave.

Given: Frequency ($\nu$) = $500 \text{ Hz}$.

Formula: $T = 1 / \nu$
$$T = 1 / 500 = 0.002 \text{ seconds.}$$
(Note: The distance of 450 m is extra information and is not needed for this calculation).

• Intensity: It is the objective measure of sound energy passing through a unit area per second. It is a physical quantity measured in $\text{Watt/m}^2$.
• Loudness: It is a subjective measure of the response of the ear to the sound. Even if two sounds have the same intensity, we may hear one as louder than the other because our ears are more sensitive to it.

Sound travels the fastest in iron (solid). The speed of sound is maximum in solids, lesser in liquids (water), and least in gases (air).

Given:
Speed of sound ($v$) = $342 \text{ m/s}$
Total time for echo ($t$) = $3 \text{ s}$

In an echo, the sound travels the distance twice (to the obstacle and back).
Formula: $2d = v \times t$
$$2d = 342 \times 3$$
$$2d = 1026$$
$$d = 1026 / 2 = 513 \text{ meters.}$$

The reflecting surface is 513 m away.

The ceilings of concert halls and cinema halls are curved so that sound, after reflection from the ceiling, reaches all corners of the hall evenly, ensuring that every person in the audience hears the sound clearly.

The audible range of an average human ear is from 20 Hz to 20,000 Hz (or 20 kHz).

SONAR stands for Sound Navigation And Ranging. It is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects like submarines, sunken ships, or the depth of the sea.

Answer: (c) Above 20,000 Hz (Difficulty: Easy)

Answer: (a) - A sound of a single frequency is called a tone, whereas a mixture of frequencies is a note. (Difficulty: Medium)

Answer: (a) Ultrasonic