Chapter 7 – Sound | Chapter Solution Class 9

Sound is a form of energy that produces the sensation of hearing. It is produced due to vibrations of a sounding body and travels through a medium as a wave.

A wave transfers energy from one point to another without the physical transport of the material through which it propagates.

A progressive wave is a disturbance that moves through a medium, transferring energy from one point to another without any physical transport of the material.

A transverse wave is a wave in which the particles of the medium vibrate perpendicular to the direction of wave propagation. Light waves and waves on a rope are examples of transverse waves.

A longitudinal wave is a wave in which the particles of the medium vibrate parallel to the direction of wave propagation. Sound waves in air are examples of longitudinal waves.

Compression: The region of a longitudinal wave where the particles are close together, resulting in high pressure and high density.

Rarefaction: The region of a longitudinal wave where the particles are spread apart, resulting in low pressure and low density.

When sound travels through a medium, the particles of the medium vibrate back and forth, transferring energy while remaining in their mean positions.

Wavelength is the shortest distance between two points in a wave that are in the same phase, such as two consecutive compressions or two consecutive rarefactions.

The SI unit of wavelength is the meter (m).

Frequency is the number of complete vibrations made by a particle in one second. It is measured in hertz (Hz).

The SI unit of frequency is hertz (Hz).

Pitch is the characteristic of sound that distinguishes a shrill or high-frequency sound from a flat or low-frequency sound. Higher frequency corresponds to a higher pitch.

Loudness is the property of sound that helps distinguish a loud sound from a faint one, both having the same pitch. It depends on the amplitude of vibrations and is measured in decibels (dB).

Quality or timbre of a sound is the characteristic that distinguishes two sounds of the same loudness and pitch but produced by different instruments or sources.

Intensity of sound is the amount of sound energy transmitted per unit area per second in a direction perpendicular to the wave propagation. It is measured in watts per square meter (W/m²).

Echo is the phenomenon in which a sound wave is reflected from a surface and heard again after a time interval.

The minimum distance required between the source of sound and the obstacle to hear an echo is 17 meters.

The principle of echo is utilized in SONAR (Sound Navigation and Ranging) to measure the depth of the sea and locate underwater objects.

Reverberation is the persistence of sound due to multiple reflections from surfaces, making the sound last for a longer duration in an enclosed space.

Reverberation time is the time taken for the sound to decay by 60 dB after the source has stopped producing sound.

The audible range for humans is 20 Hz to 20,000 Hz.

Ultrasonic waves are sound waves with frequencies greater than 20,000 Hz, which are inaudible to the human ear but can be detected by animals like bats.

SONAR stands for Sound Navigation and Ranging.

SONAR works on the principle of reflection of sound waves (echo). A sound wave is sent underwater, and the time taken for the echo to return is used to determine the depth or locate objects.

The two important uses of ultrasonics for medical purpose:

1. Ultrasonography – Used for imaging internal organs and detecting abnormalities in unborn babies.
2. Echocardiography – Used to obtain images of the heart.

Pitch depends on the frequency of the sound wave. Higher frequency results in a higher pitch.

Time period is the time required for a particle in the medium to complete one vibration. It is denoted by T and is measured in seconds (s).

An echo cannot be heard in a small room because the minimum required distance for the reflected sound to be heard as an echo is 17 meters, which is usually not available in a small room. Instead, the reflected sound merges with the original sound, causing reverberation.

A vibrating body produces sound when the vibrations are rapid enough to be detected by the human ear. A simple pendulum oscillates at a very low frequency, which is below the audible range of human hearing (20 Hz to 20,000 Hz). Since the frequency of the pendulum’s oscillation is too low, it does not produce audible sound.

The experiment with the electric bell in a vacuum demonstrates that sound cannot travel through a vacuum:

• An electric bell is placed inside a bell jar, which is connected to a vacuum pump.
• When the bell is rung, the sound is clearly heard.
• As the air inside the bell jar is pumped out, the loudness of the sound gradually decreases.
• When a near-vacuum is created, the sound almost disappears, proving that sound needs a medium (like air) to travel.

The speed of sound (V) is related to its wavelength (λ) and frequency (n) by the equation:

$$V=n\lambda$$

Where:

• V = speed of sound (m/s)
• n = frequency (Hz)
• λ = wavelength (m)

This equation shows that the speed of sound depends on both frequency and wavelength. If the frequency increases, the wavelength decreases for a constant speed of sound.

The human ear consists of three main parts:

Outer Ear:

• Pinna: Collects sound waves and directs them to the auditory canal.
• Auditory Canal: Passes sound waves to the eardrum.

Middle Ear:

• Eardrum (Tympanic Membrane): Vibrates when sound waves strike it.
• Ossicles (Hammer, Anvil, Stirrup): Amplify vibrations and pass them to the inner ear.

Inner Ear:

• Cochlea: Converts vibrations into electrical signals.
• Auditory Nerve: Sends signals to the brain for interpretation.

The speed of sound depends on:

1. Medium of propagation – Sound travels fastest in solids, slower in liquids, and slowest in gases.
2. Temperature – Sound speed increases with temperature.
3. Density of the medium – In general, a denser medium slows down sound.
4. Elasticity of the medium – A more elastic medium allows sound to travel faster.

Ultrasound refers to sound waves with frequencies greater than 20,000 Hz, which are inaudible to the human ear.

Detection of defects in metal blocks using ultrasound:

• Ultrasonic waves are passed through a metal block.
• If the block is defect-free, the waves pass through uniformly.
• If there is a crack or defect, the waves get reflected back and are detected using a sensor.
• The location of the defect is identified based on the time delay of the reflected waves.

The two laws of reflection of sound are:

1. The angle of incidence (∠i) is equal to the angle of reflection (∠r).
2. The incident sound wave, the reflected sound wave, and the normal to the reflecting surface all lie in the same plane.

Avoidance of excessive reverberation by using sound-absorbing materials on walls and ceilings.

Proper placement of speakers and reflectors to ensure even sound distribution.

Designing curved ceilings or walls to direct sound waves efficiently towards the audience.

The two conditions for an echo to be heard are:

1. The minimum distance between the source and the reflecting surface should be at least 17 meters.
2. The reflecting surface should be large, rigid, and capable of reflecting sound clearly.

The characteristic that makes different musical instruments recognizable even when played together is quality (timbre) of sound.

Each instrument produces a different waveform due to the presence of harmonics and overtones, making its sound unique.

V = velocity of the wave = 3 × 10⁸ m/s

λ = wavelength = 300 m

n = V / λ

n = (3 × 10⁸) / 300

n = 10⁶ Hz

∴ Frequency = 1000000 Hz

velocity of sound in air (V) = 340 m/s

total time for the echo to return (t) = 0.5 s

V = \frac{2d}{t}

or, 340 = \frac{2d}{0.5}

or, 2d = 340 × 0.5

or, d = \frac{170}{2} = 85 m

V = 340 m/s

λ = 1.7 m

n = V / λ

n = 340 / 1.7

n = 200 Hz

∴ Frequency = 200 Hz

Velocity (V) = 344 m/s

frequency (n) = 100 kHz = 100 × 10³ Hz

λ = V / n

λ = 344 / (100 × 10³)

λ = 3.44 × 10⁻³ m

Since 1 meter = 1000 mm,

λ = 3.44 mm

∴ Wavelength = 3.44 mm

For minimum wavelength (maximum frequency):

λ_min = V / n_max

λ_min = 340 / 20000

λ_min = 0.017 m

For maximum wavelength (minimum frequency):

λ_max = V / n_min

λ_max = 340 / 20

λ_max = 17 m

Range of wavelengths = 0.017 m to 17 m

(a) Frequency:

n = Number of waves / Time

n = 1500 / 3

n = 500 Hz

(b) Wavelength:

Distance covered by a compression and an adjacent rarefaction is half of the wavelength.

λ = 2 × 68 cm

λ = 2 × 0.68 m

λ = 1.36 m

(c) Velocity:

V = n × λ

V = 500 × 1.36

V = 680 m/s

(a) Frequency = 500 Hz,

(b) Wavelength = 1.36 m,

(c) Velocity = 680 m/s

(a) Wavelength:

λ = 2 × Distance between a compression and rarefaction

λ = 2 × 50 cm = 100 cm = 1 m

(b) Frequency:

n = Total waves / Time

n = (20 + 20) / 0.2 = 100 Hz

(c) Time-period:

T = 1 / n

T = 1 / 100 = 0.01 s

(i) Velocity of the pulse:

V = Distance / Time

V = 8 / 0.5 = 16 m/s

(ii) Wavelength:

Using V = n × λ,

λ = V / n

λ = 16 / 200 = 0.08 m = 8 cm

frequency (n) = 40Hz

wavelength (λ) = 8 m

V = n × λ

V = 40 × 8

V = 320 m/s

Velocity (V) = 340 ms⁻¹

frequency (f) = 20 Hz

Using formula: λ = V / n

λ = 340 / 20

λ = 17 m

∴ Wavelength = 17 m

Energy

Explanation:

Sound is a form of mechanical energy that is produced by the vibration of an object and transmitted in a medium as a wave. It requires a medium to propagate, such as air, water, or solids.

Material

Explanation:

Sound requires a material medium (solid, liquid, or gas) for its propagation because it is a mechanical wave. It cannot travel through a vacuum.

Amplitude

Explanation:

Amplitude refers to the maximum displacement of a vibrating particle from its equilibrium position. It determines the loudness of a sound; greater amplitude results in a louder sound.

Transverse

Explanation:

Light is an electromagnetic wave and moves as a transverse wave, where the vibrations of the wave are perpendicular to the direction of propagation. Unlike sound, light can travel through a vacuum.

Longitudinal

Explanation:

In gases, sound propagates as a longitudinal wave, where the particles of the medium vibrate parallel to the direction of wave propagation, forming compressions and rarefactions.

Energy

Explanation:

A wave does not transport matter; it only transfers energy from one point to another. This energy is transmitted through vibrations of particles in a medium.

Hertz

Explanation:

Frequency is the number of oscillations or vibrations per second and is measured in hertz (Hz). One hertz is equal to one cycle per second.

17

Explanation:

For an echo to be distinctly heard, the reflected sound must take at least 1/10th of a second to return. Given that the speed of sound in air is approximately 340 m/s, the minimum distance required between the source and the reflector is 17 meters.

20000

Explanation:

Ultrasonic waves are sound waves with frequencies higher than 20,000 Hz, which are beyond the range of human hearing. These waves are used in medical imaging, industrial applications, and navigation.

The three main characteristics of musical sound are:

• Loudness (depends on amplitude)
• Pitch (depends on frequency)
• Quality or Timbre (depends on the waveform of the sound and helps distinguish different musical instruments playing the same note)

False

Explanation: Sound is a mechanical wave that requires a medium (solid, liquid, or gas) for propagation. Since a vacuum lacks particles to vibrate, sound cannot travel through it.

True

Explanation: Sound is generated when an object vibrates, creating compressions and rarefactions in a medium, which our ears detect as sound. Examples include a tuning fork or a plucked guitar string.

False

Explanation: Sound is a mechanical wave and needs a medium (air, water, or solids) to travel. In contrast, electromagnetic waves like light do not require a medium.

True

Explanation: Pitch is directly related to frequency. Higher frequency produces a higher pitch (shrill sound), while lower frequency produces a lower pitch (deep sound).

True

Explanation: Amplitude is the measure of how far a vibrating particle moves from its rest position. A larger amplitude results in a louder sound.

False

Explanation: Sound waves in air and fluids are longitudinal waves, where the vibrations of particles are parallel to the direction of wave travel. Transverse waves occur in solids and electromagnetic radiation, like light.

False

Explanation: This is the same as question (i). Sound cannot travel through a vacuum because there are no particles to transmit the wave.

True

Explanation: A speaking tube directs sound waves efficiently by reflecting them inside the tube walls, allowing clear communication over long distances.

True

Explanation: The human ear can detect sound frequencies between 20 Hz and 20,000 Hz. Sounds below 20 Hz are infrasonic, and those above 20,000 Hz are ultrasonic.

False

Explanation: The pinna is the outer part of the ear, which collects and directs sound into the auditory canal. The inner ear consists of structures like the cochlea and semicircular canals, responsible for converting sound into nerve signals.

(b) energy

Explanation: Wave motion does not transfer matter; it only transfers energy from one point to another through vibrations in a medium. Sound, water, and light waves all exhibit this property.

(c) λ

Explanation: A wavelength (λ) is defined as the distance between two successive compressions or two successive rarefactions in a longitudinal wave

(d) in all (solid, liquid, gas)

Explanation: Sound waves require a material medium for propagation. They can travel through solids, liquids, and gases, with the speed being fastest in solids, slower in liquids, and slowest in gases.

(d) increases with the increase in temperature

Explanation: As temperature increases, the molecular motion increases, allowing sound to travel faster. Therefore, the speed of sound increases with temperature.

(b) longitudinal in nature

Explanation: Sound waves in air and fluids are longitudinal waves, where the particles of the medium vibrate parallel to the direction of wave propagation.

(b) λ/2

Explanation: The distance between one compression and the next adjacent rarefaction is half of a wavelength (λ/2) in a longitudinal wave.

(b) sound wave

Explanation: A mechanical wave requires a medium to propagate. Sound waves need a medium like air, water, or solids to travel, making them mechanical waves. Light, X-rays, and ultraviolet rays are electromagnetic waves that can travel through a vacuum.

(d) iron

Explanation: Sound travels fastest in solids due to the higher elasticity and density of solid particles. Among the given options, iron is a solid and has the highest speed of sound propagation.

(d) ultrasonic

Explanation: SONAR (Sound Navigation and Ranging) uses ultrasonic waves (sound waves with frequencies above 20,000 Hz) to detect objects underwater, such as submarines and ocean depths.

(b) rhinoceros

Explanation: Infrasound refers to sound waves with frequencies below 20 Hz, which are inaudible to humans. Animals like rhinoceroses and elephants can detect infrasound, helping them communicate over long distances.