Sound

An echo is the sound produced by the reflection of sound waves from a surface and back to the listener.

The repeated reflection of sound that causes its persistence until it is reduced to an inaudible value is called reverberation.

SONAR stands for Sound Navigation And Ranging. It is a device used to measure the distance, direction and speed of underwater.

The audible range of a normal human being is 20 Hz to 20 KHz. But a vibrating pendulum does not produce sound in this range. So, we are not able to hear this.

The audible range of a normal human being is 20 Hz to 20 KHz.

The waves that require a medium to propagate are called mechanical waves. They require a medium to transport energy from one point to another. Sound waves are characterised by their motion in a medium. They cannot travel in vacuum(without any medium). So, they are called mechanical waves.

The audience in a concert hall should hear the sound clearly. Thus, the ceilings of a concert hall are made curved. It is done so that the sound after reflection reaches every corner of the concert hall and the audience can listen to it clearly.

In a longitudinal wave, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles oscillate back and forth on their position of rest. A sound wave propagate in the same way. So, they are called longitudinal waves.

A sound persists in our ear for 0.1 s. At temperature 22℃ in air and speed of sound 344 ms^-1, the total distance covered by sound between the point of generation and reflecting surface should be at least (344 m/s)x0.1 s = 34.4 m. Thus, the minimum distance between the point of generation and reflecting surface should be half of it i.e. 17.2 m. So, we cannot hear an echo in a small room as the minimum distance for the echo will not be satisfied. But we can hear an echo in a long gallery and big halls.

A sound persists in our ear for 0.1 s. At temperature 22℃ in air and speed of sound 344 ms^-1, the total distance covered by sound from the point of generation and reflecting surface and back should be at least (344 m/s)x0.1 s = 34.4 m. Thus, the minimum distance between point of generation and reflecting surface should be half of it i.e. 17.2 m. So, in a long gallery and big halls, we can hear the echo due to the distance between the point of generation and reflecting surface more than the minimum distance required in given conditions.

Transverse wave: In a transverse wave, particles oscillate up and down about their mean position in a direction perpendicular to the direction of propagation of wave. e.g. light waves

Longitudinal wave: In a longitudinal wave, the individual particles of the medium move in a direction parallel to the direction of propagation of the wave. The particles oscillate back and forth about their position of rest. e.g. Sound wave

Crest: It is the highest point the medium rises to while propagation of wave.

Trough: It is the lowest point a medium falls to while propagation of wave.

Infrasonic vibrations: Their frequency is below 20 Hz.

Ultrasonic vibrations: Their frequency is above 20 KHz.

Compressions: The particles are closest to each other in this. The density of the medium is maximum at compression.

Rarefaction: The particles are farthest from each other. The density of the medium is minimum for rarefaction.

Loudness defines how loud a sound is to a listener.

Intensity is a measure of the energy contained in a wave. Its unit is decibel(dB).

Working of human ear:

The outer ear is called ‘pinna’. It collects the sound from the surroundings. The auditory canal passes this sound to a thin membrane called the eardrum or tymphanic membrane.

When a compression or rarefaction of the medium reaches the eardrum, it moves inward or outward. In this way, the eardrum vibrates. These vibrations are amplified several times by three bones- the hammer, anvil and stirrup in the middle ear. The middle ear transmits the amplified pressure variations to the inner ear. The cochlea converts these variations into the electrical signals in inner ear. These signals are sent to the brain via the auditory nerve. The brain interprets them as sound.

● Take an electric bell and an airtight bell jar.

● Suspend the bell inside the bell jar.

● Connect a vacuum pump to the bell jar as shown in the figure.

● Press the switch to hear the bell.

● Now, start the vacuum pump.

● When the air in the jar is pumped out gradually, the sound becomes fainter even though the same current is passing through the bell.

● As the amount of air decreases inside the bell jar, the sound becomes fainter.

● When there is no air inside the jar, there is no sound of the bell.

This shows that sound requires a medium to propagate.

Given,

distance travelled = 840 m

time taken = 2.5 sec

length of wave = 70 cm = 0.7 m

Given,

time for echo = 0.8 sec

height of building = 132.8 m

distance travelled by sound = 2×height

distance travelled = 2×132.8 = 265.6 m

Given,

There are 40 crests and 40 troughs.

One crest and and one trough constitutes one cycle.

So, total number of cycles = 40

One cycle denotes one time period.

A sound persists in our ear for 0.1 s. At temperature 22℃ in air and speed of sound 344 ms^-1, the total distance covered by sound between the point of generation and reflecting surface should be at least (344 m/s) x 0.1 s = 34.4 m. Thus, the minimum distance between point of generation and reflecting surface should be half of it i.e. 17.2 m. So, we cannot hear echo in a smaller room of 10 m.

Distance between crest and trough is called its wavelength.

Given,

wavelength = 15 cm = 0.15 m

frequency = 1000 Hz

velocity = wavelength × frequency

velocity = 0.15×1000 = 150 ms^-1

Sound is a mechanical wave and requires medium to propagate. As the moon has no atmosphere, it has absence of air. So, there is no medium for sound to propagate. So, two astronauts cannot hear each other on the moon.

Dogs can hear sounds which are beyond our audible range. So, whenever they hear a sound which is inaudible to us, they bark in a response to it.

The speed of sound does not depend on the amplitude of the sound. So, both the sounds- the humming of bee and the roaring of a lion travel at the same speed but differ in loudness due to difference in amplitudes.

The speed of sound depends on the properties of a medium. It travels faster when the temperature is high as the particles vibrate more due to the heat energy provided to them.

Elephants are able to hear the infrasound range. This range lies below 20 Hz. So, they are able to hear sound below 20 Hz.

Energy is transferred from one place to another by the propagation of waves.

In a longitudinal wave, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles oscillate back and forth on their position of rest. Sound waves are longitudinal waves.

A slinky produces transverse waves when one end is fixed and the other end is stretched and given a jerk at right angle to its length.

It produces longitudinal waves when compressions are given at regular intervals of time at its free end.

The distance between consecutive crests is called one wavelength.

Hertz is a unit of frequency named after H.R. Hertz.

A rarefaction during the propagation of sound wave takes place when the density of the medium is minimum.

A compression during the propagation of sound wave takes place when the density of the medium is maximum.

The audible range for a normal human being is 20 Hz to 20 KHz.

Ultrasonic are used in SONAR, sibigraphy, CUSA but not radio waves.

As the moon has no atmosphere, there is absence of air. Sound cannot travel without medium because it is mechanical wave. So, it will never be able to reach earth due to vacuum between earth and moon in space.

The walls of a hall built for musical concert should be able to absorb sound and reduce reverberation.

Quality or timbre is the property of sound which distinguishes it from other sounds.

A larger bell has low resonant frequency and the sound is able to travel more distance and is louder.

As the moon has no atmosphere, there is absence of air. Sound cannot travel without medium because it is mechanical wave. So, an astronaut cannot hear his companion on the surface of the Moon.

The loudness of a sound is directly proportional to its amplitude.

Sound requires medium to propagate. So, it faints as the air is removed from jar and its intensity decreases.

False

Explanation:

Sound waves are longitudinal waves. They propagate when the particles of the medium oscillate back and forth about their position of rest and do not move from one place to another.

Light waves are transverse wave but the particles in oscillation are not of the medium as it is not a mechanical wave.

False

Explanation:

Distance between a crest and the next trough in a wave motion is 3λ/2.

True

Explanation:

Human ears can detect frequencies of 20 Hz to 20 kHz only.

True

Explanation:

Given,

Time period(T) = 0.01 sec

Frequency(f) = ?

False

Explanation:

The total distance covered by the sound from the point of generation and reflecting surface and back should be at least (344 m/s)x0.1 s = 34.4 m. Thus, the minimum distance between point of generation and reflecting surface should be half of it i.e. 17.2 m.

False

Explanation:

The speed of sound decreases as we go from solid to gaseous state. Thus, sound travels faster in water than in air.

True

Explanation:

The speed of sound depends on the properties of a medium. It travels faster at a higher temperature than at a lower temperature.

False

Explanation:

Only sound travels in a medium in the form of crests and troughs. Light travels in the form of wave.

True

Explanation:

Light is not a mechanical wave. They can travel without any medium.

True

Explanation:

Given,

Frequency(f) = 300 Hz

No. of vibration = 30

Distance travelled by sound = 34 m

After 30 vibrations,

Total time taken = No. of vib.×Time period

Distance travelled = speed âtime

Distance = 340×0.1 = 34 m

False

Explanation:

In a given medium, the speed of sound is almost same for all frequencies.

In a given medium, if the wavelength is same then the speed increases with frequency.

False

Explanation:

wavelength of ultrasound = 5 MHz

velocity of ultrasound = 1540 ms^-1

Then, for sound wave