Sound

Sound can travel through wood, water and air but cannot through vacuum. Sound needs a medium to travel, but in vacuum there is no medium.

Note: When air is completely removed from a vessel, it is said that there is a vacuum in the vessel.

The loudness of a sound depends upon the amplitude.

⇒ When the amplitude of the vibration is large, the sound produced is loud.

⇒ When the amplitude is small, the sound produces is feeble and soft.

⇒ Thus, loudness of sound depends upon the amplitude.

Option (iii) is incorrect. As light do not need any medium for propagation. It can also travel in vacuum too. On the other hand, sound needs a medium for propagation. All other options are correct.

Given: Frequency of object = 50 hertz

Time period =?

As we know that,

⇒

⇒

⇒ Time period = 0.02 s

Thus, the time period is 0.02s

Note: The number of oscillations per second is called the frequency.

The loudness of a sound depends upon the amplitude of vibration. Hence, in order to reduce the loudness of a sound, we have to decrease its amplitude of vibration. Lesser the amplitude, lesser will be the sound produced.

The loudness of the sound is expressed in decibel (dB). The loudness sound of normal breathing is 20 dB, soft whisper is 30 dB, normal conversation 60 dB. Above 80 dB the noise becomes physically painful.

Loudness of the sound is determined by its amplitude of vibration. For example, if the amplitude becomes, the loudness increases by a factor of 4. That’s how we can determine the loudness of sound by its amplitude.

Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1Hz is one oscillation per second. This means:

1 hertz = 1 oscillation per second = 60 vibrations per minute

∵ (1 second = 60 minutes)

The frequency determined the pitch of a sound.

⇒ If the frequency of vibration is higher, this means the sound has a higher pitch.

⇒ If the frequency of vibration is lower, this means the sound has a lower pitch.

⇒ Thus, pitch of a sound is determined by its frequency

Ultrasound has frequency of vibration higher than 20,000Hz. Some animals like dogs can hear sounds of frequencies higher than 20,000 Hz. Thus, dogs are used in investigation process.

Given: Time period = 5s

Speed of sound = 330 m/s

To calculate distance, we will apply the formula given below:

Distance = Speed × time

⇒ Distance = 330 m/s × 5s

⇒ Distance = 1650 m

⇒ Distance = 1.65 km (∵ 1km = 1000m)

Thus, she is standing at 1.65 km.

The vocal cords in our throat vibrate when we speak.

⇒ In human, the sound is produced by the voice box (larynx)

⇒ The voice box is at the upper end of the windpipe.

⇒ Two vocal cords are stretched across the voice box.

⇒ When the lungs force air through the slit, the vocal cords vibrate and produce sound.

Boojho heard the sound of cracker after seeing the cracker because:

⇒ The speed of light is faster than sound.

Speed of sound in air = 330 m/s

Speed of light in air = 3 × 10⁸ m/s

⇒ Thus, Boojho saw burst of cracker (light) first, then after some times, he heard the sound.

When we ear a sound, our eardrums vibrate:

⇒ When sound enters in the ear, it travels down a canal at the end of which a thin membrane is stretched tightly.

⇒ The thin membrane is called the eardrum.

⇒ Sound vibrations make eardrum vibrate.

⇒ The eardrums send these vibrations to the inner ear. From there, the signal goes to the brain.

⇒ That’s how we hear a sound.

Guitar and Sitar are two musical instruments which produce sound by vibrating strings.

The guitar and sitar produce sound when their strings are vibrated.

Given: No. of oscillations = 10

Time = 20s

First we will calculate the frequency by applying the formula:

⇒

⇒ Frequency = 0.5 Hz

Thus, the frequency of its oscillation is 0.5 Hz

Now, we will calculate the time period:

⇒

⇒ Time period = 2s

Thus, the time period is 2s.

We need a vibrating body for the production of sound. But we cannot hear the sound of all vibrating bodies.

⇒ Range of frequency for every vibrating body is different.

⇒ The sounds of frequencies less than about 20 vibrations per second cannot be heard by us. Such sounds are called inaudible sounds.

⇒ The sounds of frequencies higher than about 20,000 vibrations per second are also cannot be heard by us.

⇒ The range of frequency which is audible to us is 20 Hz – 20,000 Hz. This range is called audible range.

Yes, the frying pan will vibrate. No we will not be able to hear the sound because:

⇒ The vibrations need a medium to travel.

⇒ In vacuum, sound cannot travel and there is no medium by which vibrations (sound) can produced.

⇒ Thus, we will not be able to hear the sound.

No, the two astronauts cannot talk to each other without using any special device because:

⇒ In space, there is no medium for sound to travel.

⇒ Thus, the astronauts cannot talk to each other without using any special device.

Presence of excessive or unwanted sounds in the environment is called noise pollution.

The three sources of noise pollution in our locality are:

⇒ High volumes of televisions and transistors.

⇒ Bursting of crackers.

⇒ Loudspeakers in parties.

⇒ Horns pf vehicles.

⇒ Generators without silencers.

⇒ Bikes without silencers. (Any three)

In the first case in which the string is plucked in the middle produce a louder sound because:

⇒ The loudness of a sound depends upon the amplitude of vibration.

⇒ Greater force means greater amplitude.

⇒ This means, when the string is plucked with greater force, the amplitude of a string is larger.

⇒ Thus, the sound will be louder in this case.

Sound is produced by vibrating any object by any means.

⇒ In short vibrating object produces sound.

⇒ These vibrations travel through their medium and finally reached to the ear.

⇒ These vibrations enter in the ear and travels down a canal at the end of thin membrane called eardrum.

⇒ The eardrum performs an important function.

⇒ Sound vibrations make the eardrum vibrate.

⇒ The eardrum sends vibrations to the inner ear.

⇒ From there, the signal goes to the brain.

⇒ That’s how, we hear sound.

First when the alarm bell is kept inside a vessel, the person standing near it can hear the sound coming from air and water separately.

But when the air is removed from the bottle, this means there is vacuum in the bottle. As we know that sound cannot travel through vacuum and as a result, the person cannot hear the sound of alarm.

Boojho has noticed that the sound of the clock appears to be much clearer at night because:

⇒ During the day, there are number of vibrations travel in the surroundings.

⇒ So the sound coming from the alarm gets distracted and amplitude of vibrations becomes small.

⇒ As a result, the loudness of sound become slow.

⇒ During the night, the noise level is quite low. There are no multiple vibrations travel in the surroundings.

⇒ Thus, the sound becomes clear and loud.

Three measures to limit noise pollution in the city are:

⇒ Silencing devices must be installed in air craft engines, vehicles, machines etc.

⇒ Use of horns must be minimized.

⇒ T.V. and music system must be run at low volumes.

⇒ The industries which produce excessive noise should be set up far away residential areas.

⇒ Trees must be planted along the roads and around the buildings.

(Any three)