Sound

Question (1) How does the sound produced by a vibrating object in a medium reach your ear?

Answer

Vibration is back and forth rapid motion of an object.

When a vibrating object goes forward, it creates the region of high pressure in the medium. This region of high pressure is called compression.

And when this vibrating object goes back it creates a region of low pressure in the medium. This region of low pressure is called rarefaction.

As vibrating object moves back and forth rapidly, it creates a series of compression and rarefaction in the medium, such as air.

Figure: Propagation of sound¹

This series of compression and rarefaction makes the sound wave reaches to our ear through medium.

Question (2) Explain how sound is produced by your school bell.

Answer

School bell or any bell produce sound after hitting it. School bell starts vibrating after hitting by a hammer.

Vibration means rapid back and forth motion of an object. This back and forth motion of bell creates series of compression and rarefaction in air. And because of these compression and rarefaction sound of bell propagates all around in air.

Thus, because of vibration school bell produces sound and series of compression and rarefaction in air particles propagates sound of bell all around.

Question (3) Why are sound waves are called mechanical waves?

Answer

Sound wave is produced because of vibration of an object. This vibration set the particles of medium into back and forth motion. This back and forth motion takes place because of compression and rarefaction of particles of medium which propagates the sound wave through the medium.

Thus, as sound wave propagates through medium because of mechanism of interaction between particles, hence sound wave is called mechanical wave.

All mechanical waves require medium to propagate. Since sound wave does not propagate in vacuum and hence clearly is called mechanical wave.

Question (4) Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer

Sound wave is mechanical wave. And mechanical wave needs medium to propagate.

We know that, there is no atmosphere on the surface of moon and hence there is no medium present in the atmosphere for the propagation of sound wave.

Thus, on moon we will not able hear the sound produce by one of our friend.

Question (5) Which wave property determines (a) Loudness, (b) Pitch?

Answer

(a) Loudness is determined by the amplitude of sound wave.

The magnitude of the maximum disturbance in the medium on either side of the mean value is called The Amplitude of the wave.

Figure: Wave of louder sound²

Figure: Wave of soft sound³

Loudness of sound increases with increase in amplitude. Sound having shorter amplitude is softer.

(b) Pitch of the sound is determined by the Frequency of sound wave.

The faster the vibration of the source, the higher is the frequency and the higher is the pitch.

Objects of different sizes and conditions vibrate at different frequencies to produce sound of different pitch.

Question (6) Guess which sound has a higher pitch: guitar or car horn?

Answer

It is the Frequency which determines the pitch of a sound wave. Higher is the frequency higher will be pitch of sound wave.

Since guitar has higher frequency than a car horn, thus sound of guitar has higher pitch than a car horn.

Question (7) What are wavelength, frequency, time period and amplitude of a sound wave?

Answer

Wavelength

Distance between two consecutive compressions or rarefaction or two peak of a wave is called Wavelength. Wavelength is represented by Greek Letter "λ (Lambda)".

The SI unit of wavelength is meter (m) .

Frequency

Frequency is the number of an event takes place in unit time.

When sound propagates through a medium, the density of the medium oscillates between a maximum value and a minimum value. The change in density from the maximum value to the minimum value, again to the maximum value, makes one complete oscillation.

The number of oscillations per unit time is called the Frequency of the sound wave.

The Frequency is generally denoted by Greek Letter "ν (Nu)". Sometimes frequency is denoted by English letter "f".

The SI unit of Frequency is hertz. Hertz is denoted by symbol "Hz".

Time Period

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the Time Period of the wave.

In other words, time taken for one complete oscillation in the density of the medium is called the Time Period of the Sound wave.

Time Period is denoted by English letter "T".

The SI unit of Time Period is second (s).

Amplitude

The magnitude of the maximum disturbance in the medium on either side of the mean value is called The Amplitude of the wave.

The amplitude is generally represented by letter "A".

The amplitude of sound wave is associated with loudness of sound. Higher will be the amplitude louder will be the sound and vice versa.

Question (8) How are the wavelength and frequency of a sound wave related to its speed?

Answer

Speed of sound is the product of wavelength and frequency of sound wave.

Or, Speed of sound = Wavelength × Frequency

Question (9) Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Solution

Given, Speed of sound (v) = 440 m/s

Frequency (ν) = 220 Hz

Thus, Wavelength (λ) =?

We know that, Speed of sound (v) = Wavelength (λ) × Frequency (ν)

⇒ 440 m/s = λ × 220 Hz

⇒ λ = 440/220 = 2 m

Thus, wavelength of the given sound wave = 2 m Answer

Question (10) A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Solution

Given, Frequency (ν) = 500 Hz

Distance (d) = 450 m

Thus, time interval or Time period (T) = ?

We know that, Frequency (ν) = 1/Time period (T)

⇒ 500 Hz = 1/T

⇒ T = 1/500 Hz = 0.002 s

Thus, time interval of given sound wave between successive compression from the source = 0.002 s Answer

Question (11) Distinguish between loudness and intensity of sound.

Solution

The amount of sound energy passing each second through unit area is called the Intensity of Sound. While Loudness is a measure of the response of the ear to the sound.

Loudness is associated with amplitude of sound wave while intensity is associated with frequency.

Sometimes "Loudness" and "Intensity" are used in the same sense, but they are not same.

Even when two sounds are of equal intensity, one may hear one as louder than the other simply because our ear detects it better.

Question (12) In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Answer

Speed of sound is fastest in solid and slowest in air.

Thus, at particular temperature among air, water or iron, sound travel the fastest in iron.

Question (13) An echo returned in 3s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 ms^–1?

Solution

Given, time (t) = 3s

Speed of sound (v) = 342 m s^–1

Thus, distance of reflecting surface = ?

We know that, distance = speed × time

⇒ Distance = 342 m s^–1 × 3 s

⇒ Distance = 1026 m

Thus, distance travelled by sound in given time of 3 s = 1026

Now, since sound has to travel two ways, i.e. from source to reflecting surface and from reflecting surface to listener.

Thus, distance of reflecting surface = distance travelled by sound / 2

⇒ Distance of reflecting surface = 1026/2 = 513 m

Thus, distance of reflecting surface from the source = 513 m Answer

Question (14) Why are the ceiling of concert halls curved?

Answer

Law of reflection is applied to sound wave also. This means sound wave is reflected after falling to a surface.

To use the reflecting nature of sound waves, ceiling of concert halls are made curved.

The curved ceiling or floor sends the sound to all direction after reflection. Going in all direction after reflection from curved ceiling of concert hall reaches properly to audiences sitting in hall and facilitate to hear clear sound.

Some time curved sound board is also used installed behind the stage for reflection of sound.

Question (15) What is audible range of the average human ear?

Answer Range of frequency of sound which the average human ear can hear is called audible range or hearing range of sound.

The audible range for average human ear is 20 Hz to 20000 Hz frequency of sound wave.

Question (16) What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?

Answer

(a) Infrasound

The sound wave of frequency below 20 Hz is called Infrasound or Infrasonic sound.

Human cannot hear the infrasound.

(b) Ultrasound

Sound waves having frequency above the 20000 Hz are called Ultrasound or Ultrasonic sound.

Human cannot hear the ultrasound. This means ultrasound is beyond the range of hearing or audible range.

Question (17) A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?

Solution

Given, Time taken to return the sound wave from the cliff = 1.02 s

Speed of sound in salt water = 1531 m/s

Distance of given underwater cliff = ?

We know that,

Speed = Distance × time

⇒ Distance = 1531 m/s × 1.02 s

⇒ Distance = 1561.62 m

Now, since SONAR pulse have to travel two ways, i.e. from submarine to cliff and from cliff to submarine.

Thus, distance of cliff = Distance travelled by SONAR pulse / 2

⇒ Distance of cliff = 1561.62/2 = 780.81 m

Thus, cliff is 780.81 m far Answer