The Music of Sound – Maharashtra Board Class 9 Solutions for Science and Technology

The Music of Sound – Maharashtra Board Class 9 Solutions for Science and Technology (English Medium)

AlgebraGeometryScience and TechnologyHindi

Solution 1:

1. Sound waves need a material medium for propagation.
2. Technique of echocardiography is based on reflecting property of ultrasonic waves.
3. Sound produced by vibration of simple pendulum is infra sound.
4. For determination of the depth of the sea SONAR technique is used.
5. Pitch of a sound depends upon the frequency of wave.

Solution 2.1:

Sound requires a material medium for its propagation. On the Moon, there is no atmosphere which can serve as a medium for propagation. So, we will not be able to hear any sound produced by our friend on the Moon.

Solution 2.2:

To hear a distinct echo, the minimum distance between the source of sound and the obstacle should be 17.2 m at room temperature. This condition is not satisfied in our classroom as the ceiling is not so high and the distance between the opposite walls is less than 17.2 m. Therefore, we cannot hear an echo in our classroom.

Solution 2.3:

The ceilings of concert halls and cinema halls are curved to avoid undesirable reverberation. This helps one to hear the pitch of music distinctly and clearly.

Solution 2.4:

1. Sound waves are longitudinal waves propagating through a material medium.
2. Wavelength, frequency, intensity, time period, pitch, amplitude, loudness and quality or timbre are the various characteristics of sound waves.
3. We can distinguish different voices from one another because of difference in their frequencies
4. The higher the frequency of sound, the higher is the pitch. As the intensity of sound increases the loudness of the sound also increases.
5. The quality or timber of sound is the characteristic of sound which enables us to distinguish one sound from another having the same pitch and loudness.

Solution 2.5:

1. The human ear is an important organ through which we can hear sound. The three major parts of the ear are the (i) external, (ii) middle and (iii) inner ear.
2. Pinna, the external part of the ear, collects the sound which is carried by the auditory canal to the ear drum.
3. The ear drum is a thin membrane which starts vibrating when sound waves fall on it. When a compression of medium reaches the ear drum, the pressure outside the membrane increases and forces the ear drum inwards.
4. The ear drum moves outwards and vibrates when a rarefaction reaches it. These vibrations are amplified and carried to the inner part of the ear.
5. A part of the auditory nerve connects the inner part of the ear to the brain. The pitch and loudness of the sound are recognised by a snail shell-like chamber called cochlea in the inner ear.
6. The vibrations received from the eardrum are converted into electrical signals or nerve impulses in the cochlea. The auditory nerve carries these signals to the brain which interprets them as sound. The amplitude of the vibration of the membrane gives the brain a measure of loudness of the sound being heard.

Solution 3.1:

Data: υ = 320 Hz, λ = 0.25 m, t = 10 s, s = ?

= 320 Hz × 0.25 m × 10 s
= 800 m
The wave will travel a distance of 800 m in 10 seconds.

Solution 3.2:

Data: υ = 336 m/s, λ = 3 cm = 0.03 m, v = ?

As it lies in the range 20 Hz to 20,000 Hz, it will be audible.

Solution 4:

No	Column 1	Column 2	Column 3
1	Transverse wave	a) Particles oscillate perpendicular to direction of propagation	i) Wave produced in a string
2	Longitudinal wave	b) Particles oscillate parallel to direction of propagation	ii) Wave produced in slinky
3	Ultrasound	c) High frequency waves	iii) Frequency greater than 20000 Hz
4	Infrasound	d) Low frequency waves	iv) Frequency less than  20 Hz
5	Audible frequency	e) Echo formation is heard under particular conditions	v) Frequency between 20 Hz and 20000 Hz

Solution 5.1:

• Sound with frequency greater than 20000 Hz is called ultrasound. We cannot hear ultrasound. It is produced by bats, rats and dolphins.
• Ultrasounds are used for the following purposes:

            1) To establish ship to ship communication
            2) For welding plastic surfaces
            3) To kill bacteria in liquids such as milk to preserve them for longer periods
            4) In echocardiography
            5) For imaging internal organs of the human body
            6) In industries to clean parts of a machine which are difficult to reach
            7) To detect cracks and flaws in metal blocks

Solution 5.2:

1. SONAR is Sound Navigation and Ranging. It is the technique used to determine the distance and location of underwater objects such as submarines and icebergs using ultrasonic waves. It can be used to determine the depth of the sea. In SONAR, a transmitter and detector are installed on a ship. The transmitter produces and transmits ultrasonic waves which travel through water to the bottom of the sea. The reflected waves from the sea bed are received by the detector which converts them into an electrical signal.
2. If t is the time recorded between the transmission of waves, the time taken by the waves to cover the distance between the ship and bottom of the sea is t/2. If v is the speed of the waves in seawater, then d is = v t/2 gives the depth of the sea.

Solution 5.3:

1. The persistence of sound due to repeated reflection of sound from the walls of a big room which combine is called reverberation.
2. In reverberation, the intensity of sound and the time interval between the successive arrivals of the same sound go on decreasing with time.
3. Excessive reverberation is not desirable as the speech of the speaker or the pitch of the music cannot be heard clearly because of the mixing of reflected waves and direct waves produced after the production of earlier waves.
4. To avoid these problems, reverberation should be controlled. For this, the roof and walls of auditoriums are covered with sound-absorbent materials such as compressed fibre boards, curtains or rough plaster. Even the material of the seats is chosen such that it absorbs sound waves to the desired extent.

Solution 6.1:

The distance between two consecutive compressions or two consecutive rarefactions is called wavelength.

Solution 6.2:

he number of oscillations (vibrations) per unit time is called the frequency of the wave.

Solution 6.3:

The magnitude of maximum displacement of particles in the medium on either side of the mean value is called amplitude of the wave.

Solution 6.4:

Echo is the repetition of sound due to reflection of original sound from any surface.