Acoustics Online Test 10th Science Lesson 5 Questions in English

Acoustics is a branch of physics that deals with production, transmission, reception, control, and effects of sound.

By touching a ringing bell or a musical instrument while it is producing music, you can conclude that sound is produced by vibrations. The vibrating bodies produce energy in the form of waves, which are nothing but sound waves.

As the Moon does not have air, you will not be able to hear any sound produced by your friend. Hence, you understand that the sound produced due to the vibration of different bodies needs a material medium like air, water, steel, etc, for its propagation. Hence, sound can propagate through a gaseous medium or a liquid medium or a solid medium.

Sound waves are longitudinal waves that can travel through any medium (solids, liquids, gases) with a speed that depends on the properties of the medium. As sound travels through a medium, the particles of the medium vibrate along the direction of propagation of the wave.

Infrasonic waves are sound waves with a frequency below 20 Hz that cannot be heard by the human ear.

Ultrasonic waves are sound waves with a frequency greater than 20 kHz, Human ear cannot detect these waves, but certain creatures like mosquito, dogs, bats, dolphins can detect these waves. e.g., waves produced by bats.

Audible waves are sound waves with a frequency ranging between 20 Hz and 20,000 Hz. These are generated by vibrating bodies such as vocal cords, stretched strings etc.

Examples of Infrasonic waves are waves produced during earth quake, ocean waves, sound produced by whales, etc.

Wavelength ranges from 1.65 cm to 1.65 m.

The SI unit of velocity is ms-1

When you talk about the velocity associated with any wave, there are two velocities, namely particle velocity and wave velocity.

If the distance travelled by one wave is taken as one wavelength (λ) and, the time taken for this propagation is one time period (T), then, the expression for velocity can be written as ∴ V = λ / T. Velocity = Distance / Time taken Therefore, velocity can be defined as the distance travelled per second by a sound wave. Since, Frequency (n) =1/T, equation can be written as V = n λ

Velocity of a sound wave is maximum in solids because they are more elastic in nature than liquids and gases. Since, gases are least elastic in nature, the velocity of sound is the least in a gaseous medium. So, VS > VL > VG.

In the case of solids, the elastic properties and the density of the solids affect the velocity of sound waves. Elastic property of solids is characterized by their elastic moduli. The speed of sound is directly proportional to the square root of the elastic modulus and inversely proportional to the square root of the density. Thus, the velocity of sound in solids decreases as the density increases whereas the velocity of sound increases when the elasticity of the material increases.

The velocity of sound in a gas is directly proportional to the square root of its temperature. The velocity of sound in a gas increases with the increase in temperature. v ∝ √T. Velocity at temperature T is given by the following equation: vT = (vo + 0.61 T) m s–1. Here, vo is the velocity of sound in the gas at 0° C. For air, vo = 331 m s–1. Hence, the velocity of sound changes by 0.61 m s–1 when the temperature changes by one degree Celsius.

Solid medium>> Copper = 5010 m s-1, Iron = 5950 m s-1 and Aluminium = 6420 m s-1.

Liquid medium>>> Kerosene = 1324 m s-1, Water = 1493 m s-1, Sea water = 1533 m s-1, Air (at 0o C) = 331 m s-1 and Air (at 20o C) = 343 m s-1.

Let To C be the required temperature. Let v1 and v2 be the velocity of sound at temperatures T1 K and T2 K respectively. T1 = 273K (0o C) and T2 = (To C + 273) K. v2/ v1 = (√T2 /T1) = √ ((273 + T) /273) = 2 Here, it is given that, v2 / v1 = 2. So, (273 + T) / 273 = 4, T = (273 × 4) – 273 = 819o C.

When sound waves travel in a given medium and strike the surface of another medium, they can be bounced back into the first medium. This phenomenon is known as reflection. In simple the reflection and refraction of sound is actually similar to the reflection of light. Thus, the bouncing of sound waves from the interface between two media is termed as the reflection of sound.

The waves that strike the interface are termed as the incident wave and the waves that bounce back are termed as the reflected waves.

Like light waves, sound waves also obey some fundamental laws of reflection. The following two laws of reflection are applicable to sound waves as well. The incident wave, the normal to the reflecting surface and the reflected wave at the point of incidence lie in the same plane. The angle of incidence ∠i is equal to the angle of reflection ∠r.

A perpendicular line drawn at the point of incidence is called the normal. The angle which the incident sound wave makes with the normal is called the angle of incidence, ‘i’. The angle which the reflected wave makes with the normal is called the angle of reflection, ‘r’.

The Clapping portico in Golconda Fort is a series of arches on one side, each smaller than the preceding one. So, a sound wave generated under the dome would get compressed and then bounce back amplified sufficiently to reach a considerable distance.

Christian Doppler (1803-1853), an Austrian Mathematician and Physicist first observed that the frequency of the sound as received by a listener is different from the original frequency produced by the source whenever there is a relative motion between the source and the listener. This is known as Doppler effect.

Doppler effect relative motion could be due to various possibilities as follows: (i) The listener moves towards or away from a stationary source (ii) The source moves towards or away from a stationary listener (iii) Both source and listener move towards or away from one other (iv) The medium moves when both source and listener are at rest.

Let n and n' be the frequency of the sound produced by the source and the sound observed by the listener respectively. Then, the expression for the apparent frequency n' is n'=V+VLV-VS n Here, v is the velocity of sound waves in the given medium. Let us consider different possibilities of motions of the source and the listener.

When source and listener are both moving towards each other, the apparent frequency is n'=V+VLV-VS n n'=V+V10V-V10 n n’= (11/9) f n’= 1.22 f

n' = VV+VSn n2 = VV+VSn Vs = V.

When S and L move in such a way that distance between them remains constant.

The frequency of radio waves emitted by a satellite decreases as the satellite passes away from the Earth. By measuring the change in the frequency of the radio waves, the location of the satellites is studied.

In SONAR, by measuring the change in the frequency between the sent signal and received signal, the speed of marine animals and submarines can be determined.

In RADAR, radio waves are sent, and the reflected waves are detected by the receiver of the RADAR station. From the frequency change, the speed and location of the aeroplanes and aircrafts are tracked.

The medium in which the velocity of sound increases compared to another medium is called rarer medium. (Water is rarer compared to air for sound).

The medium in which the velocity of sound decreases compared to another medium is called denser medium. (Air is denser compared to water for sound).

Consider a wave travelling in a solid medium striking on the interface between the solid and the air. The compression exerts a force F on the surface of the rarer medium. As a rarer medium has smaller resistance for any deformation, the surface of separation is pushed backwards.

In elliptical surfaces, sound from one focus will always be reflected to the other focus, no matter where it strikes the wall. This principle is used in designing whispering halls. In a whispering hall, the speech of a person standing in one focus can be heard clearly by a listener standing at the other focus.

One of the famous whispering galleries is in St. Paul’s cathedral church in London. It is built with elliptically shaped walls. When a person is talking at one focus, his voice can be heard distinctly at the other focus. It is due to the multiple reflections of sound waves from the curved walls.

When the source is moving towards the stationary listener, the expression for apparent frequency is n’ = VV-VSn = VV-110Vn = ( 109)n = (109) × 90 = 100 Hz.

When sound waves are reflected from a plane surface, the reflected waves travel in a direction, according to the law of reflection. The intensity of the reflected wave is neither decreased nor increased.

Parabolic surfaces are used when it is required to focus the sound at a particular point. Hence, many halls are designed with parabolic reflecting surfaces.

When the sound waves are reflected from the curved surfaces, the intensity of the reflected waves is changed. When reflected from a convex surface, the reflected waves are diverged out and the intensity is decreased. When sound is reflected from a concave surface, the reflected waves are converged and focused at a point. So, the intensity of reflected waves is concentrated at a point.

When the source is moving towards the stationary listener, the expression for apparent frequency is n’ = VV-VSn n’ = 330330-30×500 = 550 Hz.

An echo is the sound reproduced due to the reflection of the original sound from various rigid surfaces such as walls, ceilings, surfaces of mountains, etc. If you shout or clap near a mountain or near a reflecting surface, like a building you can hear the same sound again. The sound, which you hear is called an echo. It is due to the reflection of sound.

The persistence of hearing for human ears is 0.1 second. This means that you can hear two sound waves clearly, if the time interval between the two sounds is a least 0.1 s. Thus, the minimum time gap between the original sound and an echo must be 0.1 s.

When the source is moving towards the stationary listener, the expression for apparent frequency is n’ = VV-VSn 1000 = 330330-50n n = 1000× 280330 = 848.48 Hz The actual frequency of the sound is 848.48 Hz. When the source is moving away from the stationary listener, the expression for apparent frequency is n' = VV+VSn = 330330 + 50× 848.48 = 736.84 Hz.

The sound pulse emitted by the source travels a total distance of 2d while travelling from the source to the wall and then back to the receiver. The time taken for this has been observed to be ‘t’. Hence, Speed of sound = distance travelledtime taken = 2dt

Sound board are basically curved surfaces (concave), which are used in auditoria and halls to improve the quality of sound. This board is placed such that the speaker is at the focus of the concave surface. The sound of the speaker is reflected towards the audience thus improving the quality of sound heard by the audience.

Ear trumpet is a hearing aid, which is useful by people who have difficulty in hearing. In this device, one end is wide and the other end is narrow. The sound from the sources fall into the wide end and are reflected by its walls into the narrow part of the device. This helps in concentrating the sound and the sound enters the ear drum with more intensity. This enables a person to hear the sound better.