# Acoustics Online Test 10th Science Lesson 5 Questions in English

## Acoustics Online Test 10th Science Lesson 5 Questions in English

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 Question 1
Which is a branch of physics that deals with production, transmission, reception, control, and effects of sound?
 A Optics B Classis Mechanics C Quantum Statistic D Acoustics
Question 1 Explanation:
Acoustics is a branch of physics that deals with production, transmission, reception, control, and effects of sound.
 Question 2
By touching a ringing bell or a musical instrument while it is producing music, you can conclude that sound is produced by what?
 A Air B Vibrations C Vacuum D None of the above
Question 2 Explanation:
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.
 Question 3
Which among the following is not the medium for sound propagation?
 A Air B Water C Vacuum D Steel
Question 3 Explanation:
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.
 Question 4
Which among the following statement is correct
1. Sound waves are latitudinal waves that can travel through any medium (solids, liquids, gases) with the same speed irrespective of 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.
2. This displacement involves the longitudinal displacements of the individual molecules from their mean positions. This results in a series of high- and low-pressure regions called compressions and rarefactions.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 4 Explanation:
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.
 Question 5
Which among the following wave are sound waves with a frequency below 20 Hz that cannot be heard by the human ear?
 A Audible waves B Ultrasonic waves C Infrasonic waves D All the above
Question 5 Explanation:
Infrasonic waves are sound waves with a frequency below 20 Hz that cannot be heard by the human ear.
 Question 6
Which among the following 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?
 A Audible waves B Ultrasonic waves C Infrasonic waves D All the above
Question 6 Explanation:
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.
 Question 7
What was the frequency range of Audible waves?
 A 20 Hz to 200 Hz B 20 Hz to 1000 Hz C 20 Hz to 10000 Hz D 20 Hz to 20000 Hz
Question 7 Explanation:
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.
 Question 8
Which among the following is not the Infrasonic waves?
 A Waves produced during earth quake B Ocean waves C Sound produced by whales D Waves produced by bats
Question 8 Explanation:
Examples of Infrasonic waves are waves produced during earth quake, ocean waves, sound produced by whales, etc.
 Question 9
Which among the following is not the property of sound?
 A Medium is required for the propagation B Sound waves are longitudinal C Wavelength ranges from 4 × 10–7 m to 7 × 10–7 m D Sound waves travel in air with a speed of about 340 m s–1 at NTP
Question 9 Explanation:
Wavelength ranges from 1.65 cm to 1.65 m.
 Question 10
Which among the following is the SI unit of velocity?
 A m s B m s-1 C m s-2 D m-1 s-1
Question 10 Explanation:
The SI unit of velocity is ms-1
 Question 11
When you talk about the velocity associated with any wave, there are how many velocities?
 A Two B Three C Five D Six
Question 11 Explanation:
When you talk about the velocity associated with any wave, there are two velocities, namely particle velocity and wave velocity.
 Question 12
Which among the following statement is correct
1. The velocity with which the particles of the medium vibrate in order to transfer the energy in the form of a wave is called particle velocity.
2. The velocity with which the wave travels through the medium is called wave velocity. In other words, the distance travelled by a sound wave in unit time is called the velocity of a sound wave.
 A Only 1 B Only 2 C Both 1 and 2 D None
 Question 13
Which among the following represent the expression for velocity?
 A V = λ / T B V = T / λ C V = λ T D V = λ T-2
Question 13 Explanation:
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
 Question 14
Velocity of sound is maximum in which among the following medium?
 A Solid B Liquid C Gases D All the above
Question 14 Explanation:
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.
 Question 15
Which among the following statement is incorrect
 A The speed of sound is directly proportional to the square root of the elastic modulus B The speed of sound is inversely proportional to the square root of the density C The speed of sound is directly proportion to the square root of the density D None of the above
Question 15 Explanation:
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.
 Question 16
Which among the following statement is correct
1. The velocity of sound in a gas is inversely proportional to the square root of the density of the gas. Hence, the velocity decreases as the density of the gas increases. V α (√1/d)
2. The velocity of sound in a gas is inversely proportional to the square root of its temperature. The velocity of sound in a gas increases with the decrease in temperature. v ∝ √1/T. Velocity at temperature T is given by the following equation: vT = (vo + 0.61 T) m s–1.
3. When humidity increases, the speed of sound increases. That is why you can hear sound from long distances clearly during rainy seasons.
 A Both 1 and 2 B Both 1 and 3 C Both 2 and 3 D All 1, 2 and 3
Question 16 Explanation:
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.
 Question 17
In which among the following medium sound travels at high speed?
 A Copper B Iron C Aluminium D All the above
Question 17 Explanation:
Solid medium>> Copper = 5010 m s-1, Iron = 5950 m s-1 and Aluminium = 6420 m s-1.
 Question 18
In which among the following medium sound travels at high speed?
 A Sea water B Kerosene C Water D Air (at 0o C)
Question 18 Explanation:
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.
 Question 19
At what temperature will the velocity of sound in air be double the velocity of sound in air at 0o C?
 A T = 628o C B T = 736o C C T = 783o C D T = 819o C
Question 19 Explanation:
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.
 Question 20
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 _____
 A Reflection B Convention C Echo D Suspension
Question 20 Explanation:
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.
 Question 21
The sound waves that strike the interface are termed as ______
 A Transmitted wave B Incident wave C Dimension wave D Narrow wave
Question 21 Explanation:
The waves that strike the interface are termed as the incident wave and the waves that bounce back are termed as the reflected waves.
 Question 22
Which among the following is the incorrect law of reflection
1. The incident wave, the normal to the reflecting surface and the reflected wave at the point of incidence lie in different plane
2. The angle of incidence ∠i is greater than the angle of reflection ∠r.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 22 Explanation:
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.
 Question 23
A perpendicular line drawn at the point of incidence is called the ____
 A Medium B Normal C Conjunction D Tune
Question 23 Explanation:
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’.
 Question 24
Which among the following fort there is Clapping portico which is a series of arches on one side, each smaller than the preceding one?
 A Gwalior Fort B Golconda Fort C Amber Fort D Bidar Fort
Question 24 Explanation:
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.
 Question 25
Which among the following statement is incorrect
1. A longitudinal wave travels in a medium in the form of compressions and rarefactions. Suppose a compression travelling in air from left to right reaches a rigid wall. The compression exerts a force F on the rigid wall.
2. In turn, the wall exerts an equal and opposite reaction R = – F on the air molecules. This results in a compression near the rigid wall. Thus, a compression travelling towards the rigid wall is reflected back as a compression. That is the direction of compression is reversed.
 A Only 1 B Only 2 C Both 1 and 2 D None
 Question 26
Who 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?
 A James Maxwell B Edwin Hubble C Christian Doppler D Ezra Pound
Question 26 Explanation:
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.
 Question 27
Which among the following is the possibility of doppler effect relative motion?
1. The listener moves towards or away from a stationary source. The source moves towards or away from a stationary listener.
2. Both source and listener move towards or away from one other. The medium moves when both source and listener are at rest.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 27 Explanation:
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.
 Question 28
Which among the following statement is correct
1. Let S and L be the source and the listener moving with velocities vS and vL respectively. Consider the case of source and listener moving towards each other. As the distance between them decreases, the apparent frequency will be more than the actual source frequency.
2. 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.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 28 Explanation:
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.
 Question 29
Which among the following Expression for apparent frequency is wrong based Position of source and listener
1. Both source and listener move They move towards each other n' =V+VLV-VSn
2. Both source and listener move They move away from each other n' =V-VLV+VSn
3. Both source and listener move They move one behind the other. Source follows the listener       n' = ( V+VLV+VS) n
4. Both source and listener move They move one behind the other Listener follows the source          n' = V-VLV-VS n
 A Both 1 and 2 B Both 1 and 3 C Both 3 and 4 D Both 2 and 4
 Question 30
A source and listener are both moving towards each other with a speed v/10 where v is the speed of sound. If the frequency of the note emitted by the source is f, what will be the frequency heard by the listener?
 A n' = 1.22 f B n' = 2.44 f C n' = 3.66 f D n' =4.88 f
Question 30 Explanation:
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
 Question 31
Which among the following Expression for apparent frequency is correct based Position of source and listener?
1. Source at rest. Listener moves towards the source n' =  V-VLVn
2. Source at rest. Listener moves away from the source n' = V+VLVn
3. Listener at rest. Source moves towards the listener n' = VV-VSn
4. Listener at rest Source moves away from the listener n' = VV+VSn
 A Both 1 and 2 B Both 1 and 3 C Both 3 and 4 D Both 2 and 4
 Question 32
At what speed should a source of sound move away from a stationary observer so that observer finds the apparent frequency equal to half of the original frequency?
 A Vs =2 V B Vs = 1.5V C Vs = V D Vs = 0.5V
Question 32 Explanation:
n' = VV+VSn n2 = VV+VSn Vs = V.
 Question 33
Which among the following circumstance there is a doppler effect occurs
 A When source (S) and listener (L) both are at rest B When S and L move in a opposite way that distance between them remains change continuously. C When source S and L are moving in mutually perpendicular directions. D If the source is situated at the centre of the circle along which the listener is moving.
Question 33 Explanation:
When S and L move in such a way that distance between them remains constant.
 Question 34
Which among the following statement is correct
1. An electromagnetic wave is emitted by a source attached to a police car. The wave is reflected by a moving vehicle, which acts as a moving source. There is a shift in the frequency of the reflected wave. From the frequency shift, the speed of the car can be determined. This helps to track the over speeding vehicles.
2. The frequency of radio waves emitted by a satellite increase 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.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 34 Explanation:
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.
 Question 35
In what by measuring the change in the frequency between the sent signal and received signal, the speed of marine animals and submarines can be determined?
 A SONAR B RADAR C LIDAR D None of the above
Question 35 Explanation:
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.
 Question 36
In what radio waves are sent, and the reflected waves are detected by the receiver of the station?
 A SONAR B RADAR C LIDAR D AIDAR
Question 36 Explanation:
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.
 Question 37
The medium in which the velocity of sound increases compared to another medium is called ____
 A Denser medium B Closer medium C Rarer medium D Pointer medium
Question 37 Explanation:
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).
 Question 38
The medium in which the velocity of sound decreases compared to another medium is called ____
 A Denser medium B Closer medium C Rarer medium D Pointer medium
Question 38 Explanation:
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).
 Question 39
Which among the following statement is correct
1. 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 larger resistance for deformation, the surface of separation is pushed forward.
2. As the particles of the rarer medium are free to move, a rarefaction is produced at the interface. Thus, a compression is reflected as a rarefaction and a rarefaction travels from right to left.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 39 Explanation:
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.
 Question 40
In which surface sound from one focus will always be reflected to the other focus, no matter where it strikes the wall?
 A Convex surface B Concave surface C Parabolic surface D Elliptical surface
Question 40 Explanation:
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.
 Question 41
One of the famous whispering galleries is in St. Paul’s cathedral church is located in which among the following city?
 A Paris B Berlin C London D Cape town
Question 41 Explanation:
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.
 Question 42
A source producing a sound of frequency 90 Hz is approaching a stationary listener with a speed equal to (1/10) of the speed of sound. What will be the frequency heard by the listener?
 A n' = 9 Hz B n' = 90 Hz C n' = 100 Hz D n' = 110 Hz
Question 42 Explanation:
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.
 Question 43
From which surface, the reflected waves travel in a direction, according to the law of reflection thus intensity of the reflected wave is neither decreased nor increased?
 A Plane surface B Curved surface C Convex surface D Concave surface
Question 43 Explanation:
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.
 Question 44
Which surface are used when it is required to focus the sound at a particular point?
 A Plane surface B Parabolic surface C Convex surface D None of the above
Question 44 Explanation:
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.
 Question 45
Which among the following statement is incorrect
1. 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 increased.
2. When sound is reflected from a concave surface, the reflected waves are increased and dispersed at various point. So, the intensity of reflected waves is concentrated at various point.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 45 Explanation:
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.
 Question 46
A source producing a sound of frequency 500 Hz is moving towards a listener with a velocity of 30 m s–1. The speed of the sound is 330 m s–1. What will be the frequency heard by listener?
 A n' = 450 Hz B n' = 550 Hz C n' = 650 Hz D n' = 500 Hz
Question 46 Explanation:
When the source is moving towards the stationary listener, the expression for apparent frequency is n’ = VV-VSn n’ = 330330-30×500 = 550 Hz.
 Question 47
Which is the sound reproduced due to the reflection of the original sound from various rigid surfaces such as walls, ceilings, surfaces of mountains, etc.
 A LIDAR B Echo C Mail D All the above
Question 47 Explanation:
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.
 Question 48
Which among the following Conditions necessary for hearing echo is incorrect
1. The persistence of hearing for human ears is 0.5 second. This means that you can hear two sound waves clearly, if the time interval between the two sounds is at least 0.5 s. Thus, the minimum time gap between the original sound and an echo must be 0.5 s.
2. The above criterion can be satisfied only when the distance between the source of sound and the reflecting surface would satisfy the following equation:
Velocity (v) = distance travelled by sound/time taken     = 2d/t d = vt/2, since, t = 0.1 second, then d = v × 0.1/  2 = v/20 Thus, the minimum distance required to hear an echo is 1/20th part of the magnitude of the velocity of sound in air. If you consider the velocity of sound as 344 m s–1, the minimum distance required to hear an echo is 17.2 m.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 48 Explanation:
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.
 Question 49
Which among the following statement is correct
1. Some animals communicate with each other over long distances and also locate objects by sending the sound signals and receiving the echo as reflected from the targets. Echo is used to determine the velocity of sound waves in any medium.
2. The principle of echo is used in obstetric ultrasonography, which is used to create real-time visual images of the developing embryo or fetus in the mother’s uterus. This is a safe testing tool, as it does not use any harmful radiations.
 A Only 1 B Only 2 C Both 1 and 2 D None
 Question 50
A source of sound is moving with a velocity of 50 m s–1 towards a stationary listener. The listener measures the frequency of the source as 1000 Hz. what will be the apparent frequency of the source when it is moving away from the listener after crossing him? (velocity of sound in the medium is 330 m s–1)
 A n' = 848.48 Hz. B n' = 736.84 Hz C n' = 682.15 Hz D n' = 562.52 Hz
Question 50 Explanation:
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.
 Question 51
Which among he following statement is incorrect regarding Measuring velocity of sound by echo method
1. Measure the distance ‘d’ between the source of sound pulse and the reflecting surface using the measuring tape. The receiver is also placed adjacent to the source. A sound pulse is emitted by the source.
2. The stopwatch is used to note the time interval between the instant at which the sound pulse is sent and the instant at which the echo is received by the receiver. Note the time interval as ‘t’. Repeat the experiment for three or four times. The average time taken for the given number of pulses is calculated.
3. The sound pulse emitted by the source travels a total distance of d 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 travelled/time taken =  dt
 A Both 1 and 2 B Both 2 and 3 C Both 2 and 3 D All 1, 2 and 3
Question 51 Explanation:
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
 Question 52
Which among the following statement is correct
1. Sound board are basically square surfaces (plain), 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 plane surface. The sound of the speaker is reflected towards the audience thus improving the quality of sound heard by the audience.
2. A megaphone is a horn-shaped device used to address a small gathering of people. Its one end is wide and the other end is narrow. When a person speaks at the narrow end, the sound of his speech is concentrated by the multiple reflections from the walls of the tube. Thus, his voice can be heard loudly over a long distance.
 A Only 1 B Only 2 C Both 1 and 2 D None
Question 52 Explanation:
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.
 Question 53
Which among the following is a hearing aid, which is useful by people who have difficulty in hearing?
 A Ear spinner B Ear copper C Ear pole D Ear trumpet
Question 53 Explanation:
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.
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