WBBSE Solutions For Class 9 Physical Science Chapter 7 Some Properties Of Sound And Characteristics Of Sound

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Synopsis

Laws Of Reflection Of Sound:

1. The incident sound wave, the reflected sound wave, and the normal to the reflector at the point of incidence, all lie on the same plane.
2. The angle of incidence is equal to the angle of reflection.
3. Regular reflection of light waves occurs from a small-sized smooth reflector, whereas sound waves are not reflected by it. Again, regular reflection of sound waves occurs from a large-sized rough reflector.
4. If the frequency of sound is within the range of 20 Hz to 20 kHz, it produces a sensation of hearing in the human ear and is known as audible sound.
5. Sound having a frequency lower than 20 Hz is known as infrasonic sound.
6. Sound having frequency higher than 20 kHz is known as ultrasonic sound.
7. The wavelength of audible sound approximately ranges from 1.5 cm to 16 cm, hence a large reflector is required for the reflection of such sound waves.
8. If a sound produced from a source is reflected by a reflector and reaches the listener in addition to the original sound, then the reflected sound is called an echo of the original sound.
9. If we hear any sound, its trace remains in the brain for approximately 0.1s. During this time duration of 0.1s, any other sound entering the ears cannot be distinguished. This time duration is called persistence of hearing.
10. If the velocity of sound in air at 27°C is 348 m/s, the minimum distance between the source and the reflector for hearing an echo of an inarticulate sound is 17.4 m. The minimum distance between the source and the reflector for hearing the echoes of monosyllabic, bisyllabic, and trisyllabic sounds are 34.8m, 69.6m, and 104.4m respectively.
11. The persistence of sound within a closed space after its source ceases to produce sound, caused by multiple reflections of the sound from walls, roof, etc. is known as reverberation.
12. SONAR is an acronym for SOund Navigation And Ranging. With the help of ultrasonic sound, SONAR is used for the detection of ruins of a sunken ship or existence of a mountain deep inside an ocean.
13. When sound of a single frequency is emitted from a tuning fork or different musical instruments, then the emitted sound is known as tone. Sound due to a mixture of more than one frequency is called a note.
14. The sound which is produced by regular and periodic vibrations of the source and appears euphonious to our ears is known as musical sound. Sound of guitar, sitar, flute, tuning fork etc. are examples of musical sound.
15. The sound which is produced by irregular and non-periodic vibrations of the source and appears cacophonous to our ears is known as noise. Sound of crackers, automobiles, uproar of crowd etc. are examples of noise.

There Are Three Characteristics Of A Musical Sound. These Are:

1. Loudness,
2. Pitch and
3. Quality or timbre.

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

1. Amount of sound energy passing through an unit area around a point in one second, perpendicular to the direction of sound is called intensity of the sound at that point.
2. The dimensional, formula of intensity of sound is MT^-3 and its unit in SI is W/m².
3. The characteristic property of a musical sound due to which a listener can differentiate between a high-pitched and a low-pitched sound is known as pitch of the sound.
4. Frequency is the cause, pitch is its effect. Frequency is a measurable physical quantity. But pitch is a sensation or feeling which is not measurable.
5. The characteristic property by which sound of same intensity and pitch emitted by different sources can be distinguished in known as the quality or timbre of the musical sound.

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Short And Long Answer Type Questions

Question 1. Write down the laws of reflection of sound.

Answer:

The Laws Of Reflection Are:

1. The incident sound wave, the reflected wave and the normal to the reflector at the point of incidence, all lie on the same plane.
2. The angle of incidence is equal to the angle of reflection.

Question 2. Demonstrate with an experiment that during reflection of sound, angle of incidence = angle of reflection. Or, Demonstrate by an experiment that sound wave is reflected similar to a light wave.

Answer:

The Angle Of Incidence = Angle Of Reflection:-

Two hollow long pipes, open at both ends, are taken. Also, one table clock and one raised partition are taken. One long table is kept touching the wall and at a little distance from the wall, the partition is kept in a perpendicular way.

Now, the pipes are kept on the two sides of the partition in such a way that the axes of both the pipes intersect at the point O of the wall.

Now, a table clock is kept at the end A of the left pipe. The right pipe is now rotated slowly with respect to the point O so that a position is obtained, where tick-tock sound of the clock is heard loudly. Suppose, sound is heard loudly at the point D.

This is the reflected sound. The tick-tock sound of the clock passes through pipe AB, reflected at point O, and comes to the ear through the pipe CD and is heard. Now pencil marks are made at the points O, A, D, and E.

Next, pipe and partition are removed, and A,0; D,0; E,0 are connected. If ∠AOE and ∠DOE are measured, it is found that ∠AOE = ∠DOE, i.e., angle of incidence = angle of reflection.

So, it can be concluded that sound wave is reflected similar to a light wave.

Question 3. How is the property of reflection of sound utilised in a stethoscope?

Answer:

Property Of Reflection Of Sound Utilised In A Stethoscope:-

1. The stethoscope is a medical instrument using which doctors examine the heart or lungs of a patient. There are two rubber pipes in this instrument which are attached to a round metallic disc covered with a thin diaphragm.
2. If the two open ends of the rubber pipes are fitted in the ears and the disc is placed on the chest of the patient, sound of the heart of the patient is reflected repeatedly through the rubbers tubes and reaches the ears of the doctor.

Question 4. Why do we curve our palms by the side of our ear to clearly hear a faint sound from a distant place?

Answer:

We Curve Our Palms By The Side Of Our Ear To Clearly Hear A Faint Sound From A Distant Place:-

To clearly hear a faint sound from a distant place, we curve our palm by the side of our ear. In addition to the direct faint sound, sound reflected from the palm also enters the ear. As a result, loudness of the sound increases to some extent. Here, palm of the hand works like a concave reflector.

Question 5. Why is the roof of a modern auditorium curved like an arch?

Answer:

The Roof Of A Modern Auditorium Curved Like An Arch Because:-

The roof of a modern auditorium is curved like an arch so that it works as a concave reflector and if the source of sound is kept at its focus, sound spreads evenly throughout the auditorium. As a result, the loudness of the sound increases.

Question 6. How can one talk in a speaking tube?

Answer:

One Talk In A Speaking Tube As Follows:-

If we talk at one end of a long metallic tube, the produced sound advances through the tube without scattering and reaches the other end after repeated reflections. The listener on the other side hears this sound very clearly. This arrangement is called a speaking tube. A funnel is attached to the front side for the speaker.

Question 7. What do you mean by audible sound?

Answer:

Audible Sound Means:-

One can hear sound only when the number of vibrations per second, i.e., frequency of a vibrating body lies within the range of 20 Hz to 20000 Hz. This sound is called audible sound.

Question 8. What do you mean by infrasonic sound and ultrasonic sound? give examples.

Answer:

Infrasonic Sound And Ultrasonic Sound:-

1. Infrasonic sound is the sound whose frequency is less than 20 Hz.
2. Ultrasonic sound is the sound whose frequency is more than 20 kHz.

When a pendulum oscillates, its frequency remains less than 20 Hz. Sound produced in this case is infrasonic sound, hence inaudible. Again, when a bat flies, it produces a type of sound from its mouth whose frequency is more than 20kHz, that is, ultrasonic sound.

Question 9. A flying bat makes a sound from its mouth. What type of sound wave does it produce?

Answer:

The sound wave emitted by a flying bat from its mouth has a frequency more than 20 kHz. Hence, it is an ultrasonic sound. If there is any obstacle in front of the sound, sound waves get reflected. By hearing that reflected sound, the bat can locate the position of any obstacle. We can not hear this ultrasonic sound emitted from the mouth of a bat.

Question 10. Give some practical applications of ultrasonic sound.

Answer:

These Are Some Of The Practical Applications Of Ultrasonic Sound:

1. In the process of ultrasonography, ultrasound is used to detect the location of a tumour in a human body.
2. Ultrasonic sound is used to clean dirty clothes.
3. Ultrasonic sound is also used to measure depth of the sea, to locate submarine or a shoal of fish, or to understand the presence of aquatic animals inside sea.

Question 11. Mention two applications of ultrasonic sound in medical science.

Answer:

Applications Of Ultrasonic Sound In Medical Science:-

It has been confirmed by experiments that there are some bacteria which can be destroyed by the application of ultrasonic sound. Also in the process of ultrasonography or USG, ultrasonic sound is used to know more about the internal muscles, different joints, arteries, etc. of the body.

Question 12. Write about a few animals in nature who normally make use of ultrasonic sound in their manner of living.

Answer:

Animals In Nature Who Normally Make Use Of Ultrasonic Sound In Their Manner Of Living Are Given Below:-

It has been observed that animals like bats, dolphins, whales, etc. utilise ultrasonic sound. Bats emit ultrasonic sound of very high frequency (100 kHz – 200 kHz) from their mouth at the time of flying.

This sound is reflected back from some obstacle in front of it and this helps the bat in recognising the obstacle. It thus flies freely without any impediment. Whales, dolphins, etc. living deep inside the sea communicate with each other through the use of ultrasound.

Question 13. Light wave is reflected regularly from a small and smooth reflector whereas sound wave is not reflected. Why?

Answer:

LightWave Is Reflected Regularly From A Small And Smooth Reflector Whereas Sound Wave Is Not Reflected

Reflection of a wave is possible only when the size of the reflector is larger than its wavelength. Again, whether reflection of a wave is regular or scattered, depends on the roughness of the surface of the reflector. If the wavelength of the wave is greater than the roughness of the surface of the reflector, there is regular reflection, otherwise the reflection is scattered.

The wavelength of sound in the audible range is from 1.5 cm to 16 m and wavelength of light in the visible range is from 1000 A (or 4 x 10^-5 cm) to 8000 A (or 8 x 10^-5 cm ). The size of a small, smooth reflector is comparatively larger than the wavelength of visible light.

So, in this case, light is reflected and the value of roughness being less than the wavelength of light, the reflection is regular. But as the size of a reflector is smaller than the wavelength of a sound wave, there is no reflection of sound wave.

Question 14. What are the differences between reflection of sound and reflection of light?

Answer:

The Differences Between The Reflection Of Sound And Reflection Of Light Are:

Question 15. What are the differences between sound wave and light wave?

Answer:

The Differences Between Sound Wave And Light Waves Are:

Question 16. What is the persistence of hearing?

Answer:

Persistence Of Hearing:-

If we hear any sound, its trace remains in the brain for approximately 0.1s. During this time duration of 0.1 s, more than one sound entering the ears cannot be distinguished. This time duration is called persistence of hearing.

Question 17. Calculate the minimum distance between the source and the reflector for hearing an echo of a short-lived sound; taking time of persistence of hearing as 0.1 second. [Given, velocity of sound in air = 348 m/s.]

Answer:

1. As the velocity of sound in air is 348 m/s, it traverses a distance of 348 x 0.1 m = 34.8 m in 0.1s.
2. So, the minimum distance between the source and the reflector for hearing an echo of a short-lived sound = \(\frac{34.8}{2}\) = 17.4 m.

Question 18. Calculate the minimum distance between the source and the reflector to hear an echo for a syllabic and intelligible sound, taking time of persistence of sound as 0.1 s. [Given that velocity of sound in air is 348 m/s and a human being cannot pronounce more than five syllables in one second.]

Answer:

Intelligible sound consists of one or more than one syllable. A human being cannot pronounce more than five syllables in one second. So, sound has to reach the ear after 1/5 s = 0.20 s, in order to hear an echo of a monosyllabic word. As the velocity of sound in air is 348 m/s, sound traverses a distance of 348 x 0.2 m * 69.6 m during this time.

In this case, minimum distance between the source and the reflector = \(\frac{69.6}{2}\) m = 34.8 m

Similarly, minimum distances between the source and the reflector, to hear an echo for a bisyllabic and trisyllabic sound are 34.8 x 2 = 69.6 m and 34.8 x 3 = 104.4 m respectively.

Question 19. What do you mean by an echo?

Answer:

Echo:-

If a sound produced from a source is reflected by a reflector situated at some distance and reaches the listener in addition to the original sound, then the reflected sound is called an echo of the original sound.

Question 20. State two conditions for the echo to be heard.

Answer:

Echo Can Be Heard If:-

1. The minimum distance between the source of sound and the obstacle will be 17 m if velocity of sound in air be 340 m/s.
2. The obstacle will be rigid, hard, and big in size (compare to wavelength of sound wave).

Question 21. Echo can’t be heard in a small room. Explain.

Answer:

Echo Can’t Be Heard In A Small Room:-

The minimum distance between source of sound and the obstacle to hear an echo should be 17 m if the velocity of sound in air be 340 m/s. Therefore echo can’t be heard in a small room.

Question 22. How is the depth of a sea measured with the help of echo?

Answer:

Depth Of A Sea Measured With The Help Of Echo As Follows:-

The depth of a sea can be measured with the help of echo. From the bottom of the ship, a source of sound (S) and a hydrophone (H) (a hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound) are hung from the two sides at the same depth no.

If an intense and short-lived sound is produced from the source, one sound traverses along the straight path SH in time t₁ to reach the hydrophone and another sound reaches the hydrophone in time t₂, after getting reflected from the bottom at P. The first sound is the original sound and the second one is the reflected sound. A perpendicular PQ is drawn on SH from P.

Now if V is the velocity of sound in water, then SH = Vt₁

or, SQ = \(=\frac{1}{2} S H=\frac{V t_1}{2}\)

Again, SP + PH = Vt₂

or, SP = \(\frac{V t_2}{2}\) [SP = PH]

∴ PQ = \(\sqrt{S P^2-S Q^2}\)

= \(\sqrt{\left(\frac{V t_2}{2}\right)^2-\left(\frac{V t_1}{2}\right)^2}=\frac{V}{2} \sqrt{t_2^2-t_1^2}\)

As h₀ is the depth of the source from the water surface, so depth of sea at that place from the surface of sea = \(h_0+P Q=h_0+\frac{v}{2} \sqrt{t_2^2-t_1^2}\)

Question 23. How is the height of an airplane measured with the help of echo?

Answer:

Height Of An Airplane Measured With The Help Of Echo:-

The height of an airplane can be measured from the surface of the earth with the help of echo. Suppose, an airplane is flying parallel to the earth with a velocity u.

When it comes to position A, an intense sound of very short duration is made and when the airplane reaches the position C, then an echo is heard. If the sound is reflected at point 6, a perpendicular BD is drawn from B to AC.

If f is the difference of time between creating of sound and hearing of echo, then AC = ut.

∴ AD = \(\frac{AC}{2}{/latex] = [latex]\frac{ut}{2}{/latex]

Again, time taken for sound to reach B from A = [latex]\frac{t}{2}{/latex]

∴ if V is the velocity of sound in air, AB = [latex]\frac{Vt}{2}{/latex]

height of the airplane from the surface of the earth,

h = BD = [latex]\sqrt{A B^2-A D^2}\)

= \(\sqrt{\left(\frac{V t}{2}\right)^2-\left(\frac{u t}{2}\right)^2}=\frac{t}{2} \sqrt{V^2-u^2}\)

Question 24. What is reverberation?

Answer:

Reverberation

The persistence of sound within a closed space after its source ceases to produce sound, caused by multiple reflections of the sound from walls, roof, etc. is known as reverberation.

Question 25. What measures are taken to prevent reverberation inside an auditorium?

Answer:

Measures To Take For The Prevention of Reverberation Inside An Auditorium:-

Reverberation takes place if a lot of reflectors are present within a small distance. For this reason, sound persists for a considerable amount of time inside an empty hall even after the original sound stops. This creates great difficulty for the audience as they cannot hear the original sound.

To remove this difficulty, walls of the auditorium are covered with soft pads or rubber-type materials. These work as sound absorbers. As a result, the problem of reverberation does not arise. Furniture and material of the seats are also made of materials which can absorb sound.

Question 26. What is SONAR?

Answer:

SONAR:-

SONAR is an acronym for Sound Navigation And Ranging. With the help of ultrasonic sound, SONAR is used for detection of ruins of a sunken ship or existence of a mountain deep inside an ocean.

Question 27. How can we measure the depth of a sea with the help of SONAR?

Answer:

We Can Measure The Depth Of A Sea With The Help Of SONAR As Follows:-

One sound emitting machine and a detector of SONAR are present in a ship. A very powerful ultrasonic sound wave is sent inside the sea with – the help of the sound emitter. This wave is reflected by the sea bed and comes back to the detector instrument.

Suppose, t is the time difference between the time of sending the wave and time of receiving the echo of the wave. If the velocity of the ultrasonic sound is V, then the depth of sea at that place is \(\frac{Vt}{2}\).

Question 28. What are the differences between a tone and a note?

Answer:

Differences Between A Tone And A Note:-

Tone is the sound of a single frequency, emitted from different musical instruments or tuning fork. In other words, it may be stated that the sound emitted from any musical instrument is a mixture of sounds of different frequencies. Each one of these frequencies is a tone. In the language of music, a summation of several tones is a note. Therefore, the note is a mixture of several frequencies.

Question 29. What do you mean by fundamental tone, overtone, and harmonic?

Answer:

Fundamental Tone, Overtone, And Harmonic:-

Fundamental tone: Fundamental tone is defined as the tone of minimum frequency present among the tones of a note.

Overtone: Overtone is defined as any tone present in a note other than the fundamental tone.

Harmonic: Harmonic is defined as any tone whose frequency is an integral multiple of the fundamental tone.

Question 30. The following tones with given frequencies are mixed in a sound emitted from a musical instrument: 256Hz, 512Hz, 1020Hz, and 1280Hz. Identify them as:

1. Fundamental tone
2. First harmonic
3. First overtone
4. Second overtone
5. Second harmonic.

Answer:

1. Fundamental tone: 256 Hz
2. First harmonic: 256 Hz
3. First overtone: 512 Hz
4. Second overtone: 1020 Hz
5. Second harmonic: 512 Hz

Question 31. What do you mean by musical sound and noise?

Answer:

Musical Sound And Noise:-

Musical sound: Musical sound is defined as that sound which is produced by regular and periodic vibrations of the source and appears euphonious to our ears. Sound of guitar, sitar and flute, tuning fork etc. are examples of musical sound.

Noise: Noise is defined as that sound which is produced by irregular and non-periodic vibration of the source and appears cacophonous to our ears. Sound of crackers, automobiles, uproar of crowd etc. are examples of noise.

Question 32. What are the characteristics of a musical sound? What is intensity of sound?

Answer:

Characteristics Of A Musical Sound:-

There are three characteristics of a musical sound. These are:

1. Loudness,
2. Pitch and
3. Quality or timbre.

Question 33. Amount of sound energy passing through an unit area around a point in one second, perpendicular to the direction of sound is called intensity of the sound at that point. what is the unit of intensity in SI? Calculate the dimensional formula of intensity.

Answer:

Unity of intensity in SI is W • m^-2.

Dimensional formula of intensity = \(\frac{\text { dimensional formula of energy }}{\text { dimensional formula of area}}\) x dimensional formula of time

= \(\frac{M L^2 T^{-2}}{L^2 \times T}=M T^{-3}\)

Question 34. What do you understand by loudness of sound?

Answer:

Loudness Of Sound:-

Among the sounds we hear everyday, some are high-pitched and some are low-pitched. Loudness of sound is such a characteristic which defines the strength with which it reaches the ears of the listener. Loudness is a feeling only and is not measurable. Loudness of sound is measured by its intensity.

Question 35. Intensity is the cause, loudness is its effect—explain.

Answer:

Intensity Is The Cause, Loudness Is Its Effect

When intensity of sound is more, more amount of sound energy reaches our ears, and hence, greater loudness is felt by us. Again, when intensity is less, less amount of sound energy enters our ears, so loudness of sound is less. This is why it may be concluded that if intensity is the cause, then loudness is its effect.

Question 36. Intensity of sound depends on which factors?

Answer:

Intensity of sound depends on several factors

Amplitude of vibration of the source of sound:

Intensity of sound is directly proportional to the square of the amplitude of vibration of the source of sound.

Size of the source:

Larger the size of the source, more is the intensity of the sound.

Distance of the listener from the source:

Intensity of sound is inversely proportional to the square of the distance of the listener from the source.

Density of the medium:

Intensity is more if the density of the medium is more.

Flow of air: Intensity of sound increases if flow of air is in the direction of motion of the sound. But if flow of air is in the opposite direction of motion of sound, intensity gets reduced.

Question 37. On a particular day, wind is blowing from east to west. A song is played through a microphone. A man is standing slightly away from the microphone in the east. Now, this man crosses the microphone and moves to the west side of the microphone at an equal distance. He hears more intense sound in which case? Explain with reasons.

Answer:

Sound spreads in every direction with equal loudness. But if wind blows along the direction of propagation of sound, loudness increases in that direction. According to this question, wind blows from east to west. As a result, the man hears sound with greater intensity while standing in the west.

Question 38. What do you mean by pitch of a sound?

Answer:

Pitch Of A Sound:-

If a male, a female and a child pronounce a sound with the same intensity, this same sound is known to create three different feelings in our ears. The voice of a female appears to be shriller than that of a male, again the voice of a child appears to be shriller than that of a female. This characteristic of sound is called pitch.

Pitch is defined as that characteristic of a musical sound due to which a listener can differentiate between a high-pitched and a low-pitched sound.

Question 39. Explain the statement — frequency is the cause of a sound and pitch is its effect.

Answer:

Frequency Is The Cause Of A Sound And Pitch Is Its Effect

Pitch depends mainly on frequency. If the frequency of a tone is high, its pitch is also high. On the other hand, if frequency is low, pitch is also low. So, it is said that frequency is the cause and pitch is the effect.

But frequency is a physical quantity and is measurable whereas pitch is a feeling which is not measurable.

Question 40. Loudness and pitch of sound change mainly due to which physical quantities?

Answer:

Due to change of intensity of a musical sound, loudness of sound perceived by a person changes. It has been found by experiment that this sensation increases as intensity of sound increases.

On the other hand, due to change in frequency of a musical sound, the pitch of a sound perceived by ear changes. Generally, as the frequency of the source of sound increases, the sound appears to be more high-pitched in the ears of an individual.

Question 41. What do you mean by quality or timbre of a sound?

Answer:

Timbre Of A Sound:-

The characteristic property by which sound of same intensity and pitch emitted by different sources can be distinguished is called the quality or timbre of a musical sound. Due to this characteristic of sound, tones of the same intensity and pitch emitted from harmonium, violin and table, etc. can be easily differentiated by us.

Question 42. The quality or timbre of a note depends on which factors?

Answer:

The quality of a note depends on three factors. These are:

1. The number of overtones mixed with the fundamental tone;
2. Frequencies of the overtones and
3. Relative intensity of the overtones.

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Very Short Answer Type Questions Choos The Correct Answer

Question 1. Due to which characteristic of sound, one can distinguish between the low-pitched and high-pitched sound of same intensity?

1. Loudness
2. Amplitude
3. Frequency
4. Pitch

Answer: 4. Pitch

Question 2. Which one is the first harmonic among the following frequencies: 500 Hz, 900 Hz, 550 Hz, 750 Hz, 300 Hz, 600 Hz?

1. 300 Hz
2. 500 Hz
3. 600 Hz
4. 900 Hz

Answer: 1. 300 Hz

Question 3. Out of 256 Hz, 512 Hz, 1020 Hz, and 1280 Hz, the frequency 1280 Hz is the

1. Last tone
2. Fourth overtone
3. Third harmonic
4. Second harmonic

Answer: 2. Fourth overtone

Question 4. Intensity of sound does not depend on

1. Shape of the source of sound
2. Amplitude of vibration of the source of sound
3. Density of the medium
4. Number of overtones in note

Answer: 4. Number of overtones in note

Question 5. The sound which contains more number of is more melodious.

1. Harmonics
2. Overtones
3. Fundamental tones
4. Beats

Answer: 1. Harmonics

Question 6. Characteristic of musical sound is

1. Loudness
2. Pitch
3. Quality
4. All of these

Answer: 4. All of these

Question 7. The quantity which does not produce any perception is

1. Loudness
2. Intensity
3. Pitch
4. Quality

Answer: 2. Intensity

Question 8. Among the following pairs, which one represents reciprocal quantities?

1. Frequency and wavelength
2. Frequency and time period
3. Velocity of wave and wavelength
4. Wavelength and loudness

Answer: 2. Frequency and time period

Question 9. Which of the following cannot reflect sound waves?

1. Range of mountains
2. High embankment of a river
3. A small mirror
4. Wall of an empty hall

Answer: 3. A small mirror

Question 10. Reverberation of sound may be heard

1. In a stethoscope
2. Inside an empty hall
3. In a distant wall
4. In vacuum

Answer: 2. Inside an empty hall

Question 11. Reverberation of sound is created

1. In one reflection
2. In two reflections
3. In three reflections
4. In multiple reflections

Answer: 4. In multiple reflections

Question 12. Rubber pads are fixed on the walls of a cinema hall in order to

1. Increase reflection of sound
2. Decrease reverberation of sound
3. Increase velocity of sound
4. Increase loudness of sound

Answer: 2. Decrease reverberation of sound

Question 13. If a tuning fork is struck, sound emitted from it has a frequency of 500 Hz. What is the frequency of the emitted sound when the tuning fork is struck with twice the previous force?

1. 1000 Hz
2. 500 Hz
3. 250 Hz
4. Insufficient data

Answer: 2. 500 Hz

Question 14. If the frequencies of fundamental tone, overtone, and harmonic are denoted by n₁, n₂, and n₃ respectively, then

1. n₁> n₂> n₃
2. n₁ > n₂ but n₂ > n₃
3. n₁< n₂> n₃
4. n₁< n₂ and n₁ ≤ n₃

Answer: 4. n₁< n₂ and n₁ ≤ n₃

Question 15. Which of the following is essential for a melodious sound?

1. The presence of more harmonics
2. The presence of more overtones
3. Presence of different frequencies
4. Presence of different musical instruments

Answer: 1. Presence of more harmonics

Question 16. Which of the following statements is correct?

1. Tone is a sound of single frequency
2. Note is a sound of single frequency
3. Tone is a sound composed of different frequencies
4. Tone is composed of several notes

Answer: 1. Tone is a sound of single frequency

Question 17. If the frequency of the source of sound increases, the sound heard by a person appears to be more

1. Loud
2. Discordant
3. Melodious
4. Shrill

Answer: 4. Shrill

Question 18. If the velocity of sound in air at 27°C is 348 m/s, then what is the minimum distance between the source and the reflector of sound to hear an echo of transient sound?

1. 17.4 m
2. 34.8 m
3. 69.6 m
4. 104.4 m

Answer: 1. 17.4 m

Question 19. If the velocity of sound in air at 27°C is 348 m/s, what is the minimum distance between the source and the reflector of sound to hear an echo of a monosyllabic sound?

1. 17.4 m
2. 34.8 m
3. 69.6 m
4. 104.4 m

Answer: 2. 34.8 m

Question 20. Bats detect the obstacles in their path by receiving the reflected

1. Electromagnetic waves
2. Radio waves
3. Infrasonic wave
4. Ultrasonic waves

Answer: 4. Ultrasonic waves

Question 21. Infrasonic sound can be heard by

1. Human beings
2. Dolphins
3. Rhinoceros
4. Monkey

Answer: 3. Rhinoceros

Question 22. Which of the following is used in ultrasonography?

1. Infrasonic sound
2. Ultrasonic sound
3. Audible sound
4. Radio waves

Answer: 2. Ultrasonic sound

Question 23. Which of the following is used in SONAR?

1. Infrasonic sound
2. Micro waves
3. Ultrasonic sound
4. Audible sound

Answer: 3. Ultrasonic sound

Question 24. Repeated reflection of sound produces

1. Reverberation
2. Stationary wave
3. Echo
4. Refraction

Answer: 1. Reverberation

Question 25. A mirror is a good reflector of light wave but poor reflector of sound wave because

1. Wavelength of sound is bigger than the mirror
2. Wavelength of light is bigger than the mirror
3. Velocity of sound is smaller than that of light
4. Sound can not travel in vacuum

Answer: 1. Wavelength of sound is bigger than the mirror

Question 26. Sound wave transfers a physical quantity ‘X’ from one place to another, where ‘X’ is

1. Mass
2. Velocity
3. Energy
4. Density

Answer: 3. Energy

Question 27. A sound wave strikes a wall perpendicularly. What is the angle of reflection of the sound wave?

1. 90°
2. 0°
3. 45°
4. 30°

Answer: 2. 0°

Question 28. A supersonic jet plane

1. Can fly faster than sound
2. Can fly at the highest velocity of sound
3. Cannot fly with the velocity of sound
4. Can fly with the velocity of light

Answer: 1. Can fly faster than sound

Question 29. A jet plane is flying with supersonic speed. Its Mach number is

1. Equal to one
2. More than one
3. Less than one
4. Equal to one hundred

Answer: 2. More than one

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Answer In Brief

Question 1. What is the range of frequency of audible sound?

Answer: Range of frequency of audible sound is 20 Hz to 20000 Hz .

Question 2. What is the frequency of infrasonic sound?

Answer: Frequency of infrasonic sound is less than 20 Hz.

Question 3. What is the frequency of ultrasonic sound?

Answer: Frequency of ultrasonic sound is more than 20000 Hz.

Question 4. What is a tone?

Answer: Sound of a single frequency is known as a tone.

Question 5. What is a note?

Answer: Note is a sound containing more than one frequency.

Question 6. Write the SI unit of intensity of sound.

Answer: J • s^-1 • m^-2 or, W • m^-2.

Question 7. Give one application where principle of echo is utilised.

Answer: Hearing aid.

Question 8. 500 Hz is written on the body of tuning fork. Is the sound produced from this fork a tone or a note?

Answer: If the tuning fork is vibrated, the sound produced from it is a tone since sound of only 500 Hz frequency is produced.

Question 9. Which property of sound is applied in a doctor’s stethoscope?

Answer: Property of reflection of sound is applied in a doctor’s stethoscope.

Question 10. What type of sound is emitted when a tuning fork is struck?

Answer: Audible sound is emitted when a tuning fork is struck.

Question 11. What type of sound is used in the in a radar?

Answer: Ultrasound is used in a radar.

Question 12. Mention the name of an animal which makes use of ultrasound.

Answer: A bat uses ultrasound while flying.

Question 13. Which characteristic of a musical sound depends on its frequency?

Answer: Pitch of a musical sound depends on its frequency.

Question 14. Which characteristic of a musical sound depends on its intensity?

Answer: Loudness of a musical sound depends on its intensity.

Question 15. Which characteristic of a musical sound changes when the distance between the source of sound and the listener changes?

Answer: In this case, loudness of the musical sound changes.

Question 16. Sound of the same frequency and with the same intensity are emitted from different string instruments. Which characteristic of sound helps us in distinguishing the sound of one musical instrument from the other?

Answer: Quality or timbre of the musical sound helps us in distinguishing the sound of different musical instruments.

Question 17. Which is the first harmonic in a sound consisting of more than one frequency?

Answer: The sound with the minimum frequency is the first harmonic.

Question 18. What do you mean by the intensity of sound?

Answer: Sound energy per unit time per unit area is known as the intensity of sound.

Question 19. What does SONAR stand for?

Answer: SONAR stands for SOund Navigation And Ranging.

Question 20. What is the basic principle with which SONAR works?

Answer: SONAR works on the principle of reflection of waves.

Question 21. State one important use of ultrasound in industries.

Answer: Ultrasounds are used to find faults and cracks in metal.

Question 22. State one important use of ultrasound for medical purpose.

Answer: Diagnosing the diseases in human body.

Question 23. What do you understand by the reverberation?

Answer: The phenomenon of prolongation of original sound due to the multiple reflection of sound wave even after the source stops producing sound is called reverberation.

Question 24. Why is the speed of sound grater in solid than in gas?

Answer: This is because particle of solids are closer than the particles of gases.

Question 25. Which wave property determines loudness?

Answer: Amplitude of the wave determines loudness.

Question 26. Does sound follow the same lows of reflection as light does?

Answer: Yes- Sound wave are reflected just like light wave

Question 27. Why is sound not heard when a bird flies?

Answer: When a bird flies, the frequency of its wings is less than 20 Hz which is in the infrasonic range and thus the sound cannot be heard.

Question 28. How does the velocity of sound in air change when temperature is increased?

Answer: Velocity of sound in air increases when temperature is increased.

Question 29. How does the velocity of sound in air change when humidity of air increases?

Answer: Velocity of sound in air increases when the humidity of air increases.

Question 30. When a pendulum oscillates, its frequency remains less than 20 Hz. What type of sound is produced in this case?

Answer: Infrasonic sound is produced in this case.

Question 31. Which type of sound waves are used to clean dirty clothes?

Answer: Ultrasonic sound waves are used to clean dirty clothes.

Question 32. Which type of sound waves are used to detect a shoal of fish deep inside sea water?

Answer: Ultrasonic sound waves are used to detect a shoal offish deep inside sea water.

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Fill In The Blanks

Question 1. The perception of difference of pitch sound arises due to change of ______ of sound.

Answer: Frequency

Question 2. In the case of  ________ repeated reflection of sound occurs.

Answer: Reverberation

Question 3. Sound having frequency more than the frequency of the audible range is known as _______ sound.

Answer: Ultrasonic

Question 4. Sound having frequency less than the frequency of the audible range is known as ______ sound.

Answer: Infrasonic

Question 5. Stethoscope is the application of the property of ______ of sound.

Answer: Reflection

Question 6. Soft pads fixed to the walls of different auditoriums serve as __________

Answer: Sound absorbers

Question 7. SONAR uses ________ sound to determine the depth of an ocean.

Answer: Ultrasonic

Question 8. _______ of sound increases with the increase of size of the source of sound.

Answer: Intensity

Question 9. ________ of the sound increases with increase of frequency of the source of sound.

Answer: Pitch

Question 10. The intensity of sound __________ if air flows along the direction of motion of the sound.

Answer: Increases

Question 11. The difference in the voices of males and females is due to the ________ of sound.

Answer: Pitch

Question 12. Reflection of a wave is possible only when the size of the reflector is ________ compared to the wavelength of the wave.

Answer: Greater

Question 13. The trace of a sound remains in our brain for almost ________

Answer: 0.1 s

Question 14. The ratio of the velocities of a body and sound in a medium is called __________

Answer: Match number

Question 15. The velocity of a moving body is called ________ if the value of its Mach number is more than 1.

Answer: Super sonic

Chapter 7 Some Properties Of Sound And Characteristics Of Sound State Whether True Or False

Question 1. Echo and reverberation of sound refer to a single phenomenon.

Answer: False

Question 2. SONAR is an acronym for Sound Navigation and Ranging.

Answer: True

Question 3. The tone having the lowest frequency in a note is known as fundamental tone.

Answer: True

Question 4. Ultrasonic sound is used for the purpose of sterilizing of milk.

Answer: True

Question 5. Loudness of sound varies inversely to the square of the amplitude.

Answer: True

Question 6. Pitch of a note depends on its amplitude.

Answer: False

Question 7. A sound which is music to some one may be a noise to others.

Answer: False

Question 8. Sound of one frequency is tone.

Answer: True

Question 9. In a note the sound of least frequency is called the fundamental tone.

Answer: True

Question 10. A continuous noise of high level may cause migraine.

Answer: True

Chapter 7 Some Properties Of Sound And Characteristics Of Sound Numerical Examples

Useful Information

The impression of a sound persist in our brain for about \(\frac{1}{10}\) s.

If velocity of sound in air at a particular temperature = V then,

1. The minimum distance between listener and the reflector for hearing distinctly the echo of a short sound = \(\frac{1}{2}\) x V x \(\frac{1}{5}\) = \(\frac{V}{20}\)
2. Our ears can not recognise separately more than 5 syllables in one second, for monosyllabic sound the minimum distance between the listener and the reflector = \(\frac{1}{2}\) x V \(\frac{1}{5}\) = \(\frac{V}{10}\).

For disyllabic word the distance should be = \(\frac{1}{2}\) x V x \(\frac{2}{5}\) = 2 x \(\frac{V}{10}\) and so on.

Measurement of depth of a sea:

If, velocity of sound in water be V, the depth of the source and hydrophone from the surface of the sea be hQ, the depth of the sea with respect to the source be h, time taken by the sound to reach the hydrophone directly be t₁, and the time taken by the echo be t₂, then depth of the sea from its surface,

h = \(h_0+\frac{V}{2} \sqrt{t_2^2-t_1^2}\)

Measurement of the height of an airplane:

If, an airplane flying horizontally at a height h with a velocity u, produces a loud short sound and heard the echo after time t and velocity of sound in air be V then,

h = \(\frac{t}{2} \sqrt{V^2-u^2}\)

Question 1. A man fires a shot in front of a hill and hears the echo after 3 s. If the velocity of sound in air is 332 m/s, what is the distance between this man and the hill?

Answer:

Let distance of the man from the hill is L m .

2L m is traversed in 3 s.

So, \(\frac{2L}{3}\) = 332 m 3

or, L = \(\frac{332 \times 3}{2}\) = 498 m

Question 2. A pilot fires a shot when the airplane is flying parallel to the earth’s surface at a speed of 72 km/h . If the echo is heard after 4 $ and the velocity of sound in air is 332 m/s, calculate the height of the airplane from the earth’s surface.

Answer:

Velocity of the airplane, u = 72 km/h = 72 x \(\frac{5}{18}\) = 20 m/s

Velocity of sound, V = 332 m/s; time, t = 4 s

Suppose, height of the airplane from the surface of the earth =h

∴ h = \(\frac{t}{2} \sqrt{V^2-u^2}=\frac{4}{2} \sqrt{(332)^2-(20)^2}\) = 662.8

Question 3. A man standing between two parallel mountains hears the first echo after 2 s and the second echo after 5 s of firing a bullet from a gun. If the velocity of sound in air is 332 m/s, what is the distance between the two mountains?

Answer:

Suppose the distance of the man from the nearer mountain is a m and its distance from the distant one is b m.

The distance between the two mountains = (o + b) m .

According to the question, sound traverses a distance 2a in 2 seconds and a distance 2b in 5 seconds.

∴ 2a = 2 x 332 or, a = 332 and 2b = 5 x 332 or, b = 830

∴ distance between the two mountains = 830 + 332 = 1162 m .

Question 4. An airplane is flying at a height of 1200 m with a velocity Of 125 m/s. A high- pitched sound of very short duration is emitted from the plane. How much time does the echo of that sound take to reach the pilot after being reflected from the earth’s surface? Velocity of sound in air = 325 m/s.

Answer:

Suppose, the time gap between emission of sound and its audibility as echo = t.

According to the formula, h = \(=\frac{t}{2} \sqrt{v^2-u^2}\)

t = \(\frac{2 h}{\sqrt{V^2-u^2}}=\frac{2 \times 1200}{\sqrt{325^2-125^2}}\)

= \(\frac{2 \times 1200}{\sqrt{450 \times 200}}=\frac{2 \times 1200}{300}=8 \mathrm{~s}\)

So, pilot hears the echo after 8 s of its emission.

Question 5. An echo repeated four syllables. If the velocity of sound be 335 m/s , find the distance of the reflector.

Answer:

To hear echo for a word with four syllable, the minimum time required = 4 x 1/5 = 4/5 s

∴ The distance of the reflector = \(\frac{\text { distance travels }}{2}=\frac{4 \times 335}{5 \times 2}\) = 134 m

Question 6. Two children are at opposite ends of an aluminium rod. One strikes at one end of the rod with a hammer. Find the ratio of time taken by the sound waves in air and in the aluminium to reach the second child. (Speed of sound in aluminium is 6420 m/s and in air is 338 m/s.

Answer:

Let, l = length of the rod.

Time taken by sound to travel distance l in aluminium rod

⇒ \(t_1=\frac{\text { distance }}{\text { speed of sound in alumimium }}\)

= \(\frac{l}{64210} \mathrm{~s}\)

and time taken by sound to travel distance l in air

t₂ = \(\frac{l}{338}\) s

∴ The ratio of time taken by the sound wave in air and in the aluminium

= \(t_2: t_1=\frac{1}{338}: \frac{1}{6420}=3210: 169\)

Question 7. A man standing 12 m away from a high wall makes a short sound. If velocity of the sound in air is 330 m/s. Check whether he hear the echo or not.

Answer:

The persistence of hearing = 0.1 s

∴ The minimum distance for hearing echo for short sound = \(\frac{0.1 \times 330}{2}\) = 16.5 m

According to the problem as the man standing at a distance 12 m from the wall, he can not hear the echo.

Question 8. Sound beyond the human audio range is produced at a place deep inside the sea. After 5 s, echo is detected by a SONAR at the same place. Calculate the depth of the sea from the place of production of sound. Velocity of sound in sea water is 1500 m/s.

Answer:

Suppose, depth of sea = h m

∴ sound travels 2h m in 5 seconds.

So, sound travels \(\frac{2h}{5}\) m in one second.

∴ \(\frac{2h}{5}\) = 1500 or, h = 1500 x \(\frac{5}{2}\) = 3750 m