Sound

Sound, a form of energy, originates from the vibration of objects. These vibrations propagate through a medium—solid, liquid, or gas—as longitudinal waves, characterized by compressions and rarefactions of the medium’s particles oscillating parallel to the wave’s direction. Unlike light, sound necessitates a material medium for its transmission and travels at varying speeds depending on the medium’s density and elasticity, being generally fastest in solids and slowest in gases.

Sound waves possess distinct characteristics that determine how we perceive them. Frequency, measured in Hertz (Hz), represents the number of vibrations per second and dictates the pitch; higher frequencies are perceived as higher-pitched (shrill), while lower frequencies correspond to lower-pitched (grave) sounds. The time period is the duration of one complete vibration, inversely related to frequency.

Our auditory system, the human ear, is designed to detect and interpret these sound waves. The outer, middle, and inner ear work in concert to convert sound vibrations into electrical signals that the brain processes. Humans typically have an audible range of 20 Hz to 20,000 Hz. Sounds outside this range are termed infrasonic (below 20 Hz) and ultrasonic (above 20,000 Hz), often inaudible to humans but detectable by other animals or specialized equipment.

In contrast, music is typically considered pleasant, arising from regular, periodic vibrations with specific frequencies and organized patterns, forming melodies and harmonies. Sound waves also exhibit reflection, bouncing off surfaces, which can result in an echo, a discernible repetition of the original sound if the reflected sound reaches the listener after a sufficient time delay due to the distance to the reflecting surface.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) Sound can travel in vacuum.
Ans: False.
Correct — Sound requires medium to travel.

(b) Sound is a form of energy.
Ans: True.

(c) Sound can only be produced by vibrating bodies.
Ans: True.

(d) Larger is the amplitude, feeble is the sound.
Ans: False.
Correct — Larger the amplitude, greater is the sound.

(e) The frequency is measured in hertz.
Ans: True.

(f) Loudness depends on frequency.
Ans: False.
Correct — Loudness depends on the amplitude.

(g) Waveforms of two different stringed instruments can be the same.
Ans: False.
Correct—Waveforms of two different stringed instruments cannot be the same.

(h) Female voice is shriller than the male voice.
Ans: True.

(i) A ticking clock sound is heard late when heard through a metal.
Ans: False.
Correct—A ticking clock sounds is heard early when heard through a metal.

2. Fill in the blanks

(a) Sound is produced when a body ——–.

Ans : vibrates
(b) The number of times a body vibrates in one second is called its ——–.

Ans : frequency
(c) The pitch of a sound depends on its —–.

Ans : frequency
(d) Sound can travel in a ——–.

Ans : medium solid, liquid or gas
(e) We can hear sounds of frequency in the range of ——.

Ans : 20 Hz to 20,000 Hz
(f) Sound requires a ——- for propagation.

Ans : medium
(g) Sound travels faster in ——- than in liquids.

Ans : solids
(h) The sound heard after reflection is ——.

Ans : echo
(i) ——- produces sensation in ears.

Ans : Sound

3. Match the following

4. Select the correct alternative 

(a) We can distinguish a shrill sound from a flat sound by its

1.  amplitude
2.  loudness
3.  pitch
4.  none of the above.

(b) We can hear sound of frequency

1.  10 Hz
2.  500 Hz
3.  100,000 Hz
4.  50,000 Hz

(c) Sound cannot travel in

1.  gases
2.  liquids
3.  solids
4.  vacuum

(d) The minimum distance required between the source and the reflector so as to hear the echo in air is

1.  10 m
2. 17 m
3.  34 m
4.  50 m

(e) Wavelength is measured in

1.  kg
2.  second
3.  litre
4. metre

(f) The speed of sound in water is

1.  332 m
2. 1500 m
3.  5000 m s
4.  1000 m s

(g) Sound travels the fastest in

B. Short/Long Answer Questions

Question 1.
What do you mean by a vibratory motion ?
Ans:

Vibratory motion, also known as oscillation, refers to the back-and-forth or to-and-fro movement of an object from its position of rest. This movement occurs repeatedly along the same path around a central point called the mean position or equilibrium position. The object doesn’t permanently move from its location but rather oscillates around it. A key characteristic is that there’s a restoring force that always tries to pull the object back towards its mean position.

Question 2.

What is sound ?

Ans:

Acoustic energy, manifested as waves, originates from the mechanical oscillations of substances. These vibrations generate disturbances within the encompassing environment—be it gaseous, liquid, or solid—which radiate outwards and are detected by hearing mechanisms.

Question 3.

How is sound produced ?

Ans:

Acoustic phenomena originate from the mechanical oscillation of matter. When an object undergoes vibration, it imparts this motion to the adjacent medium (be it gaseous, liquid, or solid), compelling its particles to oscillate as well. These induced oscillations propagate outwards as pressure variations, constituting what we perceive as sound waves.

Question 4.

Describe an experiment to show that each source of sound is a vibrating body.

Ans:

Demonstration: A sounding tuning fork, when brought into contact with a freely hanging, low-mass object, will cause the object to move. Likewise, when a sounding tuning fork is submerged in a liquid, disturbances on the liquid’s surface become apparent.

Interpretation: These observable physical actions—the displacement of the light object and the creation of surface waves in the liquid—serve as direct evidence of the tuning fork’s mechanical vibrations occurring concurrently with sound production. This demonstrates the fundamental principle that the emission of sound is invariably associated with the vibratory motion of the sound-generating body.

Question 5.

Name two sources of sound.

Ans:

Two sources of sound are:

1. A ringing bell: The metal of the bell vibrates when struck, producing sound waves that travel through the air.
2. Human vocal cords: When we speak or sing, air passing over our vocal cords in the larynx causes them to vibrate, generating sound.

Question 6.

How do we produce sound ?

Ans:

1. Exhaled Airflow: Air expelled from the lungs initiates the process, providing the necessary energy for vocal cord vibration.
2. Vocal Cord Vibration: Muscles in the larynx draw the vocal cords closer, narrowing the airway (glottis). Air forced through this constriction causes the vocal cords to vibrate rapidly.
3. Pulsed Airflow: The rapid opening and closing of the vocal cords create pulses of air that travel through the vocal tract (throat, mouth, and nasal cavity).
4. Sound Wave Formation: These air pulses generate pressure variations that propagate outward as sound waves.
5. Vocal Tract Shaping: The configuration of the vocal tract, adjusted by the tongue, lips, and jaw, modifies these sound waves, enabling the production of various sounds for speech and singing.

Question 7.

The bees do not have voice-boxes. How do they produce sound ?

Ans:

Then is how it works 

 Wing Movement notions have  important thoracic muscles that control their  sect movement. These muscles can contract and relax  veritably  snappily, causing the  bodies to  delirium at incredibly high  frequentness –  occasionally hundreds of times per second. 

Air Disturbance As the  bodies beat  fleetly, they push against the air. This  rapid-fire movement creates pressure  swells in the air,  analogous to how a speaker cone produces sound. 

Buzzing Sound These pressure  swells propagate outwards and are what we perceive as the characteristic buzzing sound of  notions. The  frequence of the buzzing sound can vary depending on the size of the  freak and the speed of its wingbeats. Larger  notions tend to have slower wingbeats and produce a lower- pitched buzz, while  lower  notions have faster wingbeats and a higher- pitched buzz.

Question 8.

Can sound travel through a vacuum ? Describe an experiment to explain your answer.

Ans:

Sound necessitates a medium for propagation and cannot travel through a vacuum.

Demonstration: A ringing bell enclosed within a vacuum-sealed container becomes inaudible as the air is evacuated, despite the bell’s continued mechanical operation. Upon the reintroduction of air into the container, the bell’s sound becomes audible once more.

Interpretation: This observation underscores that sound waves, being mechanical in nature, rely on the presence of particles within a medium (such as air) to transmit their energy. In the absence of such particles, as in a vacuum, the vibrations cannot be conveyed, and thus, sound transmission ceases.

Question 9.

Describe an experiment to show that sound can travel in water.

Ans:

Experiment: Submerge two people in a tub of water. One strikes two stones together underwater. The other person, with ears also submerged, can clearly hear the sound.

Conclusion: Sound travels through water. The vibrations from the struck stones are carried by the water molecules to the listener’s ears.

Question 10.

Describe an experiment to show that sound can travel in a solid.

Ans:

Experiment: One person taps one end of a long metal rod; another person with their ear pressed to the other end clearly hears the tap.

Conclusion: Sound travels through solids. The vibrations from the tap are carried by the tightly packed particles of the metal to the listener’s ear.

Question 11.

Can two person hear each other on moon’s surface ? Give reason to support your answer.

Ans:

No, people can’t hear each other directly on the Moon because it has virtually no atmosphere. Sound needs a medium (like air) to travel, and the Moon’s near-vacuum lacks the particles to carry sound waves. Astronauts communicate using radio waves, which don’t need a medium.

Question 12.

What is a longitudinal wave ?

Ans:

In a longitudinal wave, the oscillations of the medium’s particles occur parallel to the wave’s direction of propagation. This motion results in alternating regions of increased density (compressions) and decreased density (rarefactions) within the medium as the wave advances. Sound waves serve as a typical illustration of this wave type.

Question 13.
Define the following terms :
Amplitude, Time period, Frequency.
Ans:

(a) Amplitude (A) : The maximum displacement of a wave on either side of its mean position is called Amplitude. A = XY is amplitude.
(b) Time Period (T) : Time taken to complete one vibration is called Time Period, i.e. from A to B

(c) Frequency (f) or u
Number of oscillations made by a wave in one second is known as its frequency.

Question 14.

Write the audible range of frequency for the normal human ear.

Ans:

The audible range of frequency for the normal human ear is typically from 20 Hertz (Hz) to 20,000 Hertz (Hz), also often written as 20 Hz to 20 kHz.

This range represents the frequencies of sound waves that a healthy human ear can perceive. However, it’s important to note that this range can vary slightly between individuals and tends to decrease with age, particularly at the higher frequency end. Young children can often hear slightly higher frequencies than adults.

Question 15.

What are ultrasonics ? Can you hear the ultrasonic sound ?

Ans:

Ultrasonics are sound waves with frequencies above the human hearing range (typically above 20 kHz).

No, humans generally cannot hear ultrasonic sounds because our ears are not designed to detect such high frequencies.

Question 16.

What are infrasonics ? Can you hear them ?

Ans:

Infrasonics are sound waves with frequencies below the human hearing range (typically below 20 Hz).

No, humans generally cannot hear infrasonic sounds. However, at very high intensities, we might feel them as vibrations or pressure rather than a distinct sound.

Question 17.

How does a bat make use of ultrasonics waves to find its way?

Ans:

Bats navigate using echolocation, a natural sonar system:

1. They produce high-frequency ultrasonic pulses.
2. These sounds reflect off objects in their environment, generating echoes.
3. Their specialized ears capture these echoes, analyzing the time taken for the echo to return, its loudness, and any changes in pitch.
4. Their brain processes this echo information to construct a real-time “sound picture” of their surroundings, enabling them to move around and hunt accurately in darkness.

Question 18.

Name the two characteristics of sound which differentiate two sounds from each other.

Ans:

Two fundamental attributes differentiate distinct sounds:

1. Magnitude (Subjectively experienced as Loudness): This denotes the perceived strength or volume of a sound, directly linked to the amplitude of its sound wave.
2. Rate of Vibration (Subjectively experienced as Pitch): This indicates the perceived highness or lowness of a sound, directly determined by the frequency of its sound wave.

Question 19.

On what factor does the loudness of a sound depend ?

Ans:

The loudness of a sound mainly depends on the amplitude of its sound wave. Larger amplitude means louder sound.

Question 20.

How does the loudness of sound produced depend on the vibrating area of the body ?

Ans:

When a greater surface area of an object vibrates, it produces a louder sound. This occurs because the larger vibrating area moves a greater quantity of the surrounding medium, generating sound waves with a higher amplitude, which we perceive as increased volume.

Question 21.

The outer case of the bell in a temple is made big. Give a reason.

Ans:

The outer case of the bell in a temple is made big for several reasons, all contributing to producing a louder, deeper, and more resonant sound that carries further:

• Larger Vibrating Surface Area: A bigger bell has a significantly larger surface area that vibrates when struck. This larger vibrating area displaces a greater volume of air, creating sound waves with a higher amplitude. As we know, a larger amplitude corresponds to a louder sound.
• Resonance and Fundamental Frequency: The size and shape of the bell influence its resonant frequencies. A larger bell tends to have a lower fundamental frequency, resulting in a deeper sound. This deeper sound can often travel further and is perceived as more solemn and powerful, which is desirable in a temple setting. The larger volume of air within the bell’s cavity also contributes to resonance, amplifying specific frequencies and sustaining the sound longer.
• Increased Mass and Energy Storage: A bigger bell typically has more mass. When struck with the same force, a more massive object stores and releases more energy as vibrations. This greater energy translates into a louder and longer-lasting sound.
• Psychological and Symbolic Significance: Beyond the purely physical aspects, a large bell can also have psychological and symbolic significance in a temple. Its imposing size can evoke a sense of grandeur, importance, and tradition. The deep, resonant sound it produces can create a more profound and spiritual atmosphere, intended to call devotees to prayer and mark significant rituals.

Question 22.

State the factors on which the pitch of a sound depends.

Ans:

The primary factor determining the pitch of a sound is the  frequence of its sound  surge. 

 A lesser  frequence results in the perception of a advanced pitch. 

 A  lower  frequence results in the perception of a lower pitch. 

Question 23.

Differentiate between a high pitch sound and a low pitch sound.

Ans:

A high-pitched sound exhibits a high frequency, meaning its sound waves oscillate rapidly. This results in a sound perceived as sharp, high-toned, or like a treble, such as that of a whistle or a violin.  

Conversely, a low-pitched sound is characterized by a low frequency, indicating that its sound waves oscillate slowly. This leads to a sound perceived as deep, low-toned, or flat, similar to that of a bass drum or a deep male voice.  

The essential difference lies in the oscillation rate of the sound waves. Faster oscillations produce higher pitches, while slower oscillations produce lower pitches.

Question 24.

How does a man’s voice differ from a woman’s voice ?

Ans:

A man’s voice typically has a lower pitch than a woman’s voice. This primary difference stems from the longer and thicker vocal cords in men, which vibrate at a slower rate.

Additionally, men generally possess a larger vocal tract, the resonating space from the vocal cords to the lips. This larger space amplifies lower frequencies, contributing to the deeper timbre or tonal quality often associated with male voices.

Question 25.

Name the characteristic which differentiates two sounds of the same pitch and same loudness.

Ans:

Timbre, also referred to as tone quality or tone color, is the attribute that allows us to differentiate between two sounds that share the same pitch and loudness. It represents the unique auditory signature of a sound-producing source.

Timbre originates from the specific combination and intensity of overtones (harmonics) that exist alongside the fundamental frequency (which dictates the pitch). These supplementary frequencies generate the distinctive tonal texture enabling us to distinguish, for instance, between a flute and a guitar playing the identical note at the same volume level. Each instrument generates a unique mixture of these overtones, bestowing upon it its characteristic sound.

Question 26.

You recognize your friend by hearing his voice on a telephone. Explain.

Ans:

You recognize your friend’s voice on the phone due to its unique timbre (tone quality), pitch patterns, speech habits, and resonance, all of which your brain has learned to associate with them. Even with slight phone distortions, these core characteristics are usually distinct enough for recognition.

Question 27.

A musician recognizes the musical instrument by hearing the sound produced by it, even without seeing the instrument. Which characteristic of sound makes this possible ?

Ans:

The characteristic of sound that makes it possible for a musician to recognize a musical instrument by hearing its sound, even without seeing it, is timbre (also known as tone quality or tone color).

Here’s why:

• Unique Harmonic Content: Each musical instrument produces a fundamental frequency (which determines the pitch) along with a unique set of overtones or harmonics. These overtones are multiples of the fundamental frequency and have different intensities depending on the instrument’s material, shape, and how it’s played.
• Distinctive Waveform: The combination of the fundamental frequency and its specific set of harmonics creates a complex and unique waveform for each instrument. This waveform is what gives each instrument its characteristic sound.
• Brain Recognition: A musician’s brain learns to recognize these subtle differences in the harmonic content and resulting waveform, allowing them to distinguish between instruments even when they play the same note at the same loudness. For example, a flute and a trumpet playing the same “A” will have vastly different timbres due to their different harmonic structures.

Question 28.

Describe an experiment to show the production of sound having low and high pitch.

Ans:

• Experiment: Take two rubber bands of noticeably different lengths and stretch them taut, applying roughly the same amount of pull to each. Pluck each rubber band separately.
• Observation: The longer rubber band will emit a sound that is lower in pitch, while the shorter rubber band will emit a sound that is higher in pitch.
• Explanation: The perceived highness or lowness of a sound (its pitch) is directly determined by the frequency of its vibrations. Shorter vibrating objects, under similar tension, oscillate at a faster rate (higher frequency), producing a higher pitch. Conversely, longer vibrating objects oscillate at a slower rate (lower frequency), resulting in a lower pitch.

Question 29.

How does a musician playing on a flute change the pitch of sound produced by it ?

Ans:

A flautist modifies the pitch of the sound produced by their instrument by manipulating the effective length of the resonating air column within the flute. This is accomplished through:

1. Selective opening and closing of tone holes: The flute’s body features several strategically placed holes. By using their fingers or keys to open or close these apertures, the musician effectively changes the termination point of the vibrating air column.
2. Decreasing the air column’s length (resulting in higher pitch): When more tone holes are opened, the effective length of the air column is reduced. A shorter air column supports higher frequency vibrations, thus generating a higher pitch.
   
3. Increasing the air column’s length (resulting in lower pitch): Conversely, closing more tone holes increases the effective length of the air column. A longer air column resonates at lower frequencies, producing a lower pitch.

Question 30.

Why are musical instruments provided with more than one string ?

Ans:

Musical instruments have multiple strings to:

• Produce a wider range of pitches.
• Enable playing harmonies and chords.
• Offer different tonal qualities (timbre).
• Increase volume and sustain in some cases.

Question 31.

How can the pitch of sound produced in a piano be changed ?

Ans:

In a piano, the pitch of the sound produced is changed by the length, thickness, and tension of the strings that are struck by the hammers.

• Length: Shorter strings produce higher pitches, while longer strings produce lower pitches. This is why the strings in a piano get progressively longer as you move from the treble (high notes) to the bass (low notes) end of the keyboard.
• Thickness (Mass per unit length): Thinner strings vibrate at a higher frequency, resulting in a higher pitch. Conversely, thicker strings vibrate at a lower frequency, producing a lower pitch. In the bass section of a piano, the strings are not only longer but also thicker, often wound with copper wire, to further lower their pitch.
• Tension: Tighter strings vibrate at a higher frequency, leading to a higher pitch. resulting in a lower pitch. Piano tuners adjust the tension of each string using tuning pins to achieve the correct pitch for each note.

Question 32.

Explain why you can predict the arrival of a train by placing your ear on the rails without seeing it.

Ans:

By placing your ear against the rails, you can detect the vibrations and resulting sound waves generated by an approaching train as they travel through the steel. This allows you to hear the train’s approach much earlier than if you were relying on sound traveling through the air, thus enabling prediction of its arrival before it is visible or audibly detectable through the air. However, this action is extremely perilous and should never be attempted.

Question 33.

Write the approximate speed of sound in (i) air, (ii) water and (iii) steel.
Ans:

Question 34.

During a thunderstorm, the sound of a thunder is heard after the lightning is seen. Why ?

Ans:

The delay between seeing lightning and hearing thunder during a storm occurs because light travels at a significantly greater speed than sound. While both phenomena originate nearly simultaneously, the light from the lightning reaches an observer almost instantaneously due to its immense velocity. In contrast, the sound of the thunder propagates through the air at a much slower pace, resulting in a noticeable time lag between the visual and auditory perception of the event.

Question 35.

Describe an experiment to estimate the speed of sound in air.

Ans:

Experiment: Two people stand a measured distance apart. One makes a sound and a visual signal simultaneously. The other times how long it takes to hear the sound after seeing the signal.

Calculation: Speed of sound = Distance / Time.

Repeat and average the time for better accuracy.

Question 36.

Can sound travel through solids and liquids ? In which of these two does it travel faster ?

Ans:

Sound travels faster in solids than in liquids. This is because the particles in solids are more closely packed together and have stronger intermolecular forces compared to liquids. This close arrangement allows vibrations (sound waves) to be transmitted more quickly and efficiently from one particle to the next.   

Solids: Fastest (e.g., steel around 5960 m/s)   

Liquids: Intermediate (e.g., water around 1481 m/s)   

Gases: Slowest (e.g., air around 343 m/s)

Question 37.

What do you mean by reflection of sound ?

Ans:

Reflection of sound is when sound waves bounce off a surface and travel back, like an echo. Hard, smooth surfaces reflect sound well. A noticeable echo happens when there’s enough distance for a slight delay between the original sound and the reflected sound.

Question 38.

State one use of reflection of sound.

Ans:

One significant use of the reflection of sound is echolocation, employed by animals like bats and dolphins to navigate and locate prey. This principle is also used in human-made technologies like sonar for underwater navigation and detection.

Question 39.

What is echo ?

Ans:

An echo is the repetition of a sound caused by the reflection of sound waves from a surface back to the listener. This time delay is determined by the distance between the sound source, the reflecting surface, and the listener.

Essentially, when a sound wave travels and hits a large enough obstacle (like a wall, a cliff, or a building), it bounces back. If this reflected sound wave arrives at your ears at least about 0.1 seconds after the original sound (in air at typical temperatures), your brain perceives it as a separate, repeated sound – an echo.

Question 40.

What minimum distance is required between the source of sound and the reflecting surface to hear an echo ? Give reason.

Ans:

A reflecting surface needs to be at least 17.2 meters away from the sound source to hear a distinct echo in air. This is because our ears need a time delay of about 0.1 seconds to distinguish the echo from the original sound, and sound travels about 344 meters per second in air.

Question 41.

List four substances which are good absorbers of sound.

Ans:

1. Acoustic Foam: Designed specifically to absorb sound waves, often used in recording studios, home theaters, and auditoriums. Its porous and irregular surface traps sound energy and converts it into heat.
2. Fiberglass Insulation: Commonly used in walls and ceilings for thermal insulation, fiberglass is also an excellent sound absorber due to its fibrous structure that allows sound waves to penetrate and lose energy through friction.
3. Heavy Curtains and Drapes: Thick, heavy fabrics with folds can absorb a significant amount of sound, especially higher frequencies. They are often used in theaters and homes to reduce echoes and reverberation.
4. Carpets and Rugs: Soft, porous materials like carpets and rugs absorb sound, particularly impact noise (like footsteps) and higher frequency sounds. Thicker carpets with padding are more effective.

Question 42.

List the measures that you will take when designing a sound-proof room.

Ans:

To design a sound-proof room, I’d focus on:

• Heavy, decoupled walls, ceiling, and floor to block sound.
• Sound-absorbing materials (acoustic panels, carpets, etc.) inside to reduce echoes.
• Airtight seals around doors and windows.
• Solid core doors and soundproof windows.
• Vibration damping using resilient mounts and floating floors.

Essentially, mass, decoupling, absorption, and sealing are key.

C. Numericals

Question 1.
A boy fires a gun and another boy at a distance of 1020 m hears the sound of firing the gun 3 s after seeing its smoke. Find the speed of sound.
Ans:

Question 2.

A boy on a hill A fires a gun. The other boy on hill B hears the sound after 4 s. If the speed of sound is 330 ms-1, find the distance between the two hills.

Ans:

A gunshot is fired by a boy on hill A. Another boy on hill B hears the sound after a 4-second delay. Given that the velocity of sound in the prevailing conditions is 330 meters per second, we can determine the separation between the two hills using the fundamental relationship between distance, speed, and time:

Distance = Speed × Time

Substituting the provided values:

Distance = 330 m/s × 4 s

 Distance = 1320 meters

Thus, the distance separating hill A and hill B is 1320 meters.