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Chapter 12: Sound
Our world is filled with sounds from diverse sources, enabling communication and interaction. Sound is a form of energy that stimulates the sensation of hearing in our ears. Like other forms of energy (mechanical, light, etc.), sound can be produced through energy conversion. For instance, when you clap your hands, you convert muscular energy into sound energy. Understanding how sound is produced, travels, and is perceived is crucial.
Production Of Sound
Sound is always produced by objects that are vibrating. Vibration refers to the rapid to-and-fro (or back-and-forth) motion of an object or a part of an object.
Observations indicating Vibration Causes Sound:
- Striking a tuning fork on a rubber pad sets its prongs into vibration, producing sound. Touching a vibrating tuning fork reveals its movement.
- Touching the surface of water with a vibrating tuning fork causes ripples. Dipping the vibrating prongs into water creates splashes. Both effects are evidence of vibration.
- The human voice is produced by the vibration of vocal cords.
- The buzzing sound of a bee is due to the vibration of its wings.
- Plucking a stretched rubber band makes it vibrate and produce sound.
- Musical instruments produce sound through the vibration of specific parts (e.g., strings of a guitar, membrane of a drum, air column in a flute).
Essentially, any action like plucking, scratching, rubbing, blowing, striking, or shaking that causes an object to vibrate can produce sound.
Propagation Of Sound
Once sound is produced by a vibrating object, it needs a way to travel from the source to our ears. Sound travels through a medium.
A medium is the substance (solid, liquid, or gas) that transmits the sound wave. Sound is generated at a point and moves through the medium to the listener.
When an object vibrates, it disturbs the particles of the medium surrounding it. These particles, in turn, disturb their neighbouring particles, transferring the vibration. The particles of the medium themselves do not travel from the source to the ear; instead, the disturbance (vibration) is transmitted through the medium as a wave.
Sound can be visualised as a wave that travels through a medium. Since sound waves require a material medium for their propagation and involve the motion of medium particles, they are classified as mechanical waves.
In air (a common medium), when a vibrating object moves forward, it pushes the air particles in front, creating a region where particles are crowded together and pressure is high. This region is called a compression (C). When the object moves backward, it creates a region where particles are spread apart and pressure is low. This region is called a rarefaction (R).
As the source vibrates back and forth rapidly, it creates a series of alternating compressions and rarefactions that propagate through the medium. This forms the sound wave. Therefore, the propagation of sound can be viewed as the propagation of variations in the density or pressure of the medium.
Sound Needs A Medium To Travel
Sound is a mechanical wave, meaning it requires a material medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum.
Demonstration (Bell Jar Experiment): If an electric bell is placed inside an airtight glass bell jar connected to a vacuum pump, the sound of the bell can be heard initially. As the air is pumped out of the jar, the sound becomes fainter. If all the air were removed (creating a vacuum), the sound of the bell would no longer be heard, even though the hammer is still striking the bell. This proves that sound needs a medium like air to propagate.
Sound Waves Are Longitudinal Waves
Waves can be classified based on the direction of vibration of the medium particles relative to the direction of wave propagation.
In longitudinal waves, the particles of the medium vibrate back and forth in a direction that is parallel to the direction in which the wave is travelling. Compressions and rarefactions are characteristic of longitudinal waves. Sound waves are longitudinal waves because the particles in the medium (like air molecules) oscillate back and forth along the path of the sound propagation.
In transverse waves, the particles of the medium vibrate in a direction that is perpendicular to the direction in which the wave is travelling. Waves on the surface of water (ripples) are examples of transverse waves, where water particles move up and down as the wave moves horizontally. Light is also a transverse wave, but it is not a mechanical wave as it does not require a medium to travel.
Characteristics Of A Sound Wave
Sound waves have several properties that describe them. The main characteristics are frequency, amplitude, and speed.
Sound propagation involves oscillations in the density or pressure of the medium. A graphical representation of a sound wave shows these variations.
- Wavelength ($\lambda$): The distance between two consecutive compressions (regions of high density/pressure, represented by peaks or crests in the graph) or two consecutive rarefactions (regions of low density/pressure, represented by valleys or troughs in the graph). Its SI unit is metre (m).
- Frequency ($\nu$ or $f$): The number of complete oscillations (changes from maximum density to minimum and back to maximum) per unit time. It tells us how frequently the medium particles oscillate. If $T$ is the time period, then $\nu = \frac{1}{T}$. Its SI unit is hertz (Hz), where 1 Hz = 1 oscillation per second. A higher frequency corresponds to a higher pitch.
- Time Period ($T$): The time taken for one complete oscillation of the density or pressure of the medium. It is the time taken for two consecutive compressions or rarefactions to pass a fixed point. Its SI unit is second (s). The relationship between frequency and time period is $\nu = 1/T$.
- Amplitude ($A$): The magnitude of the maximum disturbance or displacement of the particles in the medium from their average (mean) position on either side. It is represented by the height of the crest or depth of the trough in the graphical representation. For sound, its unit is the unit of density or pressure. A larger amplitude corresponds to a louder sound.
Properties of Sound related to Wave Characteristics:
- Pitch: How the brain interprets the frequency of sound. Higher frequency means higher pitch (e.g., a high-pitched voice or sound). Objects vibrating faster produce higher frequency sounds.
- Loudness/Softness: Determined primarily by the amplitude of the sound wave. Larger amplitude means louder sound, smaller amplitude means softer sound. Loudness decreases as the sound wave spreads out from the source and its amplitude decreases.
- Quality or Timbre: That characteristic of sound that allows us to distinguish between two sounds of the same pitch and loudness. It depends on the shape of the sound wave (waveform), which is influenced by the mixture of different frequencies present in the sound (harmonics). A pure tone is a sound of a single frequency. Musical notes are combinations of frequencies and are generally pleasant, while noise is unpleasant.
The speed of sound ($v$) is related to its wavelength ($\lambda$) and frequency ($\nu$) by the equation:
$$ \text{Speed} = \text{Wavelength} \times \text{Frequency} $$ $$ v = \lambda \times \nu $$The speed of sound in a given medium under the same physical conditions is generally constant for all frequencies.
Example 12.1. A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?
Answer:
Given:
- Frequency, $\nu = 2 \text{ kHz} = 2 \times 1000 \text{ Hz} = 2000 \text{ Hz}$
- Wavelength, $\lambda = 35 \text{ cm} = 35/100 \text{ m} = 0.35 \text{ m}$
First, calculate the speed of the sound wave using $v = \lambda \nu$:
$v = 0.35 \text{ m} \times 2000 \text{ Hz} = 700 \text{ m/s}$.
Now, calculate the time ($t$) taken to travel a distance ($d$) of 1.5 km.
Distance, $d = 1.5 \text{ km} = 1.5 \times 1000 \text{ m} = 1500 \text{ m}$.
Time = Distance / Speed
$t = \frac{d}{v} = \frac{1500 \text{ m}}{700 \text{ m/s}} = \frac{15}{7} \text{ s} \approx 2.14 \text{ s}$.
It will take approximately 2.14 seconds for the sound wave to travel 1.5 km.
Intensity of Sound: The amount of sound energy passing each second through unit area is called the intensity of sound. While often confused with loudness, intensity is an objective physical measure (energy per unit area per unit time), whereas loudness is a subjective physiological response of the ear to sound intensity.
Speed Of Sound In Different Media
Sound travels at a finite speed, which is much slower than the speed of light (e.g., the flash of lightning is seen before the thunder is heard).
The speed of sound depends on the properties of the medium (e.g., density, elasticity) and its temperature.
- Speed of sound is generally fastest in solids, slower in liquids, and slowest in gases. This is because particles are closer together and interact more strongly in solids, allowing vibrations to be transmitted more efficiently.
- In any given medium, the speed of sound increases with increasing temperature. For instance, sound travels faster in warm air than in cold air.
Sonic Boom: When an object (like a supersonic aircraft or bullet) travels faster than the speed of sound in the medium, it creates shock waves that carry a large amount of energy. These shock waves produce a very loud and sharp sound called a 'sonic boom', which can be powerful enough to cause damage.
| State | Substance | Speed in m/s |
|---|---|---|
| Solids | Aluminium | 6420 |
| Nickel | 6040 | |
| Steel | 5960 | |
| Iron | 5950 | |
| Brass | 4700 | |
| Glass (Flint) | 3980 | |
| Liquids | Water (Sea) | 1531 |
| Water (distilled) | 1498 | |
| Ethanol | 1207 | |
| Methanol | 1103 | |
| Gases | Hydrogen | 1284 |
| Helium | 965 | |
| Air | 346 | |
| Oxygen | 316 | |
| Sulphur dioxide | 213 |
Reflection Of Sound
Similar to light, sound waves can bounce off surfaces. This phenomenon is called the reflection of sound.
Sound reflection follows the same laws as light reflection: the angle of incidence equals the angle of reflection, and the incident sound direction, the reflected sound direction, and the normal to the surface at the point of incidence all lie in the same plane.
Reflection of sound requires an obstacle or surface of a sufficient size (relative to the wavelength of the sound) which can be smooth or rough.
Echo
An echo is the phenomenon of hearing the same sound again after it is reflected from a distant obstacle (like a tall building or a mountain).
The sensation of sound persists in our brain for about 0.1 seconds. To hear a distinct echo, the reflected sound must reach the ear at least 0.1 seconds after the original sound is produced. If the time interval is less than 0.1 s, the original sound and the reflected sound merge, resulting in a prolonged sound (reverberation) rather than a distinct echo.
Given the speed of sound (e.g., 344 m/s in air at 22°C), the total distance travelled by the sound (to the reflecting surface and back to the listener) must be at least: Distance = Speed $\times$ Time = 344 m/s $\times$ 0.1 s = 34.4 m.
Since the sound travels to the obstacle and back, the minimum distance between the source of sound and the reflecting surface needed to hear a distinct echo is half of this total distance: Minimum distance = 34.4 m / 2 = 17.2 m.
This minimum distance changes with the speed of sound, which varies with temperature and medium.
Multiple echoes can occur due to successive reflections from several surfaces. The rolling of thunder is often due to multiple reflections of sound from clouds and the ground.
Example 12.2. A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m s⁻¹?
Answer:
Given:
- Time taken to hear the echo, $t = 2$ s. This is the total time for the sound to travel from the source to the cliff and back.
- Speed of sound, $v = 346$ m s⁻¹.
The total distance travelled by the sound is $d_{\text{total}} = v \times t = 346 \text{ m s}^{-1} \times 2 \text{ s} = 692 \text{ m}$.
This total distance is twice the distance between the person and the cliff (sound travels to the cliff and then back).
Distance of the cliff from the person, $d_{\text{cliff}} = \frac{d_{\text{total}}}{2} = \frac{692 \text{ m}}{2} = 346 \text{ m}$.
The cliff is 346 m away from the person.
Reverberation
In large halls or auditoriums, sound created by a source may persist for a while due to repeated reflections from the walls, ceiling, and floor. This persistence of sound due to multiple reflections is called reverberation.
Excessive reverberation can make sound unclear or distorted in a hall. To reduce reverberation, the surfaces of auditoriums (walls, ceiling) are often covered with sound-absorbing materials like compressed fibreboard, rough plaster, heavy curtains, or acoustic tiles. The seat materials are also chosen for their sound absorption properties.
Uses Of Multiple Reflection Of Sound
Multiple reflections of sound can be useful in specific applications:
- Megaphones, Loudhailers, Horns, Trumpets, Shehanais: These instruments use tubes followed by conical openings. Sound waves from the source undergo successive reflections within the tube and conical part, which directs the sound waves efficiently forward, preventing them from spreading out in all directions.
- Stethoscope: A medical instrument used by doctors to listen to internal body sounds. Sound from the patient's body (like heartbeat) travels through the tubes of the stethoscope and reaches the doctor's ears via multiple reflections within the tubes.
- Curved Ceilings and Soundboards in Halls: In concert halls or cinema halls, the ceilings are often curved, or curved soundboards are placed behind the stage. These curved surfaces reflect sound waves to distribute them more evenly throughout the hall, ensuring that sound reaches all parts of the audience.
Range Of Hearing
Not all frequencies of sound are audible to the human ear. The range of frequencies that the average human ear can hear is called the audible range.
The audible range for the average human ear is approximately from 20 Hz to 20,000 Hz (or 20 kHz).
- Children and some animals (like dogs) can hear frequencies slightly higher than 20 kHz (up to about 25 kHz).
- As people age, their sensitivity to higher frequencies tends to decrease.
Sounds with frequencies below the audible range (below 20 Hz) are called infrasonic sound or infrasound. Examples of sources include vibrating pendulums (though typically too low in amplitude to hear), elephants, whales, and sounds produced by earthquakes before the main shock waves arrive (which may alert animals).
Sounds with frequencies above the audible range (above 20 kHz) are called ultrasonic sound or ultrasound. Examples of sources include dolphins, bats, and porpoises, which use ultrasound for communication and navigation. Some moths can detect bat ultrasound and take evasive action.
Applications Of Ultrasound
Ultrasound waves, with their high frequencies, have characteristics that make them useful in various industrial and medical applications. They can travel along relatively straight paths and be focused over short distances even when obstacles are present (unlike low-frequency sound, which tends to bend around obstacles).
Uses of Ultrasound:
- Cleaning: Used to clean objects with intricate shapes or parts located in hard-to-reach areas (e.g., electronic components, spiral tubes). Objects are placed in a cleaning solution, and ultrasound waves are passed through it. The high-frequency vibrations detach dust, grease, and dirt particles.
- Detecting Flaws in Metal Blocks: Ultrasound waves can penetrate metal blocks. If there are cracks or flaws inside, the ultrasound waves are reflected back from the defective regions. A detector receives the reflected waves, indicating the location and size of the flaw. This is important for ensuring the structural integrity of components used in construction and machinery.
- Echocardiography: Ultrasound waves are reflected from different parts of the heart to create images of the heart structure.
- Ultrasonography: An instrument called an ultrasound scanner uses ultrasonic waves to create images of internal organs (liver, gall bladder, kidneys, uterus, etc.). The waves reflect from regions where there is a change in tissue density, and these reflections are processed to form images. This technique is used for diagnosis (e.g., detecting stones or tumours) and for monitoring foetal development during pregnancy.
- Breaking Kidney Stones: High-intensity ultrasound can be used to break up small kidney stones into tiny fragments, which can then be passed out naturally through urine.
Sonar
SONAR (Sound Navigation And Ranging) is a technology that uses ultrasonic waves to determine the distance, direction, and speed of underwater objects. It is widely used in ships and submarines.
Working Principle: A Sonar device has a transmitter and a detector. The transmitter sends out powerful ultrasonic pulses into the water. These pulses travel through the water, reflect off underwater objects (seabed, hills, valleys, submarines, icebergs, sunken ships), and return as echoes to the detector.
The detector converts the received ultrasonic echoes into electrical signals, which are analysed. By measuring the time interval ($t$) between the transmission of the pulse and the reception of the echo, and knowing the speed of sound ($v$) in water, the distance ($d$) to the object can be calculated.
The total distance travelled by the ultrasound pulse is twice the distance to the object (down and back). So, $2d = v \times t$.
Therefore, the distance to the object is $d = \frac{v \times t}{2}$. This method is called echo-ranging.
Sonar is used for determining the depth of the sea, mapping the seabed, and locating underwater objects.
Example 12.3. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship?
Answer:
Given:
- Time interval between transmission and reception of echo, $t = 3.42$ s. This is the total time for the sound to go down to the seabed and come back up.
- Speed of ultrasound in seawater, $v = 1531$ m/s.
Let the distance of the seabed from the ship be $d$. The total distance travelled by the sound is $2d$.
Using the formula $2d = v \times t$:
$2d = 1531 \text{ m/s} \times 3.42 \text{ s}$
$2d = 5236.02 \text{ m}$
$d = \frac{5236.02 \text{ m}}{2} = 2618.01 \text{ m}$.
The distance of the seabed from the ship is approximately 2618 m or 2.618 km.
Bats also use echo-ranging (echolocation) with ultrasound to navigate and find prey in the dark. They emit high-pitched ultrasonic squeaks, and detect and interpret the returning echoes to locate objects.
Structure Of Human Ear
The human ear is a remarkable and sensitive organ responsible for detecting sound waves and converting them into signals that our brain interprets as sound.
The human ear is typically divided into three main parts:
- Outer Ear:
- Consists of the pinna and the auditory canal.
- The pinna (the visible outer part) collects sound waves from the surroundings.
- The auditory canal (ear canal) is a tube that funnels the collected sound waves towards the eardrum.
- Middle Ear:
- Separated from the outer ear by the eardrum (tympanic membrane).
- Contains three tiny bones: the hammer, anvil, and stirrup.
- Sound waves reaching the eardrum cause it to vibrate.
- These vibrations are transmitted and amplified by the three bones in the middle ear.
- Inner Ear:
- Contains the cochlea (a spiral-shaped structure) and structures related to balance.
- The middle ear bones transmit the amplified vibrations to the inner ear.
- Inside the cochlea, these pressure variations are converted into electrical signals.
- These electrical signals travel along the auditory nerve to the brain.
The brain processes these electrical signals, allowing us to perceive and interpret them as different sounds.
Intext Questions
Page No. 162
Question 1. How does the sound produced by a vibrating object in a medium reach your ear?
Answer:
Page No. 163
Question 1. Explain how sound is produced by your school bell.
Answer:
Question 2. Why are sound waves called mechanical waves?
Answer:
Question 3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Answer:
Page No. 166
Question 1. Which wave property determines (a) loudness, (b) pitch?
Answer:
Question 2. Guess which sound has a higher pitch: guitar or car horn?
Answer:
Question 1. What are wavelength, frequency, time period and amplitude of a sound wave?
Answer:
Question 2. How are the wavelength and frequency of a sound wave related to its speed?
Answer:
Question 3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
Answer:
Question 4. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?
Answer:
Question 1. Distinguish between loudness and intensity of sound.
Answer:
Page No. 167
Question 1. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Answer:
Page No. 168
Question 1. An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 $m s^{-1}$?
Answer:
Page No. 169
Question 1. Why are the ceilings of concert halls curved?
Answer:
Page No. 170
Question 1. What is the audible range of the average human ear?
Answer:
Question 2. What is the range of frequencies associated with
(a) Infrasound?
(b) Ultrasound?
Answer:
Page No. 172
Question 1. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?
Answer:
Exercises
Question 1. What is sound and how is it produced?
Answer:
Question 2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.
Answer:
Question 3. Cite an experiment to show that sound needs a material medium for its propagation.
Answer:
Question 4. Why is sound wave called a longitudinal wave?
Answer:
Question 5. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?
Answer:
Question 6. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
Answer:
Question 7. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 $m s^{-1}$.
Answer:
Question 8. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.
Answer:
Question 9. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Answer:
Question 10. Does sound follow the same laws of reflection as light does? Explain.
Answer:
Question 11. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
Answer:
Question 12. Give two practical applications of reflection of sound waves.
Answer:
Question 13. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 $m s^{-2}$ and speed of sound = 340 $m s^{-1}$.
Answer:
Question 14. A sound wave travels at a speed of 339 $m s^{-1}$. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Answer:
Question 15. What is reverberation? How can it be reduced?
Answer:
Question 16. What is loudness of sound? What factors does it depend on?
Answer:
Question 17. Explain how bats use ultrasound to catch a prey.
Answer:
Question 18. How is ultrasound used for cleaning?
Answer:
Question 19. Explain the working and application of a sonar.
Answer:
Question 20. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Answer:
Question 21. Explain how defects in a metal block can be detected using ultrasound.
Answer:
Question 22. Explain how the human ear works.
Answer: