Sound

Sound is a form of mechanical energy that produces the sensation of hearing. It travels through a material medium such as air, water, or solids in the form of longitudinal waves.

Sound is produced when an object vibrates. A vibrating object moves to and fro about its mean position. During this motion, it transfers energy to the surrounding particles of the medium.

The production of sound occurs in the following steps:

1. A source such as a guitar string, tuning fork, loudspeaker diaphragm, or drum membrane begins to vibrate.
2. These vibrations disturb the particles of the surrounding medium.
3. The particles nearest to the vibrating source move back and forth, creating regions where particles come closer together called compressions, and regions where particles move farther apart called rarefactions.
4. The disturbance is passed from one particle to the next, transferring energy throughout the medium.
5. When these sound waves reach our ears, they make the eardrum vibrate. The brain interprets these vibrations as sound.

Thus, sound is produced due to the vibrations of an object and propagates through a medium as successive compressions and rarefactions.

Examples:

• When a guitar string is plucked, it vibrates and produces sound.
• When a bell is struck, its metal body vibrates and emits sound.
• The vocal cords in our throat vibrate when we speak, producing our voice.
• The membrane of a drum vibrates when hit, generating sound waves.

A vibrating object continuously moves to and fro about its mean position. As it vibrates, it disturbs the surrounding air particles and produces alternating regions of high and low pressure.

The formation of compressions and rarefactions takes place in the following steps:

1. Suppose a vibrating object moves forward.
2. During this forward motion, it pushes the air particles in front of it closer together.
3. The air particles become crowded, increasing the pressure and density of that region. This region is called a compression (C).
4. Next, the vibrating object moves backward.
5. As it moves backward, the air particles spread farther apart.
6. The pressure and density decrease in this region, forming a rarefaction (R).
7. The object continues vibrating repeatedly, producing alternate compressions and rarefactions.
8. These regions travel through the medium from one particle to another, carrying sound energy without transporting the particles themselves over long distances.

Therefore, a sound wave consists of a continuous sequence of compressions and rarefactions produced by the vibrations of the source.

Sound waves are called longitudinal waves because the particles of the medium vibrate parallel to the direction in which the sound wave travels.

The propagation of a sound wave takes place in the following steps:

1. A vibrating source such as a tuning fork, loudspeaker, or guitar string produces vibrations.
2. These vibrations disturb the nearby particles of the medium.
3. The particles move to and fro about their mean positions in the same direction as the sound wave travels.
4. This back-and-forth motion creates alternate regions of compressions (high pressure) and rarefactions (low pressure).
5. The compressions and rarefactions move forward through the medium, carrying sound energy from one place to another.
6. Although the sound wave travels forward, the particles themselves only oscillate about their mean positions and do not travel with the wave.

Since the direction of particle vibration is the same as the direction of wave propagation, a sound wave is called a longitudinal wave.

The characteristic of sound that helps us identify our friend's voice is quality or timbre.

The identification of a person's voice can be understood in the following steps:

1. Every person has vocal cords that differ slightly in length, thickness, and tension.
2. The shape of the throat, mouth cavity, tongue, and nasal cavity also varies from one person to another.
3. These differences produce a unique waveform even if two people speak with the same pitch and loudness.
4. This unique waveform gives each voice its own characteristic quality or timbre.
5. Our ears and brain recognize this distinctive quality, allowing us to identify familiar voices even without seeing the speaker.

Therefore, even in a dark room where visual identification is not possible, we can easily recognize our friend's voice because of its unique quality (timbre).

Lightning (flash) and thunder are produced at the same time during a thunderstorm. However, we see the flash first and hear the thunder a few seconds later because the speed of light is much greater than the speed of sound.

The explanation is as follows:

1. Lightning produces an intense flash of light and generates thunder simultaneously.
2. The speed of light in air is approximately \[ 3 \times 10^{8}\ \text{m/s} \] which is extremely high.
3. The speed of sound in air at room temperature is approximately \[ 340\ \text{m/s} \] which is much slower than the speed of light.
4. Because light travels almost instantaneously over distances encountered on Earth, the flash reaches our eyes almost immediately.
5. The sound waves travel much more slowly through the air, so they require a few seconds to reach our ears.
6. Therefore, we observe the lightning first and hear the thunder after a short delay.

Hence, the delay between seeing lightning and hearing thunder is due to the enormous difference between the speeds of light and sound, not because they are produced at different times.

Lower frequency: \[ \nu_{1}=20\,\text{Hz} \]

Higher frequency: \[ \nu_{2}=20\,\text{kHz}=20\times10^{3}\,\text{Hz}=20000\,\text{Hz} \]

Speed of sound: \[ v=344\,\text{m s}^{-1} \]

\[ v=\lambda\nu \]

Rearranging the formula,

\[ \lambda=\frac{v}{\nu} \]

Let the length of the aluminium rod be \[ L. \]

Speed of sound in air: \[ v_{\text{air}}=344\,\text{m s}^{-1} \]

Speed of sound in aluminium: \[ v_{\text{Al}}=6420\,\text{m s}^{-1} \]

Frequency of the source, \[ f=100\,\text{Hz} \]

Time, \[ t=1\,\text{minute}=60\,\text{s} \]

• \(f\) = frequency
• \(n\) = number of vibrations
• \(t\) = time

Yes. Sound follows the same laws of reflection as light.

When sound waves strike a hard and smooth surface, they are reflected back into the same medium. The reflection of sound obeys the following two laws:

1. First Law of Reflection
   The angle of incidence is equal to the angle of reflection.

   \[ \boxed{\angle i=\angle r} \]

2. Second Law of Reflection
   The incident sound wave, the reflected sound wave, and the normal at the point of incidence all lie in the same plane.

Therefore, the reflection of sound is completely analogous to the reflection of light. The only important difference is that sound requires a material medium for propagation, whereas light can travel through vacuum.

Example: When a person shouts towards a large wall or a mountain, the sound waves strike the surface and are reflected back. If the reflected sound reaches the listener after a sufficient time interval, it is heard as an echo.

Yes. An echo is heard sooner on a hotter day.

The explanation is given below:

1. When a person produces a sound, the sound waves travel to the reflecting surface.
2. After striking the surface, the sound waves are reflected back towards the listener.
3. The total distance travelled by the sound is twice the distance between the source and the reflecting surface.
4. On a hotter day, the temperature of air is higher.
5. As the temperature increases, the speed of sound in air also increases according to the relation:

   \[ v=331+0.6T \]

6. Since the distance remains unchanged but the speed increases, the time taken by the sound to travel to the obstacle and back decreases.
7. Hence, the reflected sound reaches the listener earlier.

Therefore, although the distance between the source and the reflecting surface remains the same, the echo is heard earlier on a hotter day because sound travels faster in warm air.

• If speed increases, time decreases.
• If speed decreases, time increases.

Reflection of sound has numerous practical applications. Two important applications are explained below.

SONAR (Sound Navigation and Ranging)

SONAR is used in ships and submarines to measure the depth of the sea and to detect underwater objects such as rocks, submarines, icebergs, and shoals of fish.

It works on the principle of reflection of sound. A pulse of ultrasonic sound is transmitted into the water. When it strikes an underwater object, it gets reflected back as an echo. The time taken by the echo is measured to determine the distance of the object.

\[ \boxed{\text{Distance}=\frac{v\times t}{2}} \]

where

• \(v\) = speed of sound in water
• \(t\) = total time taken by the echo

Design of Auditoriums and Concert Halls

Large auditoriums, theatres, conference halls, and lecture rooms are carefully designed so that reflected sound reaches every listener clearly.

Curved ceilings, sound reflectors, and specially designed walls distribute sound uniformly throughout the hall. At the same time, sound-absorbing materials are used to reduce excessive reflections and prevent echoes or reverberation.

This improves the clarity, loudness, and quality of sound for the audience.

• Stethoscope used by doctors.
• Megaphones and loudhailers.
• Hearing aids.
• Soundboards in musical instruments.
• Whispering galleries.
• Ultrasonic imaging and industrial testing.

Height of the tower, \[ h=500\,\text{m} \]

Acceleration due to gravity, \[ g=10\,\text{m s}^{-2} \]

Speed of sound, \[ v=340\,\text{m s}^{-1} \]

Speed of sound, \[ v=339\,\text{m s}^{-1} \]

Wavelength, \[ \lambda=1.5\,\text{cm} \]

Converting centimetres into metres,

\[ 1.5\,\text{cm} =\frac{1.5}{100}\,\text{m} =1.5\times10^{-2}\,\text{m} \]

Reverberation is the persistence of sound in an enclosed space due to multiple reflections of sound waves from walls, ceilings, floors, and other hard surfaces, even after the original sound has stopped.

The phenomenon occurs in the following steps:

1. A sound is produced inside a room or hall.
2. The sound waves strike the walls, ceiling, floor, and furniture.
3. The sound waves are reflected repeatedly from these surfaces.
4. The reflected waves continue to reach the listener's ears for a short time after the original sound has ceased.
5. As a result, the sound appears prolonged or blurred. This effect is known as reverberation.

For example, in an empty classroom, auditorium, or large hall, a person's voice may continue to be heard for a short time due to repeated reflections.

Reverberation can be reduced by using materials that absorb sound instead of reflecting it.

• Cover the floor with carpets, rugs, or mats to absorb reflected sound.
• Hang thick curtains, draperies, or fabric coverings on windows and walls.
• Use cushioned chairs and upholstered furniture that absorb sound energy.
• Install acoustic panels, cork sheets, foam panels, or fibre boards on walls and ceilings.
• Provide false ceilings or wooden acoustic panels to reduce multiple reflections.
• Design auditoriums with properly shaped walls and ceilings so that sound is distributed uniformly without excessive reflection.

Loudness is the sensation produced in the ear that enables us to distinguish between a loud sound and a soft sound. It represents how strong or weak a sound is perceived by a listener.

Loudness is directly related to the intensity of sound reaching the ear. A sound having greater energy is generally heard as louder than a sound having lower energy.

For example, the sound of a drum is louder than the sound of a flute because the drum produces vibrations with much larger amplitudes.

1. Amplitude of Vibration (Most Important Factor)

   Loudness mainly depends on the amplitude of the vibrating source.

   • Greater amplitude → greater energy → louder sound.
   • Smaller amplitude → less energy → softer sound.
2. Distance from the Source

   As the distance from the source increases, the sound energy spreads over a larger area. Consequently, the intensity and loudness decrease.

3. Sensitivity of the Ear

   Different individuals may perceive the same sound differently because the sensitivity of human ears varies from person to person.

4. Nature of the Medium

   The medium through which sound travels also affects loudness. Sound generally travels more efficiently through solids than through gases because solids transmit sound energy more effectively.

5. Intensity of the Sound Wave

   A sound wave carrying greater energy has higher intensity and is perceived as louder by the human ear.

Ultrasound is widely used for cleaning delicate and complex objects by the process of cavitation.

The cleaning process takes place in the following steps:

1. The object to be cleaned is immersed in a tank containing a suitable cleaning liquid.
2. Ultrasonic waves of very high frequency are passed through the liquid.
3. These ultrasonic waves produce millions of tiny bubbles in the liquid.
4. The bubbles rapidly collapse, producing minute shock waves and high-pressure jets.
5. These shock waves remove dust, grease, oil, dirt, and other impurities even from very small holes, cracks, and narrow grooves.
6. The object becomes thoroughly cleaned without causing any damage to its surface.

Therefore, ultrasound provides an efficient and non-destructive method for cleaning delicate objects that are difficult to clean by ordinary methods.

• Cleaning surgical and medical instruments.
• Cleaning jewellery such as rings, necklaces, and watches.
• Cleaning electronic components and circuit boards.
• Cleaning laboratory equipment.
• Cleaning precision machine parts and automobile components.

Defects inside a metal block are detected using the reflection of ultrasonic waves.

The detection process takes place in the following steps:

1. An ultrasonic transmitter sends high-frequency ultrasonic waves into the metal block.
2. The ultrasonic waves travel through the metal.
3. If the metal block is free from defects, the waves travel to the opposite surface and are reflected back to the receiver.
4. If the block contains an internal crack, cavity, air gap, or other defect, the ultrasonic waves are reflected from that defect before reaching the opposite surface.
5. The reflected waves (echoes) are received by the ultrasonic detector.
6. The time taken by the echo to return is measured.
7. Using the speed of ultrasound in the metal, the position and approximate size of the defect are determined.

Thus, ultrasound makes it possible to detect internal defects without cutting or damaging the metal block. This technique is known as Non-Destructive Testing (NDT).

• Inspection of railway tracks.
• Testing aircraft wings and engine components.
• Inspection of bridges and steel structures.
• Testing pressure vessels and pipelines.
• Quality control in manufacturing industries.