Sound Class 9 Notes Science Chapter 11

Sound is an integral part of our daily lives, surrounding us in various forms—from the chirping of birds to the honking of vehicles. But what exactly is sound?

Sound is a form of energy that travels through a medium, such as air, water, or solids, to reach our ears, allowing us to hear. Similar to other energy forms like mechanical or light energy, sound is generated through vibrations and requires a medium to propagate.

• Nature of Sound: Sound is generated by vibrations and travels in waves.
• Energy and Sound: Sound production requires energy, such as when we clap our hands.
• Transmission: Sound needs a medium (air, water, solids) to travel.
• This chapter explores how sound is produced, how it travels, and the role it plays in our everyday lives.

Production of Sound

Sound is produced when the objects are set into vibration which is to and fro motion of an object which results in production of sound.

Activity:

Vibrating tuning fork just touching the suspended Table Tennis ball

• Objective: Observe how vibrations produce sound and affect nearby objects.
• Materials: Tuning fork, rubber pad, small ball (table tennis or plastic), thread, needle.
• Procedure:
  1. Strike the tuning fork on a rubber pad to set it vibrating.
  2. Bring the vibrating fork near your ear and observe the sound.
  3. Touch a vibrating prong with your finger and note the sensation.
  4. Suspend a small ball using a thread. Gently touch the ball with the vibrating fork and observe its movement.
• Observations:
  1. The vibrating fork produces sound.
  2. Touching the prong reveals vibrations.
  3. The ball moves when touched by the vibrating fork.
• Conclusion: Vibrations produce sound and can transfer energy to move other objects.

Sound can be produced by striking a tuning fork and through various actions like plucking, scratching, rubbing, blowing, or shaking different objects. These activities involve setting the objects into vibration, resulting in sound production.

• Vibration refers to the rapid to and fro motion of an object.
• The human voice produces sound through vibrations in the vocal cords.
• The buzzing sound accompanying a bee is generated through a specific mechanism.
• When a stretched rubber band is plucked, it vibrates and produces sound.

Propagation of Sound

Sound Propagation and Waves:

• Sound is created when objects vibrate and can travel through solids, liquids, or gases.
• When something vibrates, it makes the particles around it vibrate too.
• These vibrating particles then pass the vibrations to nearby particles.
• This passing on of vibrations continues, causing sound to travel through the medium.
• Sound waves are like ripples moving through water, but in this case, it's particles in the medium that carry the disturbance.
• Sound waves are mechanical waves because they rely on the movement of particles in the medium.

Sound Propagation in Air:

• Air is the most common medium for sound transmission.
• When a vibrating object moves forward, it creates a region of high pressure called compression.
• Compressions move away from the vibrating object, while backward motion creates a region of low pressure called rarefaction.
• Rapid back-and-forth motion of the object creates a series of compressions and rarefactions in the air, forming the sound wave.
• Compression represents a region of high pressure, while rarefaction represents a region of low pressure.

Compression (C) & Rarefaction (R) of sound

• Pressure is related to the density of particles in the medium: higher density results in higher pressure and vice versa.

Sounds Waves are Longitudinal Wave

• Sound propagates in the medium through a series of compressions (C) and rarefactions (R).
• These regions of closer and further apart coils create longitudinal waves.
• In longitudinal waves, particles of the medium move parallel to the direction of the wave propagation.
• The particles oscillate back and forth around their position of rest without moving from one place to another.
• Sound waves are an example of longitudinal waves.

Transverse Waves:

• Transverse waves are a different type of wave and In transverse waves, particles do not oscillate along the direction of wave propagation but move up and down about their mean position.
• The individual particles of the medium move in a direction perpendicular to the direction of wave propagation.
• Water waves on a pond's surface when a pebble is dropped are an example of transverse waves.
• Light is also a transverse wave, but its oscillations are not related to medium particles, pressure, or density.
• Light waves are not mechanical waves.

Characteristics of a Sound Wave

We can describe a sound wave by its:

• Frequency 
• Amplitude
• Speed

Key Characteristics of Sound Waves:

• Sound waves can be described by their frequency, amplitude, and speed.
• The density and pressure of the medium vary with distance as the sound wave propagates.
• Compressions are regions of high density and pressure, while rarefactions are regions of low pressure.
• Wavelength is the distance between two consecutive compressions or rarefactions. Wavelength is represented by λ (lambda) and it’s SI unit is metre.
• Frequency represents the number of oscillations per unit time and is measured in hertz (Hz) It is usually represented by ν (Greek letter, nu).
• Time period is the time taken for one complete oscillation and is represented by the symbol T.
• Frequency and time period are inversely related.
  

Pitch, Amplitude, and Loudness:

• Pitch is determined by the frequency of the sound wave, where higher frequency corresponds to a higher pitch.

• Amplitude refers to the magnitude of the maximum disturbance in the medium.
• Loudness is determined by the amplitude of the sound wave, with greater amplitude producing a louder sound.
• The loudness of a sound decreases as it travels farther from its source.

Quality and Speed of Sound:

• Quality or timber refers to the characteristic that distinguishes one sound from another with the same pitch and loudness.
• Sound waves with a single frequency are called tones, while those with a mixture of frequencies are called notes.
• The speed of sound is the distance travelled by a point on a wave per unit time.
  
  
  So, Speed of sound = wavelength × frequency.
• The speed of sound remains constant for all frequencies in a given medium under the same conditions.

Intensity of Sound:

• Intensity of sound refers to the amount of sound energy passing through a unit area per second.
• Loudness is the subjective perception of sound intensity by the ear.
• Even sounds with the same intensity can be perceived as different loudness due to variations in the ear's sensitivity.

Speed of Sound In Different Media

• Sound travels through a medium at a finite speed, which is slower than the speed of light.
• The speed of sound depends on the properties of the medium.
• The speed of sound in a medium is influenced by temperature.
• As temperature increases, the speed of sound in the medium also increases.
• The speed of sound varies in different media at a given temperature.
• The speed of sound decreases when transitioning from a solid to a gaseous state.
• Increasing the temperature in a medium generally leads to an increase in the speed of sound.

Reflection of Sound

• Sound waves behave similar to a rubber ball bouncing off a wall when they encounter a solid or liquid surface.
• Just like light, sound also follows the laws of reflection that you have learned about in previous classes.
• When sound is incident on a surface, it reflects in a way that the angles of incidence and reflection are equal with respect to the normal (a line perpendicular to the surface) at the point of incidence.
• These angles and the normal lie in the same plane.
• For sound waves to reflect, they require a relatively large obstacle, whether it is smooth or rough in texture.

Echo

• Shouting or clapping near a suitable reflecting object can produce an echo.
• An echo is the sound we hear when the original sound is reflected back to us.
• Our brain retains the sensation of sound for approximately 0.1 seconds.

Man producing echo

Conditions for Hearing a Distinct Echo:

• To perceive a clear echo, there must be a time interval of at least 0.1 seconds between the original sound and the reflected sound.
• Assuming the speed of sound is 344 m/s at a temperature of 22 ºC in air, the total distance travelled by the sound should be at least 34.4 metres (speed × time).
• For a distinct echo, the minimum distance between the sound source and the reflecting surface should be half of the total distance, i.e., 17.2 metres.
• The required distance for hearing echoes may vary with changes in air temperature.

Echoes can occur more than once due to successive reflections. The rolling of thunder is caused by the sound waves reflecting off multiple surfaces, such as clouds and land.

Reverberation

When sound is produced in a large hall, it continues to exist due to multiple reflections from the walls until its intensity decreases to the point where it cannot be heard anymore. This prolonged presence of sound caused by reflections is known as reverberation. 

Reverberation of Sound

• Excessive reverberation in an auditorium or large hall is considered undesirable. 
• To minimise reverberation, the walls and roof of the auditorium are typically covered with materials that absorb sound, such as compressed fibreboard, rough plaster, or draperies. 
• Additionally, the choice of seat materials takes into account their ability to absorb sound.

Question: What is the distance of the cliff from the person if the speed of sound, v, is taken as 340 m/s?

Solution: Given,

Speed of sound, v = 340 m/s

Time taken for hearing the echo, t = 3 s

Distance travelled by the sound:

Distance = v × t = 340 m/s × 3 s = 1020 m

In 3 seconds, sound has to travel twice the distance between the cliff and the person. 
Hence, the distance between the cliff and the person is:

1020 m2 = 510 m

Uses of Multiple Reflections of Sound

1. Megaphones, loudhailers, horns, and musical instruments like trumpets and shehanais are designed to direct sound in a specific direction instead of spreading it in all directions. These instruments have a tube and a conical opening that reflect sound waves one after another, guiding most of the sound towards the audience.
2. A stethoscope is a medical tool used by doctors to listen to sounds produced inside the body, particularly in the heart or lungs. The sound of the patient's heartbeat reaches the doctor's ears through multiple reflections of sound within the stethoscope.
3. In concert halls, conference halls, and cinema halls, the ceilings are often curved to ensure that sound reaches all corners of the hall. This helps to distribute sound evenly throughout the space. Sometimes, a curved soundboard is placed behind the stage to reflect sound and ensure it spreads across the entire width of the hall.
   Curved ceiling of conference hall

Range of Hearing

• The audible range of sound for humans is between 20 Hz to 20,000 Hz (cycles per second).
• Children under five years old and some animals, like dogs, can hear frequencies up to 25 kHz (kilohertz).
• As people age, their ears become less sensitive to higher frequencies.
• Sounds below 20 Hz are called infrasonic sound or infrasound.
• Infrasound is like hearing the vibrations of a pendulum or the wings of a bee.
• Rhinoceroses, whales, and elephants communicate using infrasound.
• Animals can sense low-frequency infrasound before earthquakes, possibly alerting them to the impending quake.
• Frequencies above 20 kHz are called ultrasonic sound or ultrasound.
• Animals like dolphins, bats, and porpoises produce ultrasound.
• Certain moths can hear high-frequency squeaks of bats, allowing them to avoid capture.
• Rats engage in games using ultrasound.

Applications of Ultrasound

1. Cleaning Hard-to-Reach Objects:

• Ultrasound is used for cleaning parts located in difficult-to-reach places.
• Objects are placed in a cleaning solution, and ultrasonic waves detach and remove particles of dust, grease, and dirt.
• This method ensures thorough cleaning, even in complex shapes like spiral tubes or electronic components.

2. Detecting Cracks and Flaws:

• Ultrasound is used to detect cracks and flaws in metal blocks used in construction.
• Ultrasonic waves pass through the metal block, and detectors detect transmitted and reflected waves.
• The presence of reflected waves indicates the presence of flaws or defects that may weaken the structure.

3. Echocardiography:

• Ultrasonic waves are made to reflect from various parts of the heart, creating an image.
• Echocardiography helps in diagnosing heart conditions and abnormalities.

4. Ultrasonography:

• Ultrasonic waves are used to image internal organs of the human body.
• Changes in tissue density cause ultrasonic waves to reflect, which are then converted into electrical signals.
• These signals generate images of organs, aiding in the detection of abnormalities, such as stones or tumours.
• Ultrasonography is particularly useful for examining the foetus during pregnancy to detect congenital defects and growth abnormalities.

5. Medical Treatment: Kidney Stone Breakage:

• Ultrasound can be employed to break small kidney stones into fine grains.
• The fragmented stones can then be flushed out with urine, avoiding the need for invasive procedures.