Sound Class 9 Notes Science Chapter 11

In our daily lives, we encounter various sounds from different sources like people, birds, machines, and more. Sound is a type of energy that allows us to hear through our ears. Similar to mechanical and light energy, sound is also a form of energy. Have you ever wondered if it's possible to make a sound without expending our own energy? Additionally, which form of energy do we use to generate sound? In this chapter, we will discover how sound is created and how it travels through a medium, such as air or water, to reach our ears.

Production of Sound

Sound is a form of energy that produces a sensation of hearing in our ears.  

• Sound is created when objects vibrate or move back and forth.
• Tuning forks and actions like plucking, scratching, rubbing, blowing, or shaking objects can produce sound.
• These activities make objects vibrate, which leads to the production of sound.
• Vibration refers to the quick back and forth motion of an object.
• Our voice produces sound through vibrations in the vocal cords.
• Bees create buzzing sounds using a unique mechanism.
• When we pluck a stretched rubber band, it vibrates and makes a sound.

Propagation of Sound

• Sound is created when objects vibrate.
• Sound can travel through solids, liquids, or gases.
• Sound moves from where it's produced to where it's heard.
• Vibrating objects cause the particles in the surrounding medium to vibrate.
• The vibrating particles push and pull on nearby particles, creating a chain reaction.
• Waves are disturbances that pass through a medium, causing neighbouring particles to move.
• When sound travels, the particles in the medium don't actually move forward but pass on the disturbance.
• Sound waves are mechanical waves that involve the motion of particles in the medium.

Sound Propagation in Air:

• When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in the following figure. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Compression(C) and Rarefaction(R) of sound

• As the object moves back and forth rapidly, a series of compressions and rarefactions are created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure.

Sounds Waves are Longitudinal Waves

Longitudinal Waves:

• Sound travels through compressions and rarefactions in the medium.
• These compressions and rarefactions create longitudinal waves.
• In longitudinal waves, particles move parallel to the direction of the wave.
• The particles oscillate around their resting position without changing their location.
• Sound waves are an example of longitudinal waves.

Transverse Waves:

• Transverse waves are a different kind of wave.
• In transverse waves, particles don't move in the same direction as the wave.
• Instead, particles move up and down while the wave moves forward.
• Examples of transverse waves include water waves when a pebble is dropped in a pond.
• Light is also a transverse wave, but it's not related to the movement of particles in a medium.
• Light waves are not mechanical waves.

Characteristics of a Sound Wave

• Wavelength: Distance between two consecutive peaks (crest) or and troughs is called wavelength. Wavelength is represented by λ (lambda) and its SI unit is metres.
• Frequency: The number of oscillations of sound waves per unit time is called its frequency. A number of peaks or troughs per unit of time will give frequency. It is represented by ν (nu) and its SI unit is Hertz (Hz).
• Time Period: The time taken to complete one oscillation is called its time period. Its unit is second and is represented by T.
• Relation of frequency and time period: Time period is the reciprocal of Frequency.

• Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. Its unit is metre.
• Softness or Loudness of Sound: If the amplitude is smaller then the sound will be softer and if it is larger then the sound will be louder. Higher amplitude helps the sound wave in traveling faster.
•  Speed of sound: It is the distance which compression or a rarefaction travels per unit of time.
  Speed of sound  is given by:
  
  
  So, Speed = Wavelength × Frequency 

Speed of Sound In Different Media

• The speed of the sound remains almost the same for all frequencies in a given medium under the same physical condition.
•  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.
  
  Speed of Sound in Different Media at 25º C
  

Reflection of Sound

Sound reflects off a surface in the same way as light reflects and follows the same rules of reflection. The incident sound and the reflected sound make equal angles with the normal and all three are in the same plane.

Reflection of Sound

Echo

In bigger monuments and in large empty halls you can hear reflections of your sound after you speak something. This effect is known as echo. The sensation of sound persists in our brain for 0.1 seconds, so to hear an echo the time difference between the original sound and reflected sound should be more than that. As you know the speed of sound in air = 344 m/s And 344 × 0.1 = 34.4 metres So, the minimum distance required to hear an echo from a reflecting wall or surface should behalf of 34.4, which means it should be 17.2 metres.

Reverberation

When sound is made in a big hall, it stays around because it bounces off the walls multiple times. This ongoing sound caused by the bouncing is called reverberation. Having too much reverberation in an auditorium or large hall is not good. To reduce reverberation, the walls and roof of the auditorium are usually covered with materials that soak up sound, like compressed fibreboard, rough plaster, or draperies. Even the choice of seat materials is considered based on how well they absorb sound.

Reverberation of Sound

Uses of Multiple Reflection 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.

Range of Hearing

Audible Sounds: Sounds which can be heard by us.

Range of Hearing in humans: From 20 Hz to 20000 Hz Sounds below 20 Hz are called infrasonic.

Inaudible Sounds: Sounds  Sounds which we are not able to hear.

Humans can not hear: Sounds above 20000 Hz are called ultrasonic and sounds below 20 Hz called infrasonic.

Applications of Ultrasound

Cleaning Hard-to-Reach Objects:

• Ultrasound cleans parts in difficult-to-reach areas.
• Objects are placed in a cleaning solution, and ultrasonic waves remove dust, grease, and dirt.
• Effective for complex shapes like spiral tubes and electronic components.

Detecting Cracks and Flaws:

• Ultrasound detects cracks and flaws in metal blocks used in construction.
• Waves pass through the block, and detectors identify transmitted and reflected waves.
• Reflected waves indicate the presence of weaknesses that can affect the structure.

Medical Imaging: Echocardiography:

• Ultrasound reflects waves from different parts of the heart, creating an image.
• Helps diagnose heart conditions and abnormalities.

Medical Imaging: Ultrasonography:

• Ultrasonic waves create images of internal organs.
• Changes in tissue density cause waves to reflect, which are converted into electrical signals.
• Aids in detecting abnormalities like stones or tumours.
• Useful for examining the foetus during pregnancy to identify defects and growth issues.

Medical Treatment: Kidney Stone Breakage:

• Ultrasound breaks small kidney stones into fine grains.
• Allows stones to be flushed out naturally, avoiding invasive procedures.