These comprehensive RBSE Class 9 Science Notes Chapter 12 Sound will give a brief overview of all the concepts.
RBSE Class 9 Science Chapter 12 Notes Sound
→ Sound : Sound is a form of energy which produces the sensation of hearing in our ears.
→ Mechanical Wave : A mechanical wave is a periodic disturbance which requires material medium (i.e. solid, liquid or gas) for its propagation. In other words, waves that are characterised by the motion of particles of a medium are called mechanical waves.
→ Examples of Mechanical Waves :
- Sound waves in air.
- Water waves.
- Waves produced due to the earthquake (known as seismic waves).
- Waves produced by supersonic jet planes (known as shock waves).
- Waves produced in a stretched string.
- Waves produced in a slinky or long spring.
→ Types of Waves : Waves are of two types – (i) Transverse wave, (ii) Longitudinal wave.
1. Transverse Wave : If the particles of a medium vibrate or oscillate about their mean positions at right angles to the direction of propagation of the disturbance then the wave is called transverse wave.
Examples : Movement of string of a sitar or violin.
2. Longitudinal Wave : If the particles of a medium vibrate or oscillate to and fro about their mean positions along the direction of propagation of the disturbance then the wave is called longitudinal wave.
Examples : Sound wave, Organ pipes, Vibration on resonance apparatus.
→ Compression : The part or region of a medium, where the density of the medium is maximum or where the particles of the medium are very close to each other is known as compression. It is denoted by C.
→ Rarefaction : The part or region of a medium, where the density of the medium is minimum or where the particles of the medium are far apart from each other is known as rarefaction. It is denoted by R.
→ Propagation of Sound: A vibrating body produces sound. When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear.
→ Sound needs a medium to travel : Sound needs a material medium for its propagation. In other words, sound cannot travel through vacuum.
→ Sound waves are longitudinal waves : The particles of the medium through which a sound wave travels vibrate to and fro about their mean positions parallel to the direction of propagation of the sound wave. Therefore, the sound waves are longitudinal waves.
→ Characteristics of Sound :
(a) Amplitude : The maximum displacement of a vibrating body or particle from its
rest position (i.e., mean position) is called amplitude. In SI, unit of amplitude is metre (m).
(b) Wavelength (or length of a wave) : The distance between two successive regions of high pressure or high density (or compressions) or the distance between two successive regions of low pressure or low density (or rarefactions) is known as wavelength of a sound wave. [Represented by a (lambda)] SI unit is metre (m).
(c) Frequency : The number of oscillations or vibrations made by a vibrating body or particles of a medium in one second is known as the frequency of a wave. It is represented by v (nu) and its SI unit is hertz (Hz).
(d) Time period : Time taken by a vibrating particle or a body to complete one vibration or oscillation in the density of the medium is known as time period.
Represented by T (SI unit is sec).
(e) Pitch : Pitch is the characteristic (i.e., typical feature) of a sound that depends on the frequency received by a human ear.
(f) Loudness : Loudness of a sound depends on the amplitude of the vibrating body producing the sound.
(g) Timbre or quality : Quality or timbre is a characteristic of a sound which enables us to distinguish between the sounds of same loudness and pitch.
(h) Intensity : Intensity of a sound is defined as the sound energy transferred per unit time through a unit area placed perpendicular to the direction of the propagation of sound.
→ Speed of sound in different media : Sound can travel through solids, liquids and gases. Sound travels at different speeds in different medium. Speed of sound in solids is greater than the speed of sound in liquids and the speed of sound in liquids is greater than the speed of sound in gases.
→ Reflection of Sound : When a sound wave travelling in a medium bounces back to the same medium after striking the second medium, reflection of sound wave is said to take place. The reflection of sound wave is similar to the bouncing back of a rubber ball after striking a wall or the surface of a floor.
→ Echo : Echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.
→ Reverberation : The repeated reflection that results in the persistence of sound in a large hall is called reverberation.
→ Uses of multiple reflection of sound:
- Hearing aid
- Sound boards
- Ceilings of concert halls are curved.
→ Audible range of hearing : The waves having frequency between 20 Hz and 20,000 Hz are known as sound waves. Thus, the audible range of frequency is 20 Hz to 20,000 Hz. The waves having frequency less than 20 Hz and greater than 20,000 Hz cannot be heard by human ear.
→ Infrasonic waves : The waves of frequency less than 20 Hz are known as infrasonic waves.
→ Ultrasonic waves : The waves of frequency greater than 20,000 Hz are known as ultrasonic waves or ultrasound.
→ SONAR (Sound Navigation and Ranging) : It is a device which is used in the ships to locate rocks, icebergs, submarines, old ships sank in sea, etc. It is also used to measure the depth of a sea. It is called echo ranging. In this way, we can determine the depth of the sea and to locate underwater hills. I Principle : It is based on the principle of the reflection of sound wave (i.e. echo).
→ The Human Ear : Human ear converts sound energy into mechanical energy and then to a nerve impulse which is transmitted to the brain.
The human ear consists of :
(i) The Outer Ear : The pinna of the outer ear collects the sound from the surroundings. The collected sound passes through the auditory canal and reaches the eardrum. When the compression (high pressure) of air medium reaches the eardrum, the pressure on the outside of the eardrum membrane increases and pushes it inwards. Similarly, the eardrum membrane moves outwards when rarefaction (low pressure) reaches it and eardrum vibrates.
(ii) The Middle Ear : Three bones of the middle ear work like a lever arrangement. The middle ear receives the vibrations from the eardrum and amplifies the amplitude of these vibrations several times. These amplified pressure variations are then passed on by the middle ear to the inner ear.
(iii) The Inner Ear : Cochlea receives the amplified pressure vibrations from the middle ear. These pressure vibrations are turned into electrical signal by the cochlea. These electrical signals are then sent to the brain via the auditory canal. The brain interprets these signals as sound.