Rajasthan Board RBSE Class 9 Science Notes Chapter 11 Sound
Production of Sound:
Sound is a form of energy which produces a sense of hearing in our ears. Sound is produced by striking, plucking, scratching, rubbing, blowing or shaking of different objects.
Propagation of Sound:
- Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. The vibration of all the particles of a material medium, sets up the vibration in the object(s) placed in the medium. The medium around its vibrating the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result, the adjacent particle gets displaced from its position of rest. Disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium. A wave is a disturbance that moves through a medium. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.
- 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 compression. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction.
- A series of compressions and rarefaction’s facilitates propagation of sound waves through the medium.
- Sound needs a material medium like air, water, etc. for its propagation. It can not travel through vacuum.
Nature of Sound Waves:
- All waves-mechanical and non mechanical waves are classified into two types. These are: Longitudinal waves (b) Transverse waves.
- Longitudinal waves: A wave in which the particles of the medium oscillate to and fro about their mean positions, in the direction of propagation of the wave is called a longitudinal waves. Examples:
- Sound waves
- Waves produced in a spring.
- Transverse waves: A wave in which the particles of the medium oscillate about their mean position in a direction perpendicular to the direction of propagation, of the wave is called a transverse wave.
- All electromagnetic waves, i.e., light waves, etc.
- When a string of sitar or guitar is plucked, transverse waves are produced in the string.
- Transverse wave occurs in the form of crests and troughs.
- As we have already discussed, in transverse waves the particles of the medium oscillates in the direction perpendicular to the direction of propagation. The highest point on the hump in a transverse wave is called crest or in other words, we can say that the point of maximum positive displacement on a transverse wave is called crest.
- The lowest point on the depression in a transverse wave is called trough, or in other words we can say that the point of maximum negative displacement on a transverse wave is called trough.
Characteristics of a sound wave:
A wave is characterized mainly in terms of the following parameters.
- Frequency: The number of complete waves (or oscillations) produced in one second is called frequency of the wave. It is the number of vibrations that occur per second. It is denoted by the Greek letter v. Its S.l. unit is Hertz (symbol, Hz), named in honor of Heinrich Rudolf Hertz, who discovered photoelectric effect.1 Hz = 1 vibration per second 1 kHz = 1000 Hz
- Amplitude: The maximum displacement of the particles of ti medium from their original mean position, on passing a wave through the medium is called amplitude of the wave. It is used to describe the size of the wave. It is usually denoted by the letter A. Its S.l. unit is meter.
- Speed: The distance traveled by a wave in one second is called speed of the wave under the same physical conditions. The speed of sound is same for all frequencies. It is represented by the letter V. Its S.l. unit is meter/ second.Relationship between speed, frequency and wave length of a wave From the definition of velocity
So for a wave,
- A wave takes time equal to its time period (T), to travel a distance equal to its wave length.
So, wave velocity
As per definition:
Frequency of the wave,
From (i) and (ii) , we have
⇒ wave velocity = wave length × frequency
- The above equation is known as wave equation. This statement is true for all the types of waves. Velocity of the wave depends on the medium.
The amount of sound energy passing each second through unit area is known as the intensity of sound. Intensity of sound can be measured. The relation of intensity of sound is with its energy. The S.l. unit of intensity is watt per square meter (w/m2).
- The loudness of a sound depends on the amplitude of the vibration producing that sound. Greater the amplitude of vibration, louder is the sound produced by it.
- The loudness of a sound also depends on the quantity of air that is made to vibrate.
- Loudness of sound is measured in decibels (dB) unit.
The characteristic of sound by which a shrill note can be distinguished from a flat note is called pitch. The pitch of the sound depends upon its frequency. Higher the frequency of sound, higher is its pitch.
Quality OR Timber:
- Quality of a sound is also known as timber.
- The quality or timber of sound is that characteristic which enables us to distinguish one sound from another, having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality.
Reflection of sound:
Sound bounces off a solid or a liquid, just like a rubber ball bounces off the wall. The directions in which the sound is incident and reflected, make equal angles with the normal to the reflecting surface at the point of incidence and they are in the same plane.
If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again, a little later. This sound which we hear is called an echo. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1s.
A sound created in a big hall will persist by repeated reflections from the walls, until it is reduced to a value where it is no longer audible. This persistence of sound is called reverberation.
The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s). Sound of frequencies below 20 Hz are called infrasonic sound or infra sound. Sound frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.
Applications of Ultrasound:
Ultrasound are used extensively in industries and for medical purposes. Ultrasound is generally used to clean hard-to-reach places; to detect cracks or flaws in metal blocks; echo-cardiology; scanning; etc.
The acronym SONAR Stands for Sound Navigation and Ranging. This technique is used by ships to detect the presence of submarines, icebergs, sunken ships, etc. in the sea. SONAR is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. Sonar can also be used for determining the depth of the sea.
Determination of the depth of a sea using SONAR:
Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through seawater be v. The total distance, 2 d traveled by the ultrasound is 2 d = v x t
Structure of Human Ear:
Sound waves from outside are collected by external ear (known as Pinna) and reaches the eardrum, through the auditory canal. When the sound waves strike the eardrum, it starts vibrating. These vibrations are transmitted across the middle ear by the three ossciles called (the hammer, anvil and stirrup). The vibrations produced by the eardrum are amplified. The amplified vibrations are then transmuted to the cochlea. Cochlea is coiled and fluid filled tube, having the sense organ for thousands of auditory nerves, which send messages to the brain.
- The word RADAR is an acronym derived from the words Radio Detection and Ranging. It refers to the technique of using radio waves to detect the presence of objects in the atmosphere. The electronic principle on which radar operates is very much similar to the principle of sound wave reflection. In 1922, A. Hoyt Taylor and Leo.C.Young discovered the scientific device RADAR. RADAR is an object detection system that uses radio waves to determine range, angle or velocity of objects. It can be used to detect air craft, ships, space craft,guided missiles, formation of weather focuses and terrains.
- Once the radio waves have been generated, an antenna working as a transmitter, focuses them into the air, in front of it. The antenna is usually curved. So, it focuses the waves into a precise, narrow beam, but RADAR antennas also typically rotate so. they can detect movements over a large area.
- The radio wave travel out ward from the antenna at the speed of light (186000 miles or 300000 km/s) and keep going, until they hit something. Then, some of them bounce back towards the antenna in a beam of reflected radio waves, also travelling at the speed of light. If an enemy jet plane is approaching at over 3000 km/h, the RADAR beam needs to travel much faster than this to reach the plane, so it returns to the transmitter and trigger the alarm in time. That is no problem, because radio waves travels fast enough to go seven times around the world in a second. Any reflected radio waves picked up by the antenna are directed into a piece of electronic equipment that processes and displays them in a meaningful form, on a television like screen, watched all the time by a human operator. Using RADAR an operator can see nearby ships,planes, where they are, how quickly they are travelling and where they are heading.
Summary of how does a RADAR works:
- An instrument, magnetron generates high frequency radio waves.
- Duplexer switches magnetron to the antenna.
- Antenna acts as transmitter, sending narrow beam of radio waves through the air.
- Radio waves hit energy airplane and reflect back.
- Antenna picks up reflected wave during a break between transmission.
- Duplexer switches antenna through to receiver unit.
- Computer in receiver unit processes reflected waves, and draw them on a T.V screen.
- Enemy plane shows up on TV radar, also displaying any other nearly targets.
Uses of RADAR:
- In navigation of aeroplane and ships.
- In launching of rockets and missiles.
- In GP.S system.
- In locating the enemies aeroplane, missiles and ships.