CLASS 9TH DAY 6
INTRODUCTION
Welcome students to another fun filled lecture on Sound.In the last session we learnt about sound production and how it propagates, today we will understand how sound reflects, what is echo and many more interesting topics.
Reflection of Sound
Sound gets mirrored in the same manner and follows the same laws of reflection as light. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. An obstacle of large size which may be polished or rough is needed for the reflection of sound waves.
Activity:
- Take two identical pipes, as shown in figure. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown.
- Arrange them on a table near a wall.
- Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.
- Adjust the position of the pipes so that you can best hear the sound of the clock.
- Now, measure the angles of incidence and reflection and see the relationship between the angles.
- Lift the pipe on the right vertically to a small height and observe what happens.
Echo:
If we shout or clap close to an appropriate reflecting object similar to a tall building or a mountain, we are going to hear the identical sound again, a little bit later. This sound which we hear is called an echo. The sensation of sound persists in our mind for about 0.1 seconds. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s.
If we take the speed of sound to be 344 m/s at a given temperature, say at 22 ºC in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) × 0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m.
Reverberation
A sound created in a big hall will persist by repeated reflection from the walls till it’s decreased to a value where it’s not audible. The repeated reflection that results in this persistence of sound is known as reverberation. In an auditorium or big hall excessive reverberation is highly undesirable.
To reduce reverberation, the roof and partitions of the auditorium are usually coated with sound-absorbent supplies like compressed fibreboard, rough plaster or draperies. The seat materials are also chosen on the basis of their sound absorbing properties.
Example: A person clapped his hands near a cliff and heard the echo after 5 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m s–1?
Solution:
Given, Speed of sound, v = 346 m s–1
Time taken for hearing the echo, t = 5 s
Distance travelled by the sound = v × t = 346 m s–1 × 5 s = 1730 m
In 5 s sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person = 1730 m/2 = 865 m.
Uses of multiple reflection of sound
1. Megaphones or loudhailers, horns, musical instruments such as trumpets and shehnais, are all designed to send sound in a particular direction without spreading it in all directions.
2. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.
Range of Hearing:
The audible range of sound for human beings extends from about 20 Hz to 20000 Hz. Children below the age of 5 and a few animals, such as dogs can hear as much as 25 kHz (1 kHz = 1000 Hz).
As individuals grow older their ears become less sensitive to greater frequencies. Sounds of frequencies under 20 Hz are referred to as infrasonic sound or infrasound.
If we could hear infrasound we would hear the vibrations of a pendulum just as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.
Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by dolphins, bats and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture. Rats also play games by producing ultrasound.
Applications of Ultrasound:
1. Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tubes, odd shaped parts, electronic components etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out.
2. Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are generally used in construction of big structures like buildings, bridges, machines and also scientific equipment. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back.
3. Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body. A doctor may image the patient’s organs. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs.
4. Ultrasound may be employed to break small ‘stones’ formed in the kidneys into fine grains. These grains later get flushed out with urine.
Sonar
The acronym SONAR stands for SOund Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. How does the sonar work? Sonar consists of a transmitter and a detector and is installed in a boat or a ship.
The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound.
Structure of Human Ear:
How do we hear? We are able to hear with the help of an extremely sensitive device called the ear. It allows us to convert pressure variations in air with audible frequencies into electronic signals that travel to the brain via the auditory nerve. The auditory aspect of the human ear is discussed below.
The outer ear is called ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the eardrum or tympanic membrane.
When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear.
The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound. Attempt the quiz below to test your knowledge.👇 👇
Activity : To demonstrate the necessity of Carbon Dioxide for photosynthesis.
Procedure: This apparatus consists of a hiding mouthed bottle with a small amount of KOH solution. A potted plant is kept in darkness for two days to remove starch. A leaf lamina of the plant is introduced into the bottle in such a way that half of the leaf lamina is inside the bottle and half outside. The part inside the bottle does not get CO2 because available CO2 is absorbed by KOH. The part outside the bottle gets CO2.
After a few hours of experiment, this very leaf is tested by iodine.The part of leaf outside becomes blue while part remaining in the bottle is colorless suggesting that photosynthesis occurs only in the outside leaf which produces starch and this starch gives blue colour with iodine. Thus, potassium hydroxide can be used to remove CO2 gas. To learn more, click below👇 👇
Home made projector
The days are long, we crave to go to the movie theater but can’t because of the lock-down ,so what shall we do? Let’s bring theater home, not by buying it from the market but by creating it with our own hands.. Today, we will learn to make our own projector, so that we can watch movies in theater style. Let’s start.
Things that you will need – Shoe box, Cutter and scissors, Magnifying glass, Cello tape and your favorite movie.
Step 1: Making the Lens
Take your magnifying glass and take out the glass from it. (I had to unscrew mine from the base)
Center the glass on the front of the rectangular box that you have gathered.
Make an outline of the glass on the box.
- Using the box cutter, cut the outlined circle out. (You should have a circular hole on the front of your rectangular box)
5. Put your magnifying glass into the hole that you just made. To prevent it from falling, hold it in place with your hands for now.
Take your hot glue gun and apply the glue to where the glass and cardboard touch. (Be careful to not put glue on the lens! This can affect the projection quality)
Step 2: Making the Phone Holder
Take your ruler and measure the width of your rectangular box. (The width of mine is 6 3/8 inches) (Make sure to write this down on a piece of paper so that you may remember the measurements when making this with the cardboard)
This time, using the ruler, measure the height of a rectangular box. (The height of the box was 4 3/4 inches, but I chose to make the height of the phone holder 4 inches) (Make sure to write your measurement on a piece of paper so that you may remember the measurements when making this with the cardboard)
3. To make the back support for the phone holder, use your height and width to make a rectangle. (I decided to shorten (My rectangle was 4 inches by 6 3/8 inches)
4. To make the bottom support for the phone holder, use the measurement of the width, and for the height of the rectangle, divide the height of your back support rectangle by 2. This will be your height for the bottom support. (My rectangle was 2 inches by 6 3/8 inches)
5. Cut the rectangles out by following the outline of the rectangles that you have drawn.
6. Take your hot glue gun and attach the two rectangles, the longest sides touching each other.
7. Make sure the two rectangles together, form a right angle when looking at it from the side.
Step 4: How to Use the Projector
- Take your phone holder and lay it down on a flat surface
- Tape your phone down at the center of the holder. (Try to tape down the edges of the phone to the holder)
- Open the video, photo, or movie that you want to project.
- Hold the phone holder with the phone attached to it, upside down. (The photo, video, or movie should flip its view)
5. To lock your screen, (on an IOS device) swipe up from the bottom edge of the phone and click the Portrait Orientation Lock and if it is on, it should turn red-orange.
6. On that same screen, make sure your phone is at its highest setting.
7. Now, insert your phone holder into the projector body (making sure that the photo, video, or movie is still upside down)
8. Enter a dark room and play your photo, video, or movie
9. If the projection is blurry, focus it by moving the phone holder either backwards or forwards
Task
Since, you have watched your favourite movie. Now, it’s time to analyse the movie and learn something valuable from that( we should find something valuable from everything). So, critically analyze the movie and write about your favourite character and why is he or she your favourite . Do you like the character because you relate to them or you want to become like them ? What is the one quality you want to adopt from him or her . Give us a brief using the following link:
JIVAN GYAN
Today we will learn about 3 Mudra which practicing daily for 15min we can boost up OUR Memory and Concentration power. Click on learn more to watch the video explaining the same.
