Detailed notes -3 Chapter 11- sound- class 9

Explanation of Sound Reflection:

1. Analogous to Light Reflection:

   • Sound behaves similarly to light when it encounters a solid or liquid surface.
   • Just as light bounces off a mirror, sound waves bounce off surfaces, a phenomenon known as sound reflection.

2. Laws of Reflection:

   • Sound follows the same laws of reflection as light.
   • The incident sound wave and the reflected sound wave make equal angles with the normal (an imaginary line perpendicular to the surface) at the point of incidence.
   • These angles lie in the same plane as the incident sound wave and the normal.

3. Requirements for Reflection:

   • For sound waves to reflect effectively, they require a relatively large obstacle, which can be polished or rough.
   • The surface of the obstacle acts as a reflecting surface, bouncing sound waves back towards their source.

Activity 11.5: Investigating Sound Reflection:

1. Materials Needed:

   • Two identical pipes made from chart paper, each sufficiently long.
   • A table positioned near a wall.
   • A clock placed near the open end of one of the pipes.

2. Procedure:

   • Set up the two pipes on the table, placing them near the wall.
   • Position the clock near the open end of one of the pipes and attempt to hear the sound of the clock through the other pipe.
   • Adjust the positions of the pipes until you can clearly hear the sound of the clock through the second pipe.
   • Measure the angles of incidence (angle between the incident sound wave and the normal) and reflection (angle between the reflected sound wave and the normal) to observe their relationship.
   • Lift the pipe on the right vertically to a small height and observe any changes in sound reflection.

3. Observations:

   • Through experimentation, observe how adjusting the angles and positions of the pipes affects the reflection of sound waves.
   • Note any changes in the clarity or intensity of the sound as you modify the setup.

4. Alternative Setup:

   • Instead of a clock, a mobile phone set on vibrating mode can be used to produce sound waves for the activity.

1. EXPLATION ECHO:

2. Definition:

   • When a loud sound, such as shouting or clapping, is produced near a suitable reflecting surface, like a tall building or a mountain, the sound waves bounce off the surface and return to the listener after a short delay. This returning sound is called an echo.

3. Persistence of Sound:

   • The sensation of sound persists in our brain for a short duration, approximately 0.1 seconds.

4. Requirement for Distinct Echo:

   • To hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.

5. Calculation of Minimum Distance for Echo:

   • Assuming the speed of sound is 344 m/s at a given temperature (e.g., 22°C in air), the sound must travel to the reflecting surface and back to the listener’s ear in 0.1 seconds.
   • Thus, the total distance covered by the sound is (344 m/s) × 0.1 s = 34.4 meters.
   • For a distinct echo, the minimum distance between the source of sound and the reflecting surface (half of the total distance) should be 17.2 meters.

6. Effect of Temperature on Echo Distance:

   • The minimum distance required for a distinct echo changes with the temperature of the air.
   • As the speed of sound varies with temperature, so does the distance required for the sound to travel and return within the specified time.

7. Multiple Echoes:

   • Echoes may occur more than once due to successive or multiple reflections of sound waves.
   • For example, the rolling of thunder is caused by successive reflections of sound from various reflecting surfaces such as clouds and land.

1. Explanation of Reverberation:

2. Definition:

   • Reverberation refers to the persistence of sound in a large hall or enclosed space due to repeated reflections from the walls, floor, and ceiling.

3. Cause of Reverberation:

   • When a sound is produced in a big hall, it reflects off the walls, floor, and ceiling multiple times before dissipating.
   • Each reflection prolongs the duration of the sound, creating a prolonged decay of sound in the space.

4. Undesirable Effects:

   • Excessive reverberation in an auditorium or large hall can be highly undesirable.
   • It can distort the original sound, making it difficult to hear clearly, particularly for speech or music performances.

5. Methods to Reduce Reverberation:

   • To reduce reverberation, the surfaces of the auditorium, including the walls, ceiling, and floor, are treated with sound-absorbent materials.
   • Common sound-absorbent materials include compressed fiberboard, rough plaster, draperies, and specialized acoustic panels.
   • These materials help absorb sound energy, reducing the number of reflections and minimizing the duration of reverberation.

6. Selection of Seat Materials:

   • Even the materials used for seating in the auditorium are chosen based on their sound-absorbing properties.
   • Seats made of materials with good sound absorption characteristics help further minimize reverberation and improve the acoustics of the space.

Uses of Multiple Reflection of Sound:

1. Directional Sound Projection:

   • Megaphones, loudhailers, horns, and certain musical instruments like trumpets and shehanais are designed to project sound in a specific direction without spreading it in all directions.
   • These instruments typically consist of a tube followed by a conical opening, which allows for successive reflection of sound waves to guide most of the sound from the source in the forward direction towards the audience.

2. Medical Stethoscopes:

   • Stethoscopes are medical instruments used by doctors to listen to sounds produced within the body, such as those from the heart or lungs.
   • The sound of the patient’s heartbeat or breath reaches the doctor’s ears through multiple reflections of sound within the stethoscope.

3. Acoustic Design of Halls:

   • In concert halls, conference halls, and cinema halls, ceilings are often curved to facilitate multiple reflections of sound.
   • The curved shape ensures that sound waves, after reflection, reach all corners of the hall, providing uniform sound distribution to the audience.
   • Additionally, curved soundboards may be placed behind the stage to further ensure that sound, after reflecting from the board, spreads evenly across the width of the hall.

Explanation of Range of Hearing:

1. Audible Range for Humans:

   • The audible range of sound for human beings extends from approximately 20 Hz to 20,000 Hz (20 kHz).
   • One Hertz (Hz) represents one cycle per second, indicating the frequency of the sound wave.

2. Variations in Hearing Ability:

   • Children under the age of five and certain animals, such as dogs, have a higher hearing range and can perceive frequencies up to 25 kHz (25,000 Hz).
   • However, as people age, their ears become less sensitive to higher frequencies, and they may gradually lose the ability to hear sounds above a certain range.

3. Infrasound:

   • Sounds with frequencies below 20 Hz are categorized as infrasonic sound or infrasound.
   • Infrasound is too low-pitched for humans to perceive directly, but some animals, such as rhinoceroses, whales, and elephants, are capable of producing and detecting infrasound.
   • Infrasound can serve various purposes, such as communication, navigation, and detecting natural phenomena like earthquakes.

4. Ultrasound:

   • Frequencies higher than 20 kHz are classified as ultrasonic sound or ultrasound.
   • Ultrasound is produced by animals like dolphins, bats, and porpoises, and it is beyond the range of human hearing.
   • Certain moths possess sensitive hearing equipment that allows them to detect ultrasonic sounds emitted by bats, enabling them to evade predation.

5. Animal Communication and Detection:

   • Animals, such as rats, utilize ultrasound for communication and navigation, playing games and escaping predators.
   • Some animals, including moths and rodents, have evolved strategies to detect and respond to ultrasonic signals in their environment, enhancing their survival chances.

Applications of Ultrasound:

1. Cleaning Hard-to-Reach Parts:

   • Ultrasound is used to clean objects located in hard-to-reach places, such as spiral tubes, odd-shaped parts, and electronic components.
   • Objects to be cleaned are placed in a cleaning solution, and ultrasonic waves are transmitted into the solution.
   • The high-frequency ultrasound waves cause particles of dust, grease, and dirt to detach from the objects, ensuring thorough cleaning.

2. Detection of Cracks and Flaws in Metal Blocks:

   • Ultrasonic waves are utilized to detect cracks and flaws in metal blocks used in the construction of buildings, bridges, machines, and scientific equipment.
   • These cracks or holes, which may not be visible from the outside, can weaken the structure.
   • Ultrasonic waves are passed through the metal block, and detectors are used to analyze the transmitted waves.
   • If there is even a small defect, the ultrasound gets reflected back, indicating the presence of a flaw or defect.
   • Unlike ordinary sound waves, which bend around corners, ultrasonic waves can accurately detect defects even in hidden areas.

3. Echocardiography:

   • Ultrasonic waves are employed to form images of the heart in a medical technique known as echocardiography.
   • Ultrasound waves are directed to reflect from various parts of the heart, creating a detailed image that helps in diagnosing heart conditions and abnormalities.

4. Ultrasound Scanning for Medical Diagnosis:

   • Ultrasound scanners are instruments used to obtain images of internal organs in the human body.
   • Doctors can image organs such as the liver, gallbladder, uterus, kidney, etc., to detect abnormalities like stones or tumors.
   • Ultrasonic waves travel through the body tissues and reflect from regions with changes in tissue density, providing valuable diagnostic information to healthcare professionals.