Frank Brothers Solutions for ICSE Class 9 Physics Chapter 7.2 Sound Range Of Hearing

Page No: 284

Question 1. Define the frequency of sound.
Answer: The frequency of sound is defined as the number of vibrations produced by the sound producing object per second.
In simple words: Frequency is just a count of how many times an object shakes back and forth in one single second.

📝 Teacher's Note: Use a simple ruler on the edge of a table to show slow vs fast vibrations. Explain that faster vibrations mean higher frequency.

🎯 Exam Tip: The phrase "per second" is critical for this definition. Without it, the definition of frequency is incomplete.

Question 2. What is the S.I. unit of frequency?
Answer: S.I unit of frequency is hertz.
In simple words: Hertz is the name of the unit we use to measure how fast something is vibrating.

📝 Teacher's Note: Mention that the unit is named after the physicist Heinrich Hertz. It is abbreviated as "Hz".

🎯 Exam Tip: Always write the full name "hertz" in lowercase and the symbol "Hz" with a capital H.

Question 3. Define one hertz.
Answer: One hertz is defined as one vibration produced in one second by sound producing object.
In simple words: If a bell shakes back and forth exactly once in one second, its frequency is one hertz.

📝 Teacher's Note: This is a "unit definition" question. It requires relating the unit (\( 1 \text{ Hz} \)) to the basic quantity (\( 1 \text{ vibration} / 1 \text{ second} \)).

🎯 Exam Tip: Use the formula \( \text{Frequency} = \frac{\text{Number of vibrations}}{\text{Time}} \) to verify these types of definitions.

Question 4. What is the audible range of frequency for humans?
Answer: The audible range of frequency is 20Hz to 20KHz.
In simple words: Our ears are like a radio that can only pick up sounds between the low "station" of 20 and the high "station" of 20,000.

📝 Teacher's Note: Explain that "K" in KHz stands for kilo, which means 1,000. So \( 20 \text{ KHz} = 20,000 \text{ Hz} \).

🎯 Exam Tip: Memorize both forms: \( 20 \text{ Hz} \) to \( 20,000 \text{ Hz} \) or \( 20 \text{ Hz} \) to \( 20 \text{ KHz} \). Both are correct.

Question 5. Can we hear sound if its frequency is:
(i) Less than 20Hz
(ii) More than 20KHz
Answer:
(i) Less than 20Hz – we cannot hear sound.
(ii) More than 20KHz – we cannot hear sound.
In simple words: If a sound is too slow or too fast for our ears to handle, it remains silent to us even if it is actually there.

📝 Teacher's Note: Compare this to light; we can't see "Infrared" or "Ultraviolet" because they are outside our eyes' range, just like these are outside our ears' range.

🎯 Exam Tip: These sounds are called "infrasonic" (low) and "ultrasonic" (high).

Question 6. Does the audible range of frequency vary from person to person?
Answer: No, the audible range of frequency does not vary from person to person and also with age of person.
In simple words: Generally, all humans have the same basic design for their ears, so we all start with the same range of hearing.

📝 Teacher's Note: (Note for teacher: In reality, hearing high frequencies *does* decline with age, but stick to the textbook's "No" for standard curriculum answers).

🎯 Exam Tip: Answer "No" for basic science exams unless the question specifically asks about hearing loss in the elderly.

Question 7. Distinguish between infrasonic and ultrasonic sounds.
Answer: Sound having frequency less than 20Hz is called infrasonic sound. Sound having frequency greater than 20KHz is called ultrasonic sound.
In simple words: Infrasonic is like a giant drum beating too slow to hear; ultrasonic is like a tiny whistle blowing too fast to hear.

📝 Teacher's Note: Use the prefixes to help students remember: "Infra" means below and "Ultra" means beyond.

🎯 Exam Tip: Always state the threshold values (\( 20 \text{ Hz} \) and \( 20 \text{ KHz} \)) when defining these terms.

Question 8. How much energy is carried by ultrasonic sound?
Answer: The energy carried by ultrasonic sound is very high.
In simple words: Because these waves vibrate so fast, they pack a lot of power that can even be used to clean jewelry or break kidney stones.

📝 Teacher's Note: Energy is directly proportional to frequency. Higher frequency waves carry more energy per unit of time.

🎯 Exam Tip: Associate "High Energy" with "High Frequency" (Ultrasonic).

Question 9. What are the applications of Ultrasonic sound?
Answer: Applications of Ultrasonic sound

• Ultrasonic sound is used to detect the flaws in metal castings of automobile tyres.
• It is used in hospitals to detect defects in certain parts of body.

In simple words: We use high-frequency sounds to "see" through solid metal for cracks and into the human body for health check-ups.

📝 Teacher's Note: Show an image of an "Ultrasound" scan to help students connect the term to medical technology.

🎯 Exam Tip: If asked for two uses, remember "industrial testing" and "medical diagnostics."

Question 10. What is echocardiography?
Answer: Echocardiography is used to obtain the image of the heart by the used of ultrasonic waves.
In simple words: It is a special medical test where doctors use high-pitched sounds to take a picture of your heart while it is beating.

📝 Teacher's Note: Point out that "Echo" refers to the reflection of sound, and "Cardio" refers to the heart.

🎯 Exam Tip: Identify "ultrasonic waves" as the specific technology used in echocardiography.

Question 11. Define SONAR and state its use.
Answer: SONAR stands for Sound Navigation and Ranging. Sonar helps the oceanographers and ship captains to determine the depth of the ocean.
In simple words: SONAR is like a flashlight made of sound that ships use to "feel" how deep the water is.

📝 Teacher's Note: Explain that it works by sending a signal and timing how long the "echo" takes to bounce back from the sea floor.

🎯 Exam Tip: Memorize the full form of SONAR. It is a very common one-mark question.

Question 12. How do bats use high-frequency sounds?
Answer: Bats emit the high frequencies that range between 20 KHz to 75 KHz. The echoes from these sounds help in locating the obstacles in their path and to avoid them.
In simple words: Bats shout sounds so high we can't hear them. They listen for the bounce-back to "see" walls or bugs in the dark.

📝 Teacher's Note: This process is called "echolocation." It is a biological version of the SONAR technology used by ships.

🎯 Exam Tip: Note the specific frequency range (\( 20 \text{ KHz} \) to \( 75 \text{ KHz} \)) used by bats.

Question 13. Which large animals produce infrasonic sounds?
Answer: The elephants and whales produce the sound of frequency less than 20Hz. (means infrasonic sounds)
In simple words: Huge animals like whales can "talk" to each other across long distances using very low rumbles that our ears can't pick up.

📝 Teacher's Note: Infrasonic waves travel much further than audible sounds, which is why whales can communicate across thousands of miles of ocean.

🎯 Exam Tip: Elephants and whales are the primary examples of animals that use infrasound.

Question 14. Do all animals have the same range of audible frequencies?
Answer: No, all animals does not have same range of audible frequencies.
In simple words: Different animals have different hearing limits. A dog can hear a high-pitched whistle that sounds completely silent to a human.

📝 Teacher's Note: Provide examples like dogs (up to \( 50 \text{ KHz} \)) and cats (up to \( 64 \text{ KHz} \)) to show that most mammals hear higher than humans.

🎯 Exam Tip: Contrast this with humans to show that "audible" is relative to the species.

Question 15. What is an echo?
Answer: When the sound strike any object and reflect back, the hearing of reflected sound is called echo.
In simple words: An echo is just your own sound "bouncing" off a wall and coming back to your ears, like a ball bouncing off a wall.

📝 Teacher's Note: Explain that for a clear echo to be heard, the wall must be at least \( 17 \text{ meters} \) away from the person shouting.

🎯 Exam Tip: The keyword "reflection" must be included in the definition of an echo.

Question 16. Define echolocation.
Answer: Echolocation is defined as sending out sounds to judge the location, size and motion of objects from the returning echoes. The animals use this phenomenon to locate the obstacles in their path and avoid them so it helps them in navigation and to locate their prey.
In simple words: Echolocation is "seeing with sound." It helps animals know where things are by listening to the bounces.

📝 Teacher's Note: Echolocation allows animals to build a 3D "map" of their surroundings using sound instead of light.

🎯 Exam Tip: Mention "location," "size," and "motion" as the three things animals can judge through echolocation.

Question 17. State three important uses of echolocation.
Answer: Three Important uses of echolocation:

• It helps animals to communicate with each other. e.g dolphins use the echolocation to communicate with other dolphins.
• It helps in navigation and to locate their prey.
• It is used by them to hunt for the meal.

In simple words: Animals use sound bounces to talk, find their way home, and catch their dinner.

📝 Teacher's Note: Point out that dolphins use clicks and whistles for this. Their system is so advanced they can tell different types of fish apart just by the echo.

🎯 Exam Tip: Focus on the "survival" aspects: communication, navigation, and hunting.

Question 18. What is a hearing aid?
Answer: A hearing aid is the device which increases the loudness and intensity of the incoming sound.
In simple words: It is a tiny machine worn in the ear that acts like a personal microphone and speaker to make quiet sounds louder for people who can't hear well.

📝 Teacher's Note: It works by converting sound waves into electrical signals, making them stronger, and then turning them back into sound waves.

🎯 Exam Tip: The core function is to increase "loudness" and "intensity."

Question 19. Name the main components of a hearing aid.
Answer: The main components of hearing aid are microphone, amplifier and earphone.
In simple words: 1. Microphone (picks up sound), 2. Amplifier (makes it louder), 3. Earphone (plays it into the ear).

📝 Teacher's Note: Ask students to draw a simple block diagram of a hearing aid showing the flow: Microphone \( \rightarrow \) Amplifier \( \rightarrow \) Earphone.

🎯 Exam Tip: List all three components for a complete answer.

Question 20. How is the depth of a submerged ship calculated using a ship's sonar?
Answer: A ship sends ultrasonic waves downward in the ocean. When the sound waves reaches the ocean floor, they are reflected back to the ship. The time it takes for the signal to return is used to calculate the depth of submerged ship in the ocean.
In simple words: The ship shouts a sound and waits for the echo. Since we know how fast sound moves in water, the longer the wait, the deeper the ship is.

📝 Teacher's Note: The formula is \( \text{Depth} = \frac{\text{Speed of sound in water} \times \text{Time}}{2} \). We divide by 2 because the sound traveled down AND back up.

🎯 Exam Tip: Emphasize that "time taken for the signal to return" is the key variable measured.

Question 21. What properties of ultrasonic sounds make them useful?
Answer: High Power, High Energy and good directionality are the properties of ultrasonic sounds which makes them useful.
In simple words: These sounds are strong, carry a lot of "punch," and travel in straight beams instead of spreading out in all directions.

📝 Teacher's Note: "Good directionality" is important because it means ultrasonic waves can be aimed like a laser beam, making them very precise for medical and industrial use.

🎯 Exam Tip: Mention "High Energy" and "Directionality" to get full marks on this conceptual question.

Question 22. Classify the following sounds based on their frequency:
(i) 10 Hz
(ii) 200 Hz
(iii) 2000 Hz
(iv) 45 KHz
Answer:
(i) 10 Hz – Infrasonic sound.
(ii) 200 Hz – audible sound.
(iii) 2000 Hz – Audible sound.
(iv) 45 KHz – Ultrasonic sound
In simple words: 10 is too low (below 20); 200 and 2000 are in our "sweet spot"; 45,000 is too high (above 20,000).

📝 Teacher's Note: Create a table on the board with three columns: Infrasonic (< 20), Audible (20-20,000), and Ultrasonic (> 20,000) to help students sort these easily.

🎯 Exam Tip: Be careful with the unit "KHz"—always convert it to "Hz" (\( 45 \text{ KHz} = 45,000 \text{ Hz} \)) before comparing it to the human hearing range.