Question

1. why are audio recording halls covered with soft materials (2 marks)

1. state the property of sound that makes a bat fly safely in complete darkness (1 mark)

1. a man is standing \\(85\text{ m}\\) from a cliff face infront and \\(170\text{ m}\\) from a cliff face at his back. he whistles loudly and hears two echoes, one after another

(i) explain how this can happen (1 mark)
(ii) calculate the time interval between the two echoes. (velocity of sound in air = \\(340\text{ ms}^{-1}\\)) (3 marks)

1. state one factor that affects the velocity of sound in air. (1 mark)

1. give an expression which can be used to calculate electrical energy. (1 mark)

1. state one advantage of a pinhole camera over lens camera (1 mark)

Explanation:

🆕 New Concept Discovered: Echo and Sound Reflection
How sound waves bounce off barriers to create echoes.

Step 1: Analyze Question 7 (Audio Recording Halls)

Soft materials are porous and contain air pockets. When sound waves hit these soft surfaces, the sound energy is absorbed and converted into heat energy rather than being reflected. This prevents reverberations (multiple reflections) and echoes, ensuring clear audio recordings.

Step 2: Analyze Question 8 (Bat Flight)

Bats emit high-frequency ultrasound waves that bounce off obstacles and return to their ears. This property of sound is reflection (specifically utilized in echolocation).

Step 3: Analyze Question 9(i) (Two Echoes Explanation)

The man is standing between two reflective surfaces (cliffs) at different distances:

• Cliff 1 (in front) is \( 85\text{ m} \) away.
• Cliff 2 (behind) is \( 170\text{ m} \) away.

When he whistles, the sound waves travel in both directions. Because the distances to the two cliffs are different, the sound waves take different amounts of time to travel to each cliff and reflect back to his ears, resulting in two distinct echoes heard at different times.

Step 4: Analyze Question 9(ii) (Time Interval Calculation)

An echo travels to the barrier and back, covering a total distance of \( 2d \).

The formula relating distance \( d \), speed \( v \), and time \( t \) is:

$$ t = \frac{2d}{v} $$

Given:

• Speed of sound, \( v = 340\text{ ms}^{-1} \)
• Distance to first cliff, \( d_1 = 85\text{ m} \)
• Distance to second cliff, \( d_2 = 170\text{ m} \)

Calculate the time for the first echo (\( t_1 \)):

$$ t_1 = \frac{2 \times 85}{340} = \frac{170}{340} = 0.5\text{ s} $$

Calculate the time for the second echo (\( t_2 \)):

$$ t_2 = \frac{2 \times 170}{340} = \frac{340}{340} = 1.0\text{ s} $$

Calculate the time interval (\( \Delta t \)) between the two echoes:

$$ \Delta t = t_2 - t_1 = 1.0\text{ s} - 0.5\text{ s} = 0.5\text{ s} $$

Step 5: Analyze Question 10 (Velocity of Sound in Air)

Several factors affect the speed of sound in air. One primary factor is temperature (as temperature increases, the speed of sound increases). Another acceptable factor is humidity (moist air is less dense, which increases the speed of sound).

Step 6: Analyze Question 11 (Electrical Energy Expression)

Electrical energy (\( E \)) can be expressed using electrical parameters:

$$ E = VIt $$

Where:

• \( V \) = Voltage (Potential Difference)
• \( I \) = Current
• \( t \) = Time

(Alternative forms: \( E = I^2Rt \) or \( E = \frac{V^2}{R}t \))

Step 7: Analyze Question 12 (Pinhole Camera Advantage)

A pinhole camera has a key advantage over a lens camera: all objects at any distance are in focus (it has an infinite depth of field), and the image produced is free from spherical or chromatic aberrations.

---

Answer:

• 7. Soft materials absorb sound waves instead of reflecting them, preventing echoes and reverberation to ensure clear sound quality.
• 8. Reflection of sound (Echolocation).
• 9. (i) Sound travels to both cliffs and reflects back. Since the cliffs are at different distances (\( 85\text{ m} \) and \( 170\text{ m} \)), the reflected sound waves (echoes) take different times to return to the man.
• 9. (ii) \( 0.5\text{ seconds} \)
• 10. Temperature of the air (or Humidity).
• 11. \( E = VIt \) (where \( V \) is voltage, \( I \) is current, and \( t \) is time).
• 12. It has an infinite depth of field (all objects, near and far, are always in focus without needing adjustment).