Echo MCQs Quiz | Class 9
Class IX Science (Code 086) Unit III: Motion, Force and Work. This quiz covers the topic of Echo, focusing on the necessary conditions for an echo to be heard, such as the minimum distance and time interval, as well as its practical uses in fields like SONAR and echolocation. Attempt the questions below to test your knowledge, then submit to check your score and download a detailed answer key PDF.
Overview of Echo
An echo is a distinct sound heard after the reflection of sound from a rigid surface such as a cliff or a wall. When we shout or clap near a suitable reflecting object, we hear the same sound again a little later. This phenomenon occurs because sound waves bounce back to the source.
Conditions for Hearing an Echo
For an echo to be heard clearly by the human ear, specific conditions must be met regarding time and distance:
- Persistence of Hearing: The sensation of sound persists in our brain for about 0.1 seconds. To hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.
- Minimum Distance: Since the speed of sound in air is approximately 344 m/s (at 22 degrees C), sound travels about 34.4 meters in 0.1 seconds. Therefore, the total distance from the source to the obstacle and back must be at least 34.4 meters. This implies the minimum distance between the source of sound and the obstacle must be half of this distance, i.e., 17.2 meters.
Reverberation vs. Echo
| Feature | Echo | Reverberation |
|---|---|---|
| Definition | Single reflection heard as a separate sound. | Multiple reflections causing persistence of sound. |
| Time Gap | At least 0.1 seconds. | Less than 0.1 seconds. |
| Effect | Distinct repetition. | Prolonged sound, often confusing. |
Uses of Echo Principles
The reflection of sound is utilized in various technologies and by animals:
- SONAR (Sound Navigation And Ranging): Used to measure the depth of the sea and to locate underwater objects like shipwrecks or submarines. It transmits ultrasonic waves and measures the time taken for the echo to return.
- Echolocation: Animals like bats and dolphins emit ultrasonic squeaks and listen for echoes to detect obstacles or prey in the dark.
- Medical Ultrasound: Doctors use the reflection of ultrasonic waves to create images of internal organs (ultrasonography) or to monitor the development of a fetus.
Key Formula
If d is the distance between the source and the obstacle, v is the speed of sound, and t is the total time taken for the echo to return, then:
2d = v * t OR d = (v * t) / 2
Extra Practice Questions
- 1. Why are the ceilings of concert halls curved? (Answer: To reflect sound evenly to all corners)
- 2. What is the minimum time gap required to hear an echo? (Answer: 0.1 s)
- 3. Calculate the distance of a cliff if an echo is heard after 4 seconds and speed of sound is 340 m/s. (Answer: 680 m)
- 4. Which animal uses echolocation to fly at night? (Answer: Bat)
- 5. Does the speed of sound increase or decrease with temperature? (Answer: Increase)
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