What is the difference between echo and reverberation?

Echo: A reflected sound heard distinctly at least 0.1 seconds after the original sound, requiring a reflecting surface at least 17.2 m away. Reverberation: Multiple reflections of sound in an enclosed space (like a hall) causing sound to persist — heard within 0.1 seconds so individual echoes are not distinguished. Reverberation is reduced by using sound-absorbing materials.

Why can't sound travel in vacuum?

Sound is a mechanical wave that requires a medium (matter) to propagate. It travels by causing particles to vibrate and pass energy to neighboring particles. In vacuum, there are no particles, so sound cannot propagate. This is why space is silent — even massive explosions make no sound.

What is the range of hearing for humans?

Humans can hear sounds with frequencies between 20 Hz and 20,000 Hz (20 kHz). Below 20 Hz is infrasonic (e.g., earthquakes). Above 20,000 Hz is ultrasonic (e.g., dog whistles, bat calls, medical SONAR). Hearing ability decreases with age, especially for higher frequencies.

Properties and Reflection of Sound

Sound • Class 9 Science • NCERT • CBSE

Sound is a mechanical longitudinal wave produced by vibrating objects. It requires a medium to travel and cannot propagate through vacuum. The speed of sound in air at 25°C is approximately 346 m/s. Echo is the reflection of sound heard after 0.1 seconds.

Key Formulas

v = fλ (Speed = frequency × wavelength)
T = 1/f (Time period = 1/frequency)
Minimum distance for echo = 17.2 m (in air)
Speed of sound in air = 346 m/s at 25°C
SONAR depth = v × t / 2

Frequently Asked Questions

What is the difference between echo and reverberation?: Echo: A reflected sound heard distinctly at least 0.1 seconds after the original sound, requiring a reflecting surface at least 17.2 m away. Reverberation: Multiple reflections of sound in an enclosed space (like a hall) causing sound to persist — heard within 0.1 seconds so individual echoes are not distinguished. Reverberation is reduced by using sound-absorbing materials.
Why can't sound travel in vacuum?: Sound is a mechanical wave that requires a medium (matter) to propagate. It travels by causing particles to vibrate and pass energy to neighboring particles. In vacuum, there are no particles, so sound cannot propagate. This is why space is silent — even massive explosions make no sound.
What is the range of hearing for humans?: Humans can hear sounds with frequencies between 20 Hz and 20,000 Hz (20 kHz). Below 20 Hz is infrasonic (e.g., earthquakes). Above 20,000 Hz is ultrasonic (e.g., dog whistles, bat calls, medical SONAR). Hearing ability decreases with age, especially for higher frequencies.

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