Sound - Reflection of Sound

Reflection of Sound: Like light, sound reflects off the surfaces of solids and liquids. The incident sound wave, the reflected sound wave, and the normal at the point of incidence lie in the same plane.

Laws of Reflection: The angle of incidence of sound is always equal to the angle of reflection ($i = r$).

Echo: A distinct reflected sound heard after the original sound has ceased. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least $0.1 \text{ s}$ due to the persistence of hearing.

Minimum Distance for Echo: At $22^\circ\text{C}$ where speed of sound is $344 \text{ m s}^{-1}$, the total distance traveled by sound to be heard as an echo must be at least $d = 344 \text{ m s}^{-1} \times 0.1 \text{ s} = 34.4 \text{ m}$. Thus, the obstacle must be at a distance of half of this, which is $17.2 \text{ m}$.

Reverberation: The persistence of sound in a big hall due to repeated reflections. It is reduced by using sound-absorbent materials like compressed fiberboards, rough plaster, or draperies.

SONAR (Sound Navigation and Ranging): A device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects. It works on the principle of echo-ranging.

Ultrasound Applications: Used in medical imaging (Echocardiography, Ultrasonography), cleaning hard-to-reach parts, and detecting cracks in metal blocks.