The repetition of sound caused by the reflection of sound waves is known as:

An echo is the sound heard again after it is reflected from a distant surface, such as a cliff, a tall building, a hill, a large wall or the walls of a well. Echoes follow the laws of reflection: the angle of incidence equals the angle of reflection, and the incident wave, the reflected wave and the normal lie in the same plane.

For a distinct echo to be heard, the reflected sound must reach the listener at least 0.1 second after the original sound. This is because the sensation of sound persists in the human brain for about 0.1 second (the persistence of hearing). If the reflected sound arrives earlier, it mixes with the original sound and cannot be heard separately.

At a speed of sound of about 344 m/s in air (at 22°C), sound travels about 34.4 metres in 0.1 second. Since the sound has to travel to the reflecting surface and back, the minimum distance between the source and the reflecting surface must be about 17.2 metres (half of 34.4 m). This distance changes with the temperature of the air.

Uses of echoes:

• Measuring the depth of the sea (echo sounding) and locating underwater objects using SONAR.
• Echolocation by bats and dolphins.
• Ultrasound imaging in medicine, such as echocardiography (imaging of the heart) and sonography.
• Finding the speed of sound.
• Locating deposits of oil and minerals by seismic surveys.

Reverberation is different from an echo. It is the persistence of sound in a large hall or room due to repeated reflections from the walls, ceiling and floor, when the reflecting surfaces are too close to give distinct echoes. Excessive reverberation makes speech and music unclear. To reduce it, auditoriums and cinema halls use sound-absorbing materials, such as heavy curtains, carpets, fibreboard and padded seats, and are specially designed (acoustics). The American physicist Wallace Sabine is called the father of architectural acoustics. The Gol Gumbaz at Bijapur, Karnataka, is famous for its "whispering gallery", where sound is reflected many times.

Why not the other options?

• ✗Reverberation is the prolonged persistence of sound due to multiple reflections in a closed space.
• ✗Resonance is vibration with large amplitude at the natural frequency.
• ✗Refraction is the bending of sound when it passes between media.