What name is given to the repetition of sound caused by the reflection of sound waves?

The longitudinal waves travel in a coiled spring at a rate of 4 m/s. The distance between two consecutive compressions is 20 cm. Find:

(i) Wavelength of the wave

(ii) Frequency of the wave

Which property of sound leads of the formation of echoes?

What name is given to the persistence of sound in a big hall or auditorium?

Name three devices which work on the reflection of sound.