Sound echoes well on flat surfaces such as a wall or flat cliff. Irregular surfaces break up sound waves so they do not echo as well. Reflected waves cannot be heard until the original sound is stopped. Reflected waves have lost strength and are not as strong as regular sound waves. Refraction is the bending of sound waves from their original path.
Interference occurs in sound waves.
A sound from a single source reaches your ear by two different paths and they may reinforce each other. If the exact same sound is produced by two difference sources and the original sound waves from one instrument reach your ear at the same time as the refracted waves from the other source, then they may become out of phase and no sound will be heard. If two instruments are tuned, for example three vibrations per second apart, the waves will be out of phase three times a second and in phase three times per second. Therefore, the sound will only be heard one half of the time. The Quality and Intensity of a Tone Quality of a tone is determined by the intensities of harmonics of a note and the structure of its sound waves.