Voice of your friend can be recognized by its

Which one of the following cannot be the unit of frequency of a sound wave?

Shrillness of sound is determined by -

When the same tone is played on a sitar and a fute, the sound produced can be distinguished from each other because of the diference in :

How much should minimum distance be between the source of sound and reflecting surface, so that an echo can be heard clearly?

The sound created in a big hall persists because of the repeated reflections. The phenomenon is called