Question

what condition is essential for the formation of beats?

Answer 1

If you are takling about sound in general, audibility requires vibration abobe a certian threshold of the transmitting media and the human ear only "hears" frequencies within a limited range . 

If you are talking about music, given two sound frequencies(which may or may not be audible), a tone with a frequency equal to the difference of these two will be audible(called the beat frequency). 

If you are talking about listening to the heart, it is the vibrations caused by turbulent flow that produces vibrations->sound.

Answer 2

Answer:

For beats to be audible, the difference in the frequency of the two sound waves should not exceed 10. If the difference is more than 10, we shall hear more than 10 beats per second. But due to persistence of hearing, our is not able to distinguish between two sounds as separate if the time interval between them is less than (1/10)th of a second. Hence beat here will not be distant if the number of Beats produced per second is more than 10.