Voice of the Waterfall

A young, very talented painter just finished his masterpiece of a waterfall. His friends, families, colleagues all congratulated him on his wonderful job. To get a absolute recognition, this young artist decided to bring his masterpiece to one of the reputable master for advice.
"It is a great painting, you bring out the life of the waterfall, but you did not paint its voice"
What did voice of a waterfall mean? The painter changed his painting materials, even changed its drawing paper, after several months of hard effort, he brought his every satisfied painting to the master again.
"It is a great painting, still, the voice is not drawn"
The young painter returned, tried again, failed. The master was not impressed.

Months after months, on his sixth trial, the young painter was drained, desperate and impatient. "What do you mean by voice of the waterfall? I have tried everything from materials to size to contrast of colors, people have told me it was undifferentiated compare to a real waterfall!"

The master took his paint brush, added two tiny people on each side of the river bank.
One of the people channeled his mouth with his hands, indicating he was yelling across the river.
The other, using one hand siding across his right ear, with his right side facing towards the other person across the river, indicating he was concentrating to hear what the other person was yelling about.

Suddenly the young painter heard the roaring voice from his waterfall.