Solve the puzzle

Friends discussing a question; help them

Kanchan was playing with a string. She held the string taut, plucked it with a finger and listened to the sound. She heard a low-pitched sound. She decreased the length of the string and plucked it again. This time, she heard a higher-pitched sound.

She was so excited that she shared this with her friends in the class. 
Sonu, one of her friends, found a rubber band in his pocket. He tried the same with the rubber band. But he found the opposite of what Kanchan saw with the string. As he elongated the rubber band by stretching it, the sound became higher pitched.

Listen to their conversation and help them to solve this puzzle.

  Video 5.1: friends discussing a question

Perform the experiments that Kanchan and Sonu did. You can do your own experiments as well.