Solve the puzzle

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.

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.

She was so excited, she told this to her friends in the class.

Listen to their conversation and help them solve this puzzle.

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