Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Sound Amplification by a Paper Cup

Objective

To understand the sound amplification in musical instruments.

Introduction

When a string is plucked, it begins to vibrate emitting the sound of same frequency. The vibrating string produces forced vibrations which amplifies the sound.

Apparatus

paper or plastic cup, string, paper clip

Procedure

Prick a hole at the bottom of the cup. Pass the string through the hole and attach a paper clip at its end inside the cup.

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Wet the string with water. Hold the cup in one hand. Hold the string loosely between thumb and index finger of the other hand. Gently slide the fingers down, along the string. You will hear a loud roaring sound.

Discussion

Vibrations in the string make the cup and the air inside it to vibrate. Due to cavity inside the cup, these vibrations repeatedly rebounds against the wall of the cup. In this process they get amplified, emitting a louder sound.

To confirm the role of the cup in sound amplification, rub over a string without using cup and listen to the sound.

Most musical instruments like Sitar, Violin, Veena, Tanpura etc have hollow spaces to amplify the sound.