# School of PhysicsLecture Demonstration Manual

## Wb-6 The Monochord (The Sonometer)

Published: Tuesday 14 March 2006 - Updated: Wednesday 30 March 2011

### Aim

To demonstrate the fundamental law of vibration of a stretched string.

### Apparatus

•  The monochord
•  Set of attached weights
•  Frequency meter if required

### Diagrams

Click on pictures to enlarge.

 Monochord Diagram Monochord Equation

### Description

The monochord consists of a sounding box which supports a wire whose length of vibration may be adjusted by the movable bridge B. The tension is adjustable by the weights D. The equation for the vibration of a stretched string may be verified .

The equation above shows that:

for a string under constant tension the frequency varies inversely as the length;for a string of constant length the frequency is proportional to the square root of the tension;for a given length and constant tension the frequency varies inversely as the square root of the mass/unit length.

which are Mersenne's laws.