# Optional Practical: Sliding whistle

# Speed of sound with a slide whistle

#### Introduction

In this practical the frequency of the note from a slide whistle (bought from Ebay) will be measured using the programme Audacity, this is a free programme that can be downloaded from http://audacity.sourceforge.net/ . Before starting the practical download audacity and play with it to find out how to record sounds, magnify the signal and measure the time period.

After a bit of trial and error you should be able to measure the time for 10 or more complete cycles of the sound wave by moving the cursor and hence find the frequency of the sound. Note that the wave isn’t a sine wave. Here is a helpful screencast

#### Theory

The whistle is a closed pipe so the wavelength of the 1^{st} harmonic is 4 x its length. Since you can’t see the end of the piston you can’t measure the length of the whole pipe but can measure the length of the slider that sticks out, x as in the diagram

You can see from the diagram that the length of the pipe is x+L so the wavelength is 4(x+L)

We know that v=fλ

so v = 4f(x+L)

Measure the frequency of the note for different lengths x then use a graphical method to find the velocity of sound and the length L.