Determination of the Speed of Sound
Description of the Experiment
The purpose of this experiment is to determine the speed of sound.
Experimental Apparatus
The apparatus(sometimes referred as "Kundt's tube") consist in a PVC tube with 1458 mm in length. On one end there is a fixed speaker that can produce an audio sine, triangular or a single pulse wave. On the opposite side there is a movable piston for changing the effective tube length. Along the tube there are several microphones to register the sound intensity at fixed points.
The following table shows the positions of the microphones in relation to the source (speaker):
| Designation | Distance to source (mm) |
|---|---|
| Mic 1 (reference) | 250 |
| Mic 2 (center) | 750 |
| Mic 3 (extreme) | 1250 |
| Mic 4 (embolus surface) | Between 1260 e 1480 |
| tube limit | 1450 |
The reference mic (Mic 1) should be used to verify that the emitted sound is the desired on (i.e., there is no distortion caused by the speaker). On the piston surface is another microphone (Mic 4) capable of moving between 1269mm and 1475mm with it.
The sound is acquired through 2 channels of a sound card: the left channel (CH 1) is always bounded to the reference microphone (Mic 1); the other channel (CH 2) can be connected to one of the other three microphones.
The experimental data is captured by the PC's sound-card and processed on-line (normalization) prior to be receive by the user.
Protocol
This assembly is also used for the stationary wave experiment, and thus has two modes of operation: in the "Speed of sound" mode, the amplitude of the wave is registered over time.
To determine \( v_{sound} \), the user must choose a "pulse" type of stimullus and measure the "time-of-flight" taken by the wave from Mic 1 and any other microphone. The speed can be determined with data from table 1 and the formula for speed determination:
\[ v_{sound} = \frac{\Delta s}{\Delta t} \] where \(s\) is the distance between the selected microphones. Of course other waveforms could be used but this will require a close look at the phase.
Advanced Protocol
The correlation from the acquired signals can improve with great accuracy the determination of the phase among the diferent microphones. Using an appropriate software package in mathlab or octave for instance, this phase is easily determined. Noise is very suitable for that end as it won't exhibits any phase indetermination.
