Measuring the Speed of Sound

This application note describes the use of the ADC-212 virtual oscilloscope to measure the speed of sound.

Items used :-

The experiment was setup as shown below with the two crystal microphones placed 1 metre apart.

Experiment Setup

The Oscilloscope timebase was set to 500uS/Div and the trigger position set to 10% pretrigger. This allows us to collect 500uS of data before the bang and 4.5mS of data during the event.

The balloon was burst approximately 2M away from the foremost Mic. The plot below shows the results clearly.

The lefthand "Blue" trace is from the foremost Mic (Mic1) and the righthand "Red" trace is from the furthest Mic (Mic2).

The waveform from Mic1 between -164uS and 500uS is clearly visible in the trace from Mic2 delayed by 2929uS.
There is a second variation, in the waveform from Mic1, around 1.5mS caused by an echo from one of the walls or ceiling.

Experiment Results

From this plot we can measure the time between the two waveforms - 2929uS.
This is over 1 Metre which equates to 341.4M/S.

When calculating the speed of sound through the air we need to take into account the factors which can affect the speed.

The main factors are :-

The Temperature and Relative Humidity during the test were approximately 20 DegC and 50% respectively.

We can use the table below to find the expected speed :- 344M/S or 2.907mS/M.
The matches very well with the observed results shown above.

M/S RH%
T DegC 10 20 30 40 50 60 70 80 90
0 331.5 331.5 331.5 331.6 331.6 331.6 331.7 331.7 331.7
5 334.5 334.6 334.6 334.7 334.7 334.7 334.8 334.8 334.9
10 337.5 337.6 337.7 337.7 337.8 337.9 337.9 338.0 338.0
15 340.5 340.6 340.7 340.8 340.9 341.0 341.1 341.2 341.2
20 343.5 343.6 343.7 343.9 344.0 344.1 344.2 344.4 344.5
25 346.4 346.6 346.8 347.0 347.1 347.3 347.5 347.6 347.8
30 349.4 349.6 349.9 350.1 350.3 350.5 350.8 351.0 351.2
This table is taken from "Tables of Physical and Chemical Constants" by "Kaye & Laby"
It is published here with the kind permission of "Longman Group Limited" publishers.

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