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 :-

Two Microphones - crystal Mics were used here since they are cheap and give a large output.
1M wooden rule.
Fast Digital Storage Oscilloscope - the ADC-212 was used.
A balloon - to burst for a sudden loud sound source.

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

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.

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 :-

Temperature - TC-08 used.
Water Vapour content - (Relative Humidity -RH-02 used).

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.

L.A.N.Z.O Sweden AB
Hemsida

Data
Loggrar

Oscilloskop

Experiment
Ideér

Nedladdning

Kontakta
L.A.N.Z.O