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More Phun with PhyPhox: Measuring the Speed of Sound with a Phone

We had some fun yesterday measuring the speed of sound using the acoustic stopwatch setting from PhyPhox.

The stopwatch is easily triggered by a handclap, and even quite basic phones have pretty sensitive microphones - which means that the stopwatch on a phone 3 or 4 or 5 metres away can be triggered by the same handclap - and that's what allows us to use the feature to find the speed of sound. Because it obviously takes the sound a fraction of a second to reach the more distant phone - and when we're measuring down to 1/1000th of a second, that delay is measurable.

The process involves setting up two students 5 metres apart beside their phones. One claps, triggering both stopwatches. As soon as the second student sees his timer has begun counting, he also claps - stopping both stopwatches.

Because the first phone is the first to start and the last to stop, its measured time will be greater than the second phone. And the difference between the two will equal the time it takes for the sound to travel 10 metres (from one phone to the other and back)

Its all demonstrated very well here:

The procedure works better outdoors, because there's the risk of echoes indoors affecting the timers - but I have seen it work in a lab

And this is the work we put together in class yesterday.

We found the speed of sound to be 385 m/s, which seems a reasonable result for a single measurement.

PhyPhox can be downloaded here: phyphox – Physical Phone Experiments

This and similar activities are likely to feature in an upcoming PDST Physics workshop.

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