Double Acoustic Stopwatch and Sound Speed Timers Get the time between two acoustic events measured by smartphone and arduino Nano 33 sense board BLE and determines the sound speed. This experiment allows to measure by two distante sensors the time between two loud acoustic signals produced sequentially near to each sensor location. These can be clicks, beeps, claps etc. as long as they are louder than the environment. You might want to adjust the threshold, giving the level at which the stop watch is triggered (ranging from 0 to 1 for smartphone). After launching the experiment (run), the clock will start on the first noise exceeding the threshold and will be stopped on the second noise. To repeat the experiment, clear the data and start again. Make sure that the first noise is short as a long sound might be immediately detected as a stop. New: Two more claps clear the data and let you restart the smartphone clock again. Then the sound speed is calculated by the difference of the time intervals measured by sensors: distance*2 / abs(time1 - time2). Change the threshold to be above the environmental noise level, but below the trigger noise (you can try the audio scope experiment to check these). Also set the minimum delay to avoid triggers shorter than that time (for example due to echo or reverberation). Unfortunately, this won't work on slow phones. Please compare to a regular clock first, to make sure, that the experiment is working as expected. T2S blue threshold mindelay recording rate i i2 t limit max last tlist dtlist tindex tcount tcount-1 t0 t1 t0effective t1effective tmax dt01 count index CH1 CH2 CH3 CH4 CH5 CH0 phone distance speed arduino_threshold arduino_mindelay CH1 CH2 CH3 CH4 CH5 CH0 arduino_mindelay arduino_threshold threshold mindelay dt01 CH1 arduino_threshold arduino_mindelay distance speed recording count recording i max i index i2 i2 rate t max threshold t 0 t last tlist last last mindelay limit limit t t 0 t t 0 t tlist tlist dtlist 0 dtlist tlist tcount tcount 1 tcount-1 index count index index rate tmax tcount 1 dt01 phone tcount 3 tlist tcount 3 0 index phone CH1 distance speed t0 t1 0 1 tlist t0 1 1 tlist t1 t1 0 tmax t1 t1effective t0 0 tmax t0 t0effective t1effective t0effective dt01 tlist