05-26-2023, 12:50 PM
Ah, I see. I am quite sure that the reason is that the acquisition rate is too low in this configuration.
Phyphox looks for the first zero crossing of the autocorrelation as t0 and uses it to estimate the location of the first maximum expected in the range 3*t0 to 5*t0. Then it calculates the highest multiple of that maximum that is within the autocorrelation and uses that maximum to determine the periodicity.
The problem now is that the second value already crosses zero and I assume that the actual maximum is numerically outside the 3*t0 to 5*t0 range (hard to tell from the screenshot).
However, the acquisition rate is artificially limited in this experiment, because older phones (especially several years ago when we released this experiment with phyphox) struggled with calculating the autocorrelation at higher rates while "typical" spring oscillators usually do not reach 10Hz. I have attached a version that does not limit the rate - please try it by either opening the link on your phone or scanning the QR code from the "+" menu in phyphox.
Phyphox looks for the first zero crossing of the autocorrelation as t0 and uses it to estimate the location of the first maximum expected in the range 3*t0 to 5*t0. Then it calculates the highest multiple of that maximum that is within the autocorrelation and uses that maximum to determine the periodicity.
The problem now is that the second value already crosses zero and I assume that the actual maximum is numerically outside the 3*t0 to 5*t0 range (hard to tell from the screenshot).
However, the acquisition rate is artificially limited in this experiment, because older phones (especially several years ago when we released this experiment with phyphox) struggled with calculating the autocorrelation at higher rates while "typical" spring oscillators usually do not reach 10Hz. I have attached a version that does not limit the rate - please try it by either opening the link on your phone or scanning the QR code from the "+" menu in phyphox.