Calibrate threshold

[roccomuso]

I noticed that threshold needs to be adjusted from device to device. There isn't a way to calibrate it?

[alexgibson]

If yourself or anyone else can find a reliable way to do this pull requests are welcome

[davidje13]

You take a direct difference between accelerations (https://github.com/alexgibson/shake.js/blob/master/shake.js#L100) without accounting for the sampling frequency. That means that any variance in sampling frequency will change the effective threshold. You should at a minimum divide the difference by e.interval, and ideally perform smoothing on the values to simulate a consistent low sampling rate.

Those changes should make this perform near-identically on all devices.