"Please note that while the resolution of the measurements may be as low as 1mm, the precision usually is not. While I have taken great care to make the app as reliable as possible, there are simply too many factors affecting the measurement process and the precision. That is why I want to be clear about one thing: there is absolutely no warranty that the measurements taken with Acoustic Ruler Pro are correct"
Another good example of incorrect usage of the word precision. In this case, the method is actually quite precious, as in measurements are very repeatable. What the author meant is that the accuracy is not very good. I tried out the app just now, at the range of 22 inch (width of my monitor), it under estimates the distance by 1 inch; and for something half an inch apart, it over estimates by over an inch. It is possible to measure the non-linearity using a control setup, but the result would be largely useless, as measurement of different items requires placing the item near the source and receiver, and this will likely change the acoustic behavior of the environment and affect measurement accuracy.
"Consider a spherical bear, in simple harmonic motion..." -- Professor in the UCB physics department