This app measures distance between earphone and iPhone.

https://apps.apple.com/app/id1509013330

I am not the maker of the app explicitly, but I did develop the algorithm behind it. The main problem was that any delay caused by the device disqualified the result as 1 nanosecond delayed made the measurement inaccurate. I spent 2 years and made the scientific paper on my University which began my adventure with measuring Sound Waves.

Just to add - I am not a physicist, but I like challenges.

If there are any people that are sound waves masters reading this - we can make many exciting apps together just DM me.

The next app we will make using the sound algorithms is a thermometer. I already have a proof of concept working. Have a nice day y'all!