I used one of the codes on reddit. Though it looks like it's free now.

Nice work!

Couple of thoughts:

- I only have ear phones with a mic. It wasn't immediately clear how I should measure something. Perhaps a tutorial? If I hadn't watched the video first, it would have been difficult.

- It's loud, is there a different frequency that could be used?

- Without any other background info, it leads one to think that the temperature is the only additional variable in the equation. Wouldn't it then be possible to tell the temperature with a known distance?

- Unless I have multiple iOS devices, I can only measure just past a meter (from the video). Combining this with the fact that it's not very precise, it would be much more useful if I could measure longer distances (e.g., sonar).