Tested this app on my Apple device.
The app is relatively useful but one need to input lots of data like racquet size, diameter, string diameter etc. One also need some measurement tool to measure the dimensions before getting the app to work. A ruler,caliper and tape measure would be useful for precise measurement. Many parameters to input in order for it to work properly.
Besides, one also need an awl with a plastic handle to tap on the strings. The String or racquet have to be very close to the Apple device to register a consistent sound.