The community has found that due to the way manufactures of devices running Android filter out noises within their microphones using algorithms baked into the device, the community has now deemed tuning the belts via an app on Android as inadequate. There could be device settings that you need to change however to otherwise get accurate results, but since that varies greatly amongst Android devices, we have elected to not list that here as it would be rather lengthy. It is encouraged to look up your device's model name and read about your microphone settings to perhaps correct the inaccuracies when using these apps on Android.
These apps utilize your device's microphone to show you the tension (in hertz) of your belt. As such, you need to ensure you are pointing your device's microphone closest to the belt as possible.
For iPhones, most modern models have the microphone located on the bottom of the phone.
For Android devices, this varies since Android is an operating system that powers many various phones made by different manufacturers.
To ensure you know precisely where your device's microphone is located, it is highly recommended to lookup your device's model in a search engine.