Limited capabilities
The app seems to work to generate and view spectrograms, but it has limited capabilities because of the short recording time allowed. The in-app purchase is $9.99, which allows you to compare spectrograms. (I felt the developer was not upfront about the cost of the full version of this app.) You are not able to play two spectrograms simultaneously. There are better free apps available, if you just want to create a spectrogram.