Pocket RTA HD - Spectrum Sound Analyser for the iPad: Reviews
I took a chance in buying a Shure X2U interface and it works great with your app as long as I use a USB hub and plug in the interface after starting the app. I can now use any mic I want with my iPad (even phantom powered). It's great to have a separate profile for the internal mic and external mic. I just calibrated the external mic at 105db and it passed with flying colors.
Good Pro Audio RTA
This really is a good rta tool with many features. You can easily calibrate the internal mic for a more accurate response however the noise floor of the internal mic is a little high to be used in lower volume settings. It would be really great if the Spectrogram could be logarithmic.