Many video recording apps like Acapella don’t have the ability to adjust the input settings for USB microphones. This video shows you how to break into the input settings on your iPad and adjust the audio input for a perfect recording.
Setting up your USB microphone using Garage Band: https://youtu.be/3FaMah6aang?t=3m47s
Product Link: RODE NT_USB: http://amzn.to/2aN4tDe