MONO2 from iDesignSound is a great app for audio work. With frequencies ranging from 20 to 20,000 kHZ, it’s very useful for getting equipment set up. You can optimize speaker placement while using this app for output and be more confident in your placement decisions. The app can even help with finding harmonic frequencies.
You can read more of the technical details here: http://www.idesignsound.com/mono2-iphone/