AudioTools - If you've had a desire to test, tweak, optimize, or even build your own music system from scratch, the tests included in the AudioTools App will make your job much easier. All this ergonomic elegance can be had at a price that makes stand-alone test gear almost completely obsolete. The basic AudioTools App package costs $19.99. It comes with four of its twenty-four modules enabled. These active modules include a SPL meter, real time analyzer, RTA, FFT (fast Fourier transfer), ETC (energy time curve), Impulse Response test, and Smaart™ Tools 1 test suite, (RTA), signal generator, and a recorder.
Smaart™ Tools was developed by Studio Six Digital in collaboration with Rational Acoustics and is based on their proprietary algorithms, techniques, features & color mapping. Line input tools feature a signal generator, audio scope, VU meter, THD+n (total harmonic distortion plus noise) and an amplitude sweep generator. Under the Speaker Tests menu you'll find tests for polarity, distortion, impedance, an impedance sweep, and a delay finder. The utilities section contains a signal generator, monitor, calculator, recorder, and data and audio file export functions. Under settings are the global, microphone, line input, and line output adjustments.
How do I use Audiotools? Primarily to check SPL levels, but obviously, it can do far more than that, especially if you opt for Studio Six Digital's dedicated microphone for measurement.
Tone Generator by Michael Heinz - Tone Generator does exactly what you'd think it would do - generate test tones. For the princely sum of $.99 it can create any test tone from 10 Hz to 25 kHz, and play it through your iPad/iPod/iPhone's built-in speakers (only good for a limited frequency range) or through an eternal speaker or headphone. Sure, you can use it to test your hearing, or to find out what frequencies your speakers or headphones attenuate or amplify in a non-linear manner. As their lead review says, "A Simple tone generator that works well."
PlatterSpeed - If you use a turntable, you're going to want to own this app. What it does, in a nutshell, is tell you exactly how accurate your turntable's speeds are. Instead of relying on a strobe disc or other visual cues PlatterSpeed uses a test disc with a 3150 test-tone that you play on your turntable. Just play the disc while using this app and it will give you a very accurate chart of your turntable's speed variations. The disc is available from Dr. Feickert's online store. The record costs 17 Euros plus shipping, which is quite a bit more than the app but short of pressing your own disc, this is what you'll need to use this app.
Audio Tool by Performance Audio - Yes, the name is VERY similar to the first app on my list from StudioSix, and it has some of the same functionality, but this suite of test and measurement apps is different and complementary. Although you can buy each module separately for $.99 each, the suite is a better value. The suite contains a decibel meter, metronome, recorder, oscilloscope, tone generator, and bit-rate space calculator. The decibel meter is especially nice, with easy to read graphics and excellent low-level sensitivity.
Genelec Speaker Angle - This $.99 (but sometimes free) App from the professional and active speaker manufacturer, Genelec, makes getting the toe-in on your speakers a snap. Merely point your speaker straight ahead, put your iPod or iPad on top of the speaker, turn your speaker to the toe-in you'd like, and it will tell you the exact angle. Then merely adjust your other speaker to match that angle and you're done. For anyone who wants to be precise with their toe-in angles, this is the app to have. I use it a lot when I set up a new speaker on my desktop system, where the toe-in is even more critical than in a room-based set-up.