ImproVox packs a lot of complex features in an amazingly simple interface.

Self-described "vocal instrument" ImproVox is designed to bring the complexity of recording-studio equipment to iOS devices wrapped in a very simple and friendly interface. Compared to what you would expect from studio hardware, ImproVox is as far from intimidating as you can possibly get.

The interface is broken down into three main parts: the left pad changes the harmonic content of the output as you drag the ball along it. The right pad works in the same way, but applies effects to your voice, such as reverb or echo (double-tap it to reveal more abstract effects).

You can share your creations, listen to others’, or just sing along to a track in your library.

At the bottom, you can alter the harmony style by selecting one of five icons: choral harmonies, pop, barbershop quartet, jazz, and baroque. The happy and sad faces on the right alter the mood you’re creating (from major to minor). You can also change the key you’re singing to on the left with a couple of buttons, or use your own voice to set it so that ImproVox works around your abilities and you’re perfectly in tune.

As you sing, you hear four-part harmonies around you in stereo through your headphones. It’s incredibly impressive and a lot of fun to play with.

You can use the Share button to record your song, and even import songs from your iTunes library and sing along to. Log in to SoundCloud (you can register straight from the app for free) to get tracks created by other users of ImproVox, and see just what’s possible.

Set the key by singing a note for a few seconds.

ImproVox works with your device’s built-in microphone but has trouble interfacing with external mics, especially USB ones. However this is something the developers are working on fixing as soon as possible. The universal app is really designed for the iPad, but still works very well on an iPhone (3GS or 4) or iPod touch (3rd or 4th gen), despite their much smaller screens.

The bottom line. ImproVox brings complicated sound effects and harmonization to consumers and makes it so easy, anyone can sound great with it.