Flexy Kick is a simple yet flexible Bass Drum synthesizer for the iPhone. Touch position controls the pitch envelope, in the inside of the oval the envelope is negative while outside it is positive. The inclination of the device changes the pitch and the attack.
Swipe with two finger to get to the control menu, there you can set the central pitch, pitch range from 0 to 4 octaves, decay time of pitch envelope in % of volume envelope, maximum length of the sound and touch sensibility.
Good headphones are recommended as the sound has low frequencies.