Optimized sound waves presented to the listener by headphones, notwithstanding differences in ear geometry and headphone positioning. A test signal causes an acoustic sensor to receive sound waves actually formed in the listener's ear cavity. A response from the sensor is compared with an expected ear cavity transfer function, from which desired adjustments to the audio signal are determined. The audio signal might be received from an application program, calibrated by an interface software element, and adjusted thereby, before forwarding to the headphones. Calibration might be performed from when the headphones are positioned, or dynamically in response to changes in the transfer function.