Most audio equipment testing is done on a stimulusresponse basis. A signal of known characteristics is fed to the input of the Device Under Test (DUT) and the output of the DUT is measured. The performance of the DUT is determined by degradation of the output signal from the known input signal. Often, sets of measurements are made as the stimulus is swept or stepped across the audio frequency spectrum or across an amplitude range, and the desired performance information is determined by the relationship between the corresponding set of output measurements.