Most audio equipment testing is done on a stimulus-response 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

frequency spectrum or across an amplitude range, and the desired performance

information is determined by the relationship between the corresponding set of

output measurements.