Frequency response is typically measured by connecting an audio oscillator to the input of the system and measuring the input voltage as a function of frequency. Simultaneously, the output voltage is measured as a function of frequency. An alternative measurement technique, is to measure the signal to noise ratio as a function of frequency. This includes such things as power supply, ground loops, clock leakage and so on. Audio systems will often distort signals by adding harmonics to the primary signals.
Frightening things happen when two frequencies are fed into an audio system.
A non-linear transfer function will produce harmonics of both frequencies, sums and differences of both frequencies, and harmonics of the sums and differences, and sums and differences of the harmonics. In general, the harmonic distortion is separated from the intermodulation distortion. Thus, in an intermodulation measurement, only the sum and difference terms are taken. Summing these sine wave reconstructs the original signal. This filter will eliminate any frequency components higher than a certain value.