The graph to the left shows the response to a multitone excitation. The tones are 1/3 octave spaced sine signals.
Multitone testing is a rather novel, although established method to look for nonlinearities in electronic and electroacoustic systems. It is also used to test data reduction / compression algorithms such as mp3 and the like.
Traditional measurement techniques like swept sine, THD, DIN / SMPTE two-tone intermodulation distortion measurements are limited in revealing non-linear behavior due to the simplicity of the excitation signal. Besides many other advantages, however, multitone signals are able to approximate the frequency domain energy distribution of typical program material such as music.
As far as I understand the state of affairs, optimization and standardization of excitation signals / patterns are still ongoing.