Loudspeakers suffer from linear and non-linear distortion. Linear distortion can be seen as a non-flat frequency response and/or a non-flat group delay. This is discussed in other sections. Non-linear distortion in the loudspeaker system adds new frequencies to the acoustical output that were not present in the electrical input signal. One measure of this is harmonic distortion, where a single frequency is played into the loudspeaker and the level of each harmonic measured. Another measure of distortion is to play a more complex signal into the loudspeaker and measure the additional frequencies in the output. The problem with this is that there are no internationally agreed standards for the test signal so comparison of different measurements is impossible unless the test conditions are identical. Since an intermodulation distortion measurement test signal consists of many discrete tones played at the same time, this type of broadband measurement signal is representative of real world usage of the system unlike the pure sine tones used for THD measurements.
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