Glossary
TIM

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.

The test signal is commonly known as a multi-tone because it consists of a collection of tones. It sounds like someone leaning on a church organ with both forearms. In the example below there are 43 tones logarithmically distributed between 40 and 20k Hz. All the detail on the graph between these tones is the intermodulation and harmonic distortion, and this is what should be minimized. Like harmonic distortion, a lower intermodulation distortion results in a more transparent, cleaner sound quality. Unlike harmonic distortion, intermodulation distortion levels do correlate well with perceived sound quality, the lower the better.



Intermodulation distortion of a large three-way loudspeaker

Intermodulation distortion is non-linear with level, in that an increase of 10 dB in the test signal typically results in a far greater increase in level in the intermodulation distortion. As a result one should check the test conditions before comparing measurements of different loudspeakers. In general, larger loudspeakers suffer from less intermodulation distortion than smaller loudspeakers when played at the same level. Additionally, three-way loudspeakers will suffer from less intermodulation distortion than two-way loudspeakers as each driver has less work to do.

In the graphs below, one can see the advantage of introducing a midrange driver. The midrange intermodulation distortion is reduced by 10-15 dB. 



Intermodulation distortion of
a two-way loudspeaker


Intermodulation distortion of
a three-way loudspeaker

In the graphs below, one can see the advantage of adding a subwoofer to the three-way system. The low-frequency intermodulation distortion is reduced by 10-15 dB, and the midrange intermodulation distortion is reduced by about 10 dB. Additionally, Doppler distortion seen in the 2-6kHz region is removed when a subwoofer is added. This is because the bass driver no longer modulates those frequencies as its excursion has been reduced. This is audible as a cleaning up of the high-frequency region. The result is an audio transparency from a compact three-way / subwoofer system that is comparable to that of a larger three-way system. 



Intermodulation distortion of
a three-way loudspeaker


Intermodulation distortion of
a three-way loudspeaker with a subwoofer