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 adds new frequencies to the acoustical output that were not present in the electrical input signal. To measure this, a frequency (the fundamental, say 100 Hz) is played into the loudspeaker at a particular sound pressure level and a frequency above that test frequency is measured. Double the frequency is the second harmonic (200 Hz), triple the frequency is the third harmonic (300 Hz), etc. Sweeping the fundamental frequency allows a graph of frequency-dependent harmonic distortion to be plotted. Total harmonic distortion (THD) is the relation between all sound coming out of the loudspeaker compared to all the additional sound in the output that was not present at the input: second + third + fourth + fifth + etc.
Harmonic distortion can be expressed in decibels or as a percentage. Second-order harmonic distortion is generally caused by asymmetries in the system. Third-order harmonic distortion is generally caused by “clipping” in the system, and this can come from the electronics or the acoustics, for example short voice coils. Odd-order harmonics generally sound a lot worse than even-order harmonics. Higher order harmonics will be lower than the second- and third-order harmonics, and should be at reasonably low levels in a well-designed system. Ideally the lower the harmonic distortion, the cleaner, or more transparent, the loudspeaker will sound. Less than -30 dB (3%) at low frequency and less than -40 dB (1%) at mid-high frequencies is normally considered to be good, lower values than these is of course better.
Harmonic 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 the level of harmonic distortion. As a result, one should check the test conditions before comparing measurements of different loudspeakers. In general, larger loudspeakers suffer from less harmonic distortion than smaller loudspeakers when played at the same level. Additionally, three-way loudspeakers will suffer from less harmonic distortion than two-way loudspeakers as each driver has less work to do. Both these effects can be seen together in the two examples below which are tested at the same sound pressure level.
Large three-way loudspeaker O 410
Small two-way loudspeaker KH 120
Unfortunately, the level of harmonic distortion does not correlate well with subjective sound quality, for example, an audio system with high levels of second order harmonic distortion can sound quite pleasant, whilst the same system with same level of third order harmonic distortion would sound rather poor. However, harmonic distortion plots are very useful for design engineers to use as a tool to trace problems in their designs.
Note also that even if second order harmonics created by a monitoring system might sound good (like a tube distortion, exciter, compression, etc.) the signal is changed by this distortion. The acoustic output therefore differs from the electrical input signal, which must be avoided as much as possible.