Nonlinear harmonic distortion vs. linear harmonics

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I have seen the statement (among others, by Mr. Timmermanns) that there is no discernible correlation between the sound of a speaker/driver and its distortion figures, at least as long as midrange distortion is below 1%.

You can also find the statement that 2nd can be tolerated, 3rd less so whereas higher orders sound really nasty.

Typical publications will give "harmonic distortion" figures, probably without much thought.

The whole concept of harmonic distortion assumes that there is a nonlinear transfer function. E.g. a transistor amp will deliver an output voltage that is not strictly proportional to the input voltage. Or a spring in a loudspeaker will no longer exert a force that is proportional to the displacement for sufficiently large amplitudes.

A nonlinear transfer function, as can be shown from Fourier analysis, will result in added harmonics at multiples of the original signal frequency. These harmonics increase overproportionally as the signal grows!

However, there can also be linear harmonics. A mechanical resonator such as the air in a pipe, a string in a musical instrument or the membrane of a loudspeaker will oscillate at a multiple of the fundamental, also known as harmonics. As long as the forces (compressibility of the air or stiffness of string or membrane) are linear, these harmonics will remain at a fixed percentage of the fundamental amplitude.

I suspect that linear harmonics will just change the tonal balance but will sound much less nasty than harmonic distortion products, the percentage of which changes with the amplitude.

The standard speaker test will just analyse the harmonic content and report this as "distortion".

Testing the harmonics at different volumes and separating linear and nonlinear portions would be a more meaningful test. Maybe a correlation between the subjective sound and the percentage of true nonlinear distortion can then be found...


I would be particularly interested in how the high distortion of the Newtronics Mega speaker that uses the Manger MSW is composed. A plot can be found in phase_accurate's introduction thread. I have asked Dave to move that to a separate thread.

Eric