Normalization (to the frequency response) is not allowed in this case because the measured distortion values behave in a non-linear way and thus normalized curves would not be comparable.
But I have to agree with you, the presentation of IMD of S&R (
sound and recording magazin) is not helpful in terms of comparability with other speakers.
In the PA market it might be helpful to know that for a loudspeaker A the average IMD of 10% is reached at 104dB SPL and for loudspeaker B at 107dB.
It would be much better if there were IMD measurements for fixed sound pressure levels (e.g. 85, 95, 105 dB) and then the limits for 1%, 3% and 10% IMD, based on the individual frequency response of the loudspeaker, were plotted there.
This could then look something like this:
View attachment 141754 View attachment 141755 View attachment 141757
Source: https://www.lowbeats.de - additional curves were drawn by me
In this way, the measured multitone distortions MD (HD + IMD) are directly comparable. One could also immediately see where a speaker has particularly big problems.
For me this kind of evaluation is easy to understand, but I don't know anyone who uses this (apart from me
).
It would be even easier to understand if the MD were given directly as a frequency-dependent percentage value curve - I agree with Amir on this.
Whether this kind of evaluation of IMD brings an added value or whether the measured harmonic distortions basically already anticipate the result of the IMD measurement in 99% of the cases
***, would have to be examined more closely on the basis of selected loudspeaker measurements.
*** Update:
The statement refers to "comparable" speakers. If one 2-way loudspeaker has an f3 of 80Hz and another has an f3 of 40, there are of course significant differences in IMD (for the same driver size) that are not captured by an HD measurement.
In this case, the IMD measurement provides added value.
If both speakers were equalized to the same frequency response, then most likely the speaker with the lower HD will also have the lower IMD (but that is just my experience).
I couldn't agree more. The THD evaluation of S&R does not allow a comparison of the results of the individual tested speakers, because it is not clear by which order of harmonic distortion the THD is significantly influenced.
The measurements of the harmonic distortions HD, splitted into HD2 to HD5, as shown by
@amirm or
@hardisj, are much more informative, especially because the masking decreases significantly with the increase of the order of the HD.
The Geddes and Lee studies were certainly in the right direction, but unfortunately the correlation was not very high.
Source:
Perception & Thresholds of Nonlinear Distortion using Complex Signals
There are other approaches that are more promising, but for many completely unknown - see linked study.
The study is already a few years old, I am not up to date on the latest developments.