Multitone Distortion, Measurement and Evaluation

[amirm]

I concluded that Doppler is a real limitation, that cannot be circumvented, but is mitigated by using n-ways, n as large as possible.

Doppler has been analyzed both theoretically and with controlled listening tests to be a non-issue. In rooms, doppler effect decreases as you go off axis. Since bass frequencies are omni directional, much of that energy comes from other axis than 0, sharply reducing Doppler. Even without this factor, controlled testing shows that in the type of excursions we have in woofers, it is not an audible concern.

See AES paper,
THE AUDIBILITY OF DOPPLER DISTORTION IN LOUDSPEAKERS
Roy Allison, Allison Acoustics Inc.
Natick, Massachusetts, and Edgar Villchur
Foundation for Hearing Aid Research

"Although Doppler distortion in loudspeakers has been often
viewed with alarm since Beers and Belar described it in 1943,
the question of its significance in music reproduction has not
yet been answered. In this study the audibility of Doppler distortion
in simple direct radiators is investigated theoretically
(by analogy to tape-machine flutter and by analysis of
listening-room acoustic effects), and experimentally (by double-blind
listening tests). The analysis predicts Doppler inaudibility
for any practical cone velocity, and the experimental results
provide confirming evidence."


Here are some snippets from the listening tests:

"All eight of the judges evaluated the Doppler-contrast pair
of organ music recordings as the same.
Seven of the eight judges evaluated the Doppler-contrast
pair of orchestral music recordings as the same. One judge evaluated
them as different, marking the channel with Doppler as
more accurate."

"
As in experiment 1, a listening jury was asked to compare passages
of music reproduced in two modes, one with and the other without
Doppler distortion.
[...]

"The reverberant-field correction factor that reduces the
amount of Doppler distortion in normal rooms is not operative
in this experiment, since the on-axis Doppler distortion that
was recorded in the anechoic environment is not diluted by any
off-axis sound. "
[...]

"All eight of the judges evaluated the Doppler-contrast pair
of orchestral music recordings as the same.

Five of the eight judges evaluated the Doppler-contrast pair
of organ music recordings as the same, but one of the five
added a question mark on the response form. Three judges evaluated
this pair as different; one of the three indicated that
the channel with Doppler was more accurate, one that the channel
without Doppler was more accurate, and one that neither
was more accurate than the other.

There was no agreement among the three judges who detected
a difference between the Doppler-contrast organ recordings as
to which channel was more accurate, and it appears likely to
us that their responses were determined by a factor or factors
other than Doppler distortion."

So let's not jump on these things without properly reviewing authoritative work already done.

 

[Heinrich]

I don't know where the appeal of IMD is coming from. I just don't see that in research.

First HD is masked by the harmonics of musical instruments, voice, except for percussion and a plethora of natural sounds that are not 'harmonic'. Lots of subtleties to be explored.

HD can be minimized to below the threshold of audibility with relatively low effort, keeping commonplace form factors for the speakers, see Purify, in major parts SB Acoustics, while the former class leader, Scan Speak, seems to depart from that route for some time now.

Low HD devices regularly show minimized IM likewise. But at least Purify showed that IM may overtake HD by some margin. If taken serious, IM can be seen as a new frontier, now that HD seems to be under control.

If my subjective and anecdotal experimentation (I've got some training) holds, well, the limits are easily reached. Say, the common 7" bass/midrange will inevitably exhibit 1% Doppler distortion if used from 50Hz to 2kHz @90dB spl, which is not that irregular. It may be debateable, if Doppler fully converts to IM in-room, but if it does, and IM is objectionable, there is a huge gap between very low HD and maybe audible IM.

My very subjective impression is, that low IM is beneficial, clarity and so forth ..., even, or should I say especially with small speakers. It is totally understood that my personal results are not big science. I would love to see more of this for comparison. Maybe after a scientific program is set up on that.

Doppler has been analyzed both theoretically and with controlled listening tests to be a non-issue. ...

So let's not jump on these things without properly reviewing authoritative work already done.

Thanks for addressing this. O/k, I reformulate my concern and give a practical example for everyone.

I linked to an internet page hosted on the site of Purify above, twice already?. Their objective is to show the difference of AM versus FM (Doppler), and it is advised to use headphones. O/k, that works, it shows. Once I try the same with my (virtually) IM-free speakers in-room, it dosn't work no more.
If somebody could explain, please, appreciated! (I said that all the time, I'm not a fool.)

ps: the experiment you cited states the following, so the listening room, crucial for my (and Linkwitz') argumentation, is not part of the equation?

"The reverberant-field correction factor that reduces the
amount of Doppler distortion in normal rooms is not operative
in this experiment, since the on-axis Doppler distortion that
was recorded in the anechoic environment is not diluted by any
off-axis sound. "

 

[Rja4000]

the weakness of IMD is that it creates a spray of distortion products, all of which are at lower amplitude than single harmonic spikes.
[...]
I don't know where the appeal of IMD is coming from.

But if the IMD spikes amplitude was similar to harmonics amplitude, they would probably be more audible, due to less "masking", wouldn't they ?
(Just a theoretical question)

 

[Heinrich]

But if the IMD spikes amplitude was similar to harmonics amplitude, they would probably be more audible, due to less "masking", wouldn't they ?
(Just a theoretical question)

I'm not addressed personally, but if you permit? The IMD amplitude is regularly as big, or has at least the same order of magnitude as HD. Only in the multitone it comes down to the infamous 'gras' at -60 or so dB, almost always contaminated by environmental noise. In the multitone the HD is not readily seen either ;-) Seems the multitone as such isn't that usefull. May it better have some frequency weighting similar to pop music, 20dB plus in bass etc.

Btw, did you try the Purify demo, linked by me above?

 

[Rja4000]

The IMD amplitude is regularly as big, or has at least the same order of magnitude as HD.

Beware that for IMD SMPTE, IMD % is relative to the 7kHz spike, which is 12dB below the low frequency spike.
So 1% IMD SMPTE means the distortion product is 12dB below 1% THD, somehow.

 

[Heinrich]

Beware that for IMD SMPTE, IMD % is relative to the 7kHz spike, which is 12dB below the low frequency spike.
So 1% IMD SMPTE means the distortion product is 12dB below 1% THD, somehow.

Was SMPTE ever used for speakers? I first was tempted to show you my measurements on IM, Doppler in (dis)connection to HD etc. But then I considered that you possibly see me as a random layman, and so my effort would be wasted. May I give a hint on another authority?
There's an English speaking site hificompass, that does IM measurements on selected midwoofers, e/g SB17NRX2C35-4 in the section 'measurements'. There are more, and you could compare.

It is clearly seen, that IM and HD components are pretty similar in amplitude. I could also reiterate some bits and bobs on IM, but finally I think the topic is a bit more complex.