I'd be indebted for your comment on the following. Maybe 15 years ago Bob Carver (yes, that BC) told me that listening tests support the conclusion that human hearing sensitivity to THD in the bass range is so low that humans perceive little if any difference between 1% and 30% THD.
I posted the above many times and shockingly, no one ever contradicted it. (Conversely, I presume there is similar agreement that human sensitivity is extremely high to mid range phase and THD. John Krutke of Zaph Audio posted that humans have little sensitivity to large FR aberrations in the top most octave, IIRC, windows as large as 6 dB.)
Back to bass THD: What's critical about Carver's point is that any money spent reducing THD below the threshold of audibility is wasted. IOW, THD spec below the threshold of audibility is, in effect, nothing more than window dressing. I don't know how true or false is Carver's statement, but my experience indicates a resounding yes.
A friend of mine used to build 400+# floor to ceiling line source subs with four or six active 12s. Over the years I heard several similar sub systems, including IRS III in dedicated sound room. Every time I heard them I was impressed. I became convinced their most attractive quality was exceedingly low THD, you know, woofers barely moving except for canon shots, etc. Now, with further experience of other sub systems with higher THD but natural mode damping features, I'm convinced the most audible feature of the line source subs was their natural mode damping for one of the most audible modes, being floor to ceiling bounce.
I think the only spec I've ever seen re. modal effects is FR graph or simple numeric dB. Considering that modal effects are 100% synthetic bass notes generated by distinct numeric relationships (listener or mic location, boundaries, speaker locations) I have always thought that modes expressed as THD can easily be well over 100%. Please LOL at my guess and correct at will.
But the most critical defect about modes is their effect on timing. I think of modal effect as a synthetic note only indirectly related to the original note in the music score, and this unrelated note continues playing over the next note appearing in the score.
I suspect MVW has natural mode damping quality, and this may be among it's best, most audible, and most desirable features.
Just as a point of disclosure, or disclaimer, Bob hired me as a consultant in the early 1980’s to develop the Carver Amazing Loudspeaker (Planar Magnetic Line Source // Dipole Subwoofer) for him, and then in the early 1990’s, I was the Chief Technology Officer at Carver until 1997. We did a lot of psycho-acoustic threshold testing over a two-decade period, leading up to, and during, that period.
I will attempt to give the short form answer to your question, and then we may want to take it off-line if you wish to explore it further, as this being Audio Kinesis’ forum, I don’t want to inappropriately high-jack Duke’s forum with a non-AK discussion.
In terms of the high threshold of up to approximately 30% for THD at low frequencies, all thresholds are context dependent, but, I’ll give a conditional answer of “Yes” relative to listening to music in a standard living room as the use-model where this very high threshold tends to be realized.
If you are to listen to sine waves, in an anechoic environment, the majority of listeners will have a much lower JND (Just Noticeable Detection) level threshold, by a factor of 10 or more.
So, what we have in standard living room, use-model conditions, is a situation wherein a number of parameters, such as THD and phase, for instance, are masked by more dominant issues, such as environment boundary reflections (modal and non-modal bands) and source program complexity.
“Linear” room modes at low frequencies will tend to dominate over many other “non-linear” forms of distortion (but not all).
It is somewhat of an over-simplification, but to a great degree, one can make a Pareto list of variables in order of audible and masking dominance for a given audio system interface environment. In general, as one minimizes or eliminates a top-level system global variable, the remaining local variables will move up on dominance and JND thresholds.
To restate, THD thresholds are determined to a great degree by our binaural, ear/brain psycho-acoustical hardware scanning an audio delivery system dominantly impacted by a the relationship to the environmental boundaries and the nature of the type of complexity of the particular program source (at a given moment).
Your friend's floor-to-ceiling line-source system delivering impressive sound quality is most likely, strongly influenced by, as you suggested, the vertical room mode (and if he maintained an infinite line source throughout the midrange also, it can make for even more improvement by minimizing vertical, correlated reflections, which can be dominant for voice colorations) and also for the multi voice coil derived minimization of thermal compression, which is a type of audible distortion that is more complex that simple THD.
I would suggest that Duke’s conservative approach to minimizing thermal compression is a significant benefit of his design approach.
The position that money spent reducing THD is wasted, is generally true, at least to the degree that one has not yet spent money reducing modal conditions to a point of equaling an anechoic space.
In terms of your notion that modal effects are 100% synthetic bass notes that could be expressed analogously to equal or greater than 100% THD, is I would say at least partially correct, at least as an analogy.
One has to be careful comparing linear vs. non-linear effects, as they disrupt the signal in very different ways from each other, and are also perceived differently from each other. Modal effects are, mostly (that’s another story for another time), linear where as THD is by definition, non-linear.
That said, the modal output of a room is like adding an additional set of sound sources (like having more speakers that are not very good!, each with substantial frequency dependent amplitude errors) adding their outputs to the primary “real” LF speaker(s). From this standpoint, one could say that the ‘positive’ modal output is greater than that of the primary signal, and therefore creates more than 100% “linear” distortion.
In terms of modal impact on timing, I’ll save that for later, as it is a more complex subject, both as a produced reality and a psychoacoustic perception.
In terms of MVW having natural mode damping qualities, I have to say that without your experience of the device, I am skeptical of its ability to effectively damp room modes. From my initial evaluation of the internal structure, I DO
believe, that MVW could be effective at damping the internal resonant modalities that normally are problematic with wave-resonant air-columns. In the past, I have effectively squelched negative modal effects of quarter wave air-columns by way of internal structures similar to that shown in the MVW patent. But, again, I’m not sure what effect you are experiencing with MVW relative to what you experience as “natural mode damping”, so I’m open to learning more about that effect, if it does exist.
I hope this at least partially addresses some of what you were asking about. If not, please redirect me to the more specific point you were looking to discuss.
All the best,
- Jim Croft