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(Informal) Chat with Dr. Earl Geddes of GedLee Audio

hardisj

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#1
Last week I asked Earl if he would be willing to come on my show and talk about a few different topics ranging from nonlinear distortion to how reviewers can do better. He obliged. It was a great opportunity to talk to someone who I truly admire and respect. You’ll have to excuse me for occasionally “fan-girling”. ;) :D


Personally, the discussion gave me some ideas on areas I can improve my reviews. If you watch you’ll see we talk about compression and I mention that I perform what I would consider a “dynamic” test. He suggested doing long term compression and convinced me to add that back in to my arsenal. So I’ll be incorporating that in future reviews but in a different manner than I had done before.

Here is a link from Linkwitz regarding minimum phase research he had lightly gotten in to (I don't know if there is more up-to-date; this was the most pertinent returned in my search):
http://www.linkwitzlab.com/Attributes_Of_Linear_Phase_Loudspeakers.pdf
 
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Gorganzola

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This is a longish interview that I watch in it's entirety latter on today. For now I will make brief comments based on my limited exposure to Dr. Geddes work which goes back for a decade.

Dr. Geddes is a scientist and has employed rather complex mathematical means to formulate his acoustic distortion theories -- these are well beyond my grasp. But here is my layman's, simplistic take-away. Total Harmonic Distortion (THD) is relatively useless but the spectrum of harmonics is relevant. Even more simplistically, 2nd & 3rd harmonics are benign, even pleasant, while higher harmonics are progressively harsher and more objectionable.

Certainly this is consistent with my personal exposure to subjectivist audiophiles. The folks are predominantly tube amplification enthusiasts, a fair portion of whom really like SET amps. Most of the rest prefer amplification from the likes of Pass Labs whose amps are not especially low in THD but have notably high 2nd and/or 3rd order harmonics.

E.g. check out this Stereophile-John Atkinson review of the Pass XA60.5 class A amp which Atkinson declared at the time was the best sounding amp he had ever heard:


A corroborating fact in my mind is that I know of many audiophiles who really like the Pass-designed Amp Camp amplifier. Check out it's harmonic spectrum as measured by our very own Amir ... see his measurements here ... Horrendous, the worse amp Amir has measured, but nonetheless enjoyed by many audiophiles. We can see that high distortion trails off only gradually as far as the measurement goes. But not that 2nd, followed by 3rd harmonics are the highest.

In my mind it brings up the possibility that 2nd and 3rd harmonics and not only benign in themselves but also serve to mask disagreeable higher harmonics.
 

KSTR

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#5
As for the testing a different levels and looking for FR changes, preconditioning and measuring at the same time can most easily be done with periodic pink noise (with the -3dB droop correct for). REW is perfect (once again) for this.
 

KSTR

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#6
In my mind it brings up the possibility that 2nd and 3rd harmonics and not only benign in themselves but also serve to mask disagreeable higher harmonics.
Any distortion is only halfways benign when it is lower than the ear's own distortion... but even then it is not really benign (depends on the music material, of course).
 
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hardisj

hardisj

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Thread Starter #7
As for the testing a different levels and looking for FR changes, preconditioning and measuring at the same time can most easily be done with periodic pink noise (with the -3dB droop correct for). REW is perfect (once again) for this.
I've already set up the template for testing this with Klippel software. :)
 

Gorganzola

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#8
Any distortion is only halfways benign when it is lower than the ear's own distortion... but even then it is not really benign (depends on the music material, of course).
I guess so but at what levels is the ear's own distortion? In the relativity of things lots of tube amps and others have 2% or higher distortion especially at high frequencies, that's -34 dB.
 

KSTR

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#9
The ears' distortion is really quite large and hence produces a lot of IMD.

Which is the root cause for a lot of hearing phenomena like why equal temperament tuning sound off, and many more.
 
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hardisj

hardisj

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Thread Starter #10
The ears' distortion is really quite large and hence produces a lot of IMD.
This is such a hard thing for me to grasp. And I don't know how to explain why that is. But, it is, nonetheless...

I think I just have trouble saying "oh, my ears have distortion" and that being enough. I mean, on the surface, okay... but is there more to that statement that I can do to help me understand how much distortion my ears have or why. What is this "distortion"? Is it the other things going on around us, outside of what we are focusing on? Is it some kind of self-noise our own body generates in some weird way?

Kind of like asking how to make it tangible, I suppose.
 
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Spkrdctr

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#11
This is such a hard thing for me to grasp. And I don't know how to explain why that is. But, it is, nonetheless...

I think I just have trouble saying "oh, my ears have distortion" and that being enough. I mean, on the surface, okay... but is there more to that statement that I can do to help me understand how much distortion my ears have or why. Kind of like asking how to make it tangible, I suppose.
After the ear gets done doing what it is going to do to the signal, your brain then says OK, I see there are still big problems, Here "Hold my beer" while I fix them for you. How do we quantify that? We do DBT to get the brain out of the picture as much as possible. The more we get into this the more I'm always amazed that we hear such beautiful music at all. It really is amazing how our ear/brain connection works so well. The human body is beyond amazing what it does every single day.
 

KSTR

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This is such a hard thing for me to grasp. And I don't know how to explain why that is. But, it is, nonetheless...

I think I just have trouble saying "oh, my ears have distortion" and that being enough. I mean, on the surface, okay... but is there more to that statement that I can do to help me understand how much distortion my ears have or why. What is this "distortion"? Is it the other things going on around us, outside of what we are focusing on? Is it some kind of self-noise our own body generates in some weird way?

Kind of like asking how to make it tangible, I suppose.
Please check out this paper: http://www.a3ccm-apmas-eakoh.be/ISBN978-90-816095-1-7/Book_Heerens_de_Ru_EN.pdf
While it did not manage to pass peer review to get published in a medical journal or the like (last I checked at least), most of appears to be true and many phenomena can only be explained by their theory.
The shocking bottom line is, on page 49:
Our careful analysis of the results that were obtained in the auditory pathway experiments that were executed by Wever and Lawrence [16], leads us to the conclusion that the auditory sense differentiates and squares the incoming sound pressure signal.
(bold mine).
Squaring (and this happens on the physical level before any nerves fire etc) obviously is a very heavy nonlinearity and produced tons of IMD products. The brain just factors it out, so to say, it has no other signal to work on. Therefore, adding a little distortion to a signal doesn't change much wrt what we perceive unless the external distortion is dominating.
They also have a test software to recreate their experiments : http://www.a3ccm-apmas-eakoh.be/
 
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puppet

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#14
Great interview Erin. It's particularly nice to see "another side" of Dr Geddes that isn't as restrictive as typical Forum back and forth seems to be at times. Very refreshing. You did a super job!
 

richard12511

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This is a longish interview that I watch in it's entirety latter on today. For now I will make brief comments based on my limited exposure to Dr. Geddes work which goes back for a decade.

Dr. Geddes is a scientist and has employed rather complex mathematical means to formulate his acoustic distortion theories -- these are well beyond my grasp. But here is my layman's, simplistic take-away. Total Harmonic Distortion (THD) is relatively useless but the spectrum of harmonics is relevant. Even more simplistically, 2nd & 3rd harmonics are benign, even pleasant, while higher harmonics are progressively harsher and more objectionable.

Certainly this is consistent with my personal exposure to subjectivist audiophiles. The folks are predominantly tube amplification enthusiasts, a fair portion of whom really like SET amps. Most of the rest prefer amplification from the likes of Pass Labs whose amps are not especially low in THD but have notably high 2nd and/or 3rd order harmonics.

E.g. check out this Stereophile-John Atkinson review of the Pass XA60.5 class A amp which Atkinson declared at the time was the best sounding amp he had ever heard:


A corroborating fact in my mind is that I know of many audiophiles who really like the Pass-designed Amp Camp amplifier. Check out it's harmonic spectrum as measured by our very own Amir ... see his measurements here ... Horrendous, the worse amp Amir has measured, but nonetheless enjoyed by many audiophiles. We can see that high distortion trails off only gradually as far as the measurement goes. But not that 2nd, followed by 3rd harmonics are the highest.

In my mind it brings up the possibility that 2nd and 3rd harmonics and not only benign in themselves but also serve to mask disagreeable higher harmonics.
The tube amp thing actually came up later in the video. I sounded like Dr. Geddes view is that while tube amps do have higher 2nd harmonic distortion, it's usually not high enough to overcome the very high masking our ears have for such distortion. So, people probably don't like tube amps for that higher distortion, but rather for some other reason, likely (imo) psychoacoustics(ex: placebo). Actually one of my main takeaways from the talk is just how incredible our ears are at masking low order harmonic distortion. Dr. Geddes mentioned one case where it was up to 20%, but was still too small to overcome the masking.
 
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Emlin

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#16
The tube amp thing actually came up later in the video. I sounded like Dr. Geddes view is that while tube amps do have higher 2nd harmonic distortion, it's usually not high enough to overcome the very high masking our ears have for such distortion. So, people probably don't like tube amps for that higher distortion, but rather for some other reason, likely (imo) psychoacoustics(ex: placebo). Actually one of my main takeaways from the talk is just how incredible our ears are at masking low order harmonic distortion. Dr. Geddes mentioned one case where it was up to 20%, but was still too small to overcome the masking.
And the fact that masking of harmonic distortion increases with SPL. Good news for speaker designers and manufacturers.

However, he thinks that sensitivity to IMD increases with SPL. Bad news.
 

MakeMineVinyl

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#17
In my mind it brings up the possibility that 2nd and 3rd harmonics and not only benign in themselves but also serve to mask disagreeable higher harmonics.
That's been my experience as well.
 

617

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Nice to see Dr. Geddes in an informal setting. One of the most influential public researchers in the field, I always learn something when he speaks. The comments about Ford's amplifiers intentionally distorting to create the impression of loudness is really interesting.

I can say with regards to distortion...the speakers Gedlee designed offer an interesting alternative, which is simply having really low distortion. A 12" pro sound woofer which only goes down to 80hz or so is clean and dynamic in a way which hifi woofers can only approximate. Gedlee Speakers are big, ugly and inconvenient but the reasoning behind a big woofer and waveguide in a big rounded enclosure with multiple subs is pretty sound. Besides alternative approaches like dipoles and maybe cardiods, everything else looks like a compromise.

The key insight that the really innovative researchers often cited here have is that the metrics we use to describe loudspeaker performance must come from psychoacoustics and be applied to engineering, not the other way around. That, to me, is what is provocative about the Gedlee distortion metric. Harmonic distortion as a measure comes from electrical engineering, why should it have anything to do with what we hear? Suppose harmonic distortion was a great metric for what audiophiles assume it is (grunginess, sweetness, whatever.) What a miracle it would be that a measure which can be attained with simple electronic instruments corresponds to our hearing! Same thing with so many other issues with loudspeaker design - well meaning designers fixate on physics problems without any regard to audibility, which leads to the ridiculous audiophile tendency to spend inordinate effort on infinitessimals (jitter, HD, first order crossovers, cables, capacitors) while being totally ill equipped to deal with what is obviously audible (room influence, intelligibility, bass.)

Erin I was also particularly intrigued by his comments about the transfer functions of small speakers crossovers changing due to thermal effects. Is this what you said you might test? I think you have an opportunity to innovate here, and definitively define a metric which encompasses the dynamic accuracy of a speaker and its 'smallness'. I don't think distortion vs. level tells a very useful story, your compression charts on the other hand seem closer to my experience with what speakers do when you ask them to be big.
 
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hardisj

hardisj

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Erin I was also particularly intrigued by his comments about the transfer functions of small speakers crossovers changing due to thermal effects. Is this what you said you might test? I think you have an opportunity to innovate here, and definitively define a metric which encompasses the dynamic accuracy of a speaker and its 'smallness'. I don't think distortion vs. level tells a very useful story, your compression charts on the other hand seem closer to my experience with what speakers do when you ask them to be big.
Yep. I used to test compression as part of my max SPL testing and provided the data. But I don’t think many understood the results and it was never really discussed. It just didn’t seem to catch on like I expected it to. So I decided it wasn’t worth continuing given the time and complexity of that particular test method.

However, as he and I were talking I realized I resume providing that data but instead change up the presentation a bit to make it easier for people to understand. I can use essentially the same method I already use for my current compression tests by tweaking it a little bit to add a round of wideband stimulus between sweeps at a fixed output level (maybe 86dB @1m and possibly again at 96dB or so; I haven’t decided yet). I have already created the template for it and will try it out this weekend. I will likely call the two sets of data “instantaneous” and “long term” compression testing. The former being what I already provide. The latter being the new set of data.
 

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Yep. I used to test compression as part of my max SPL testing and provided the data. But I don’t think many understood the results and it was never really discussed. It just didn’t seem to catch on like I expected it to. So I decided it wasn’t worth continuing given the time and complexity of that particular test method.

However, as he and I were talking I realized I resume providing that data but instead change up the presentation a bit to make it easier for people to understand. I can use essentially the same method I already use for my current compression tests by tweaking it a little bit to add a round of wideband stimulus between sweeps at a fixed output level (maybe 86dB @1m and possibly again at 96dB or so; I haven’t decided yet). I have already created the template for it and will try it out this weekend. I will likely call the two sets of data “instantaneous” and “long term” compression testing. The former being what I already provide. The latter being the new set of data.
I know the availability of spatial radiation data has really spoiled us, but if you can conceive of a graph that easily shows these compression effects on tonality it will be a pretty big deal. It's amazing that we can see every wiggle in a DI curve but we can't see how loud the speaker can go. Modern high xmax woofers and active dsp filters have made visual assessment of speakers as a measure of their output really unreliable. A great example is the kali in5 you reviewed...you raved about the bass..it's a 5 inch woofer! How could I have ever known how dynamic the bass is without such a measure.
 
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