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IM Distortion, Speakers and the Death of Science

One topic that often comes up is perception vs. measurements.

"If you can't measure it with common, existing measurements it isn't real."

This idea is and always will be flawed. Mind you, maybe what you perceive is not worth $1, but this is not how science works. I'm reminded of how many doctors and scientists fought against modernizing polio interventions, and how only recently did the treatment for stomach ulcers change radically due to the curiosity of a pair of forensic scientists.

Perception precedes measurement.  In between perception and measurement is (always) transference to visual data.  Lets take an example.

You are working on phone technology shortly after Bell invents the telephone. You hear one type of transducer sounds better than another.  Why is that?  Well, you have to figure out some way to see it (literally), via a scope, a charting pen, something that tells you in an objective way why they are different, that allows you to set a standard or goal and move towards it.

This person probably did not set out to measure all possible things. Maybe the first thing they decide to measure is distortion, or perhaps frequency response. After visualizing the raw data the scientist then has to decide what the units are, and how to express differences. Lets say it is distortion. In theory, there could have been a lot of different ways to measure distortion.  Such as Vrms - Vrms (expected) /Hz. Depending on the engineer's need at the time, that might have been a perfectly valid way to measure the output.

But here's the issue. This may work for this engineer solving this time, and we may even add it to the cannon of common measurements, but we are by no means done.

So, when exactly are we done?? At 1? 2? 5?  30?  The answer is we are not.  There are several common measurements for speakers for instance which I believe should be done more by reviewers:

- Compression
- Intermodulation ( IM ) Distortion
- Distortion

and yet, we do not. IM distortion is kind of interesting because I had heard about it before from M&K's literature, but it reappeared for me in the blog of Roger Russel ( http://www.roger-russell.com ) formerly from McIntosh. I can't find the blog post, but apparently they used IM distortion measurements to compare the audibility of woofer changes quite successfully.

Here's a great example of a new measurement being used and attributed to a sonic characteristic. Imagine the before and after.  Before using IM, maybe only distortion would have been used. They were of course measuring impedance and frequency response, and simple harmonic distortion, but Roger and his partner could hear something different not expressed in these measurements, so, they invent the use of it here. That invention is, in my mind, actual audio science.

The opposite of science would have been to say "frequency, impedance, and distortion" are the 3 characteristics which are audible, forever. Nelson pass working with the distortion profile, comparing the audible results and saying "this is an important feature" is also science. He's throwing out the normal distortion ratings and creating a whole new set of target behavior based on his experiments.  Given the market acceptance of his very expensive products I'd say he's been damn good at this.

What is my point to all of this?  Measurements in the consumer literature have become complacent. We've become far too willing to accept the limits of measurements from the 1980's and fail to develop new standard ways of testing. As a result of this we have devolved into camps who say that 1980's measures are all we need, those who eschew measurements and very little being done to show us new ways of looking at complex behaviors. Some areas where I believe measurements should be improved:

  • The effects of vibration on ss equipment
  • Capacitor technology
  • Interaction of linear amps with cables and speaker impedance.

We have become far too happy with this stale condition, and, for the consumers, science is dead.
erik_squires

Showing 18 responses by erik_squires

erik_squires OP
15,877 posts
 
@ivan_nosnibor
I wrote something about ESL’s that appears to be incorrect. This is a retraction while I work on better understanding the mechanics. I wrote:
All moving drivers suffer from Doppler distortion, and the distortion, including IM of ESL panels is as much if not worse. ESL’s often sound better due to how they couple to the room though.


Apparently, the large surface area of an ESL leads to small displacements and less opportunity for Doppler distortion. My mistake. That was a clear error of memory on my part and I apologize for it and subsequent confusion it may have caused.

I’m clearly conflating something I read long ago about the precision and distortion of ESL panels and need to either read it again or stop talking about something I don’t remember well enough. :)


Best,
Erik
erik_squires OP
15,877 posts
 
I wanted to post a very interesting review of a recent mid-woofer driver, not because I want you to buy that driver, but because it's very interesting how the reviewer actually does consider both Doppler and IM distortion in his review, and just slightly starts to tie it to listening.

It would be very interesting to see if these measurements ever filter to finished products.

https://hificompass.com/en/reviews/purifi-audio-ptt65w04-01a-midwoofer

Further reading here:

https://www.stereophile.com/reference/1104red/index.html


Also, if we take the measurements to heart, it explains how/why a subwoofer can greatly enhance the clarity of a 2-way speaker system.
erik_squires OP
15,877 posts
 
I can be bought with the worm in the Tequila.


That would be a truly difficult thing, since Tequila doesn't have a worm, you mean Mezcal.
erik_squires OP
15,877 posts
 
I’m not sure what I’m agreeing to, but I can be bought with tequila. Please send two cases of Dos Artes silver to my off-shore liquor account ...
erik_squires OP
15,877 posts
 
I don’t get it. Why would you assume phase or compression changes?

When you don’t know what you will find you cast a large net. I’m not assuming, I’m saying this is a starting direction based on my observations.  We were asked to show examples of how we would investigate issues currently not measured. This was one example.

When you don’t know what you will find, you don’t know ahead of time how to measure it. You pick a possible explanation, then go prove/disprove it.

Otherwise we are logically stuck back where we started from, arguing metrics and measurements that are old enough to have grandchildren.

Best,

E
erik_squires OP
15,877 posts
 
OK, someone was really determined to measure break in, you know instead of just talking about it, how would one actually go about it?

Truthfully, we would need to invent a metric, just like I was discussing at the beginning of the thread.

How would I do it?  Based on what I've heard from capacitor break-in, I would go down at least the following routes:

1.  Look for phase shift and fine comb filtering effects at small and large signal levels

2. You would measure music play back before and after then look for anomalies in phase and amplitude.

3. Look for signal compression at small and large amplitudes.

Both of these approaches would measure before and after break-in has allegedly occurred.  Then work backwards from what we find into some easy to use tests.

Best,

E


erik_squires OP
15,877 posts
 
What does "death of science" mean here?

Here I take science to mean science in the public realm, as opposed to academic or manufacturer’s research. Because science involves progress and invention, so long as the educated public is stuck with a handful of metrics set in stone by the 1980’s that we discuss, I say that this science is dead. It has not progressed much at all. It had a fruitful life from the invention of the telephone up until the 1980’s and then died quietly.

I think the general consensus here is (& @erik_squires certainly makes the case): we measure irrelevant things.


Oh, no, not saying that. I’m saying what we measure is not enough, but we take it to mean all that is knowable. Imagine measuring the earth by it’s diameter and mass and saying that’s all we must know about it, and that tells us everything we need to know about the earth. Well, if your sole interest is gravitational, momentum and orbit, then yes, I supposed that’s true, but these two metrics ignore:

  • Geology
  • Geography
  • The composition of the ecosphere including liquids, gasses and plant and animals.
  • Tectonic activity
  • Weather
  • Biology and evolution

I’m not saying the earth’s diameter and mass are irrelevant, far from it, but I do think we are stuck somewhere far from knowing everything about say, capacitors or amplifier/speaker interaction. We just accept that publishers publish mass and diameter and that science is done and then must leave the rest to popular opinion, social media and taste makers.


I disagree.

erik_squires OP
15,877 posts
 
I recall Einstein's thought experiments ... before his time, it's kind of hard to find equipment like oscilloscopes, DMM, atom smasher and stuffs.


True, Einstein theorized many things, but .... since then scientists have been in a rush to confirm or refute many of them.  Einstein was also sometimes wrong. 
erik_squires OP
15,877 posts
 

The papers were peer reviewed, which implies that the test methodology stood up to critique by those with expertise in such things.


What I meant was that the study includes data on actual user perception, and it's' not just a theoretical work. :-) 
erik_squires OP
15,877 posts
 
As others have pointed out, correctly, science actually IS continuing. The GedLee metric is in fact an example of what we need. The barrier, IMHO is us the merely educated consumer. There may be a thousand new interesting metrics proposed via the AES or used in secret, but so long as we are limited in what comes down the journalism pipeline we’ll not really get better.

If I may, we can use the GM metric to also demonstrate the fallacy we see here in our hobby circle:  The idea that all is known by existing/old metrics. GM makes the clear case that for specific uses, it does not. Kind of blows up the "if you can't measure it with moldy old metrics, you can't hear it" argument.
erik_squires OP
15,877 posts
 
OK, I’ve briefly read the AES papers. So Duke proposes a way of modelling systems so that multiple non-linear behaviors can be amalgamated at once, and then comparing that to what is known about auditory masking perception, further he’s actually tested this out with different populations of listeners? Outsanding. :)

Pretty ambitious too. The idea of modelling multiple non-linear systems at once to derive a master model of behavior could probably be it’s own thing, but hey, I've never published anything.
erik_squires OP
15,877 posts
 
I am, however, puzzled by your seeming dismissal of the one thing mentioned here that seems to represent progress along those lines, the GedLee Metric mentioned by @audiokinesis.

I didn't dismiss it, I also did not address it. He wrote:

A metric which is demonstrably far more predictive of perception than THD has been figured out. It’s called the GedLee Metric, but it has not yet gained widespread acceptance.


I know nothing about it, so I wasn't sure if he was being facetious or not. And it's the lack of widespread knowledge of these metrics which hinders us.

Like, how many knew IM distortion was ever measured for speakers?
erik_squires OP
15,877 posts
 
What specification would tell you that most amps sound better driving an easy load than they do putting out the additional power?


This at least has some measurements, Steroephile routinely publishes power vs. load and frequency response vs. a simulated speaker load.

But we're not talking about what the manufacturers may feel obligated to publish, per se.  I want to see science, as we the educated consumers know of it, to progress.

BTW, there's really interesting stuff about what makes "science" science if any of you ever read subjects on the history of science. Sometimes science is defined by the consumers as much as by the practitioners.

Phrenology was science to many.  We can laugh at it now, but at the time, without the benefit of hindsight that was real.
erik_squires OP
15,877 posts
 
@heaudio123

With all the advancements in compute, storage and data collection technology you'd think we could see many more dynamic tests being done, but in fact they are quite rare.

Best,

E


erik_squires OP
15,877 posts
 
@audiokinesis

The point was that we talk about a handful of measurements as if they were all we needed and all we should ever have, and we call this science.

It isn’t. It’s not science unless we are getting better and better at inventing measurements which reflect human perception. We cannot know from an (used correctly) arbitrary count of measurements everything about how we perceive it.

Imagine if as Bell’s assistant, I said "amplitude at 1 kHz is everything and all humans can perceive."

Well, clearly that’s not what we consider adequate, but it might be enough at this time in history when I’m trying to pick among 4 different diaphragm types.

The types of measurements we are using was not my complaint, it’s that we are happy with what we have.  From Bell's time to the 1970s/ 1980s the popular literature and the hobbyist expanded what we think of as the basic measurement suite.  When and why did it stop?

Best,

E
erik_squires OP
15,877 posts
 
I never said measurements were arbitrary:

https://www.merriam-webster.com/dictionary/arbitrary

I said they are created by necessity to meet a specific need, as opposed to measuring everything humans can hear in all cases.

To reiterate:

What is my point to all of this? Measurements in the consumer literature have become complacent.

erik_squires OP
15,877 posts
 
Duke:

@audiokinesis

Don’t focus on distortion. Focus on us in consumer land (albeit educated consumer) having a limited number of measures and assuming we can’t have or use more.

Here I don't care about distortion, I used it as an example of how we invent specifications and measurements. I could have used gain instead for the example.

Best,

E
erik_squires OP
15,877 posts
 
@cleeds -

I'm talking about what we, the moderately informed consumer perceives.

Clearly, R&D proceeds and AES continues to publish.

Hence my clear caveat: "for the consumers" meaning what we read about and discuss.
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