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
Perhaps only slightly related to AI but enlightening to me, I was at an audio meet when someone mentioned he worked at a major communications company that performed an experiment. They created a machine to run wild with AI software, left to its own devices to”create”.  They found the limiting factor in obtaining progress was  that humans were doing the inputting.  So they created a second machine for it to interact with.  After watching the evolution of the 2 machines , they lost ability to monitor what the 2 machines were up to. They had created their own language to communicate to each other and the people monitoring could not decipher it. They concluded this was going in a dangerous direction and destroyed both machines.
If you are familiar with this, pardon me if I got some of the details not exactly right, I am just relaying a story heard at a gathering.
my point is, it may be dangerous to find out if machines can replicate themselves on evolutionary level.
Hey, that’s nothing. Two Mind Lamps 💡💡will interact with each other if left alone in the room together. And they don’t even have to be programmed. Under normal circumstances Mind Lamps interact with humans. Ye olde Mind-Matter Interaction. Yes, I know what you’re thinking, How can the human mind influence inanimate objects? 😟
Here, belatedly, is a picture-perfect example of the kind of (protracted) misinformation campaign that manufacturers have, and do, engage in.


https://www.youtube.com/watch?v=kPqpuTO6Kd4

If you can watch the whole way through, you can see the story unfold in front of you. Although the video is by no means meant as indictment, per se, of this sort of behavior, you should be able to see why I for one am not automatically willing to accept at face value what any given maker has to say about their product or design. The long, slo-mo dance presented here involves certainly the initial ignorance or misapprehension on the part of the consumer, but also the deliberate manipulation of that ignorance on the part of the manufacturer - even if that can stem, quite understandably, from the maker's unwillingness to surrender technical discussion to their competition. But, this sort of thing is why I do not take, nor ever will take, nor should ever be expected to take, a manufacturer's explanation simply at face value, no matter how respectfully they may present it. This does not mean that all makers are evil and therefore must be burned at the steak, it just bears outs caveat emptor. But, a certain amount of these behavioral motives here are necessarily going to be baked in. 

The harder any manufacturer works at being polished at appearing to make all the above out to be a non-issue, the more suspicious of their ultimate motives I typically become. In general, there can be reasons to doubt such highly polished explanations as well as those that are not nearly trying so hard or certainly even those who offer no explanation or rationale at all. But, the video happens to be an excellent encapsulation of human nature in the marketplace.
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  I couldn't follow many of the graphs and such, but I found the summary and discussions pretty intriguing. 

Didn't realize it was known that (dynamic) drivers had that kind of "memory"...(and yet somehow it does not entirely surprise me??...maybe I absorbed something by osmosis over the years somehow [by listening I mean]).

Thanks for the links. I'll need to revisit them going forward and see if I can go a little deeper into it.