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
@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.
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@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
While your comments on IM distortion are quite valid and overlooked, for the most part, science is dead because there is so little correlation between bench measurements and sound quality. This is why all those charts in Stereophile, etc. are worthless. Don't really understand why that is but The Audio Critic (before Peter Aczel went off the deep end) and The Absolute Sound showed that over and over for the last 40 years.
Of all the measurements, IM distortion seems to be the only really important one as far as sound quality goes. I think it is ignored by manufacturers because it IS important and their numbers are so bad.
IM distortion seems to be the only really important one as far as sound quality goes.
It is one of many variables.  Two drivers, one with lower IM does not necessarily really mean it is the better, although having low IM is important.  Another important variable is the material of the driver such as a paper, metallic, ceramic ... and so on.  Most modern drivers have fairly good IM so the determining factor may be what material the driver is made of. 

For example, a very good paper cone is still a paper cone and cannot compete with more exotic material such as ceramic.