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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
ivan_nosnibor's avatar
ivan_nosnibor
1,055 posts
 
Erik, I don’t think it is so much the death of science that has stopped the adoption of better measurement methods so much as it has been the rise of marketing. Not just the sheer amounts of advertising, but journalism and also the advent of the internet of plenty have conspired to distract us (the consumers) from the notion or concept of what a relevant set of measurements might look like or the significance of its role. In the 80’s the entire hobby was accessed through b&m stores and magazines. It was a lot easier for most everyone to point to an article now and again that might propose or explore new test techniques, since that put it still within the bounds of usual discussion. But, since then the sheer amount of information that has been made available to us average consumers has mushroomed. Combined with the internet, and suddenly ’everyone is an expert’ in this information glut. And an informed/distracted (from the original idea of a standardizing of test methodology, in this case) public, in that regard, will almost certainly be marketed to differently. Once that notion fell out of public consciousness, then the pressure was off of manufacturers or advertisers to maintain it.

But, dial back the clock on any audio discussion and you might find people talking about at least Some issues that...well, maybe..we might ought to be still talking about even today. Time and innovation march on, but, in our haste to get to the future sometimes, it seems there may not be anyone in charge of making sure that we remember to bring along Everything that might be important...or that might prove to be.

Manufacturers might listen though - IF everyone were telling them the same thing. But, which comes first, the manufacturers talking about the right specs, or the consumers...the chicken or the egg??
mike_in_nc
1,173 posts
 
Erik, I agree with the premise that we need more informative measurements (or, really, more informative measures, the difference being that the latter are derived from the former, using some formula that relates to perception). 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.
erik_squires OP
16,654 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
16,654 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
16,654 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.
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