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
Andy2 wrote: "One cannot judge a pair of speaker with the freq and phase plot."

Agreed, a frequency plot and a phase plot are insufficient.

But there is fairly good evidence that one can judge a pair of speakers with a sufficiently comprehensive suite of the right measurements. This is the guy who wrote the book on the subject:

https://youtu.be/zrpUDuUtxPM?t=259 (Up to about 4:20 is just introduction.)

And here is the book: https://www.amazon.com/Sound-Reproduction-Psychoacoustics-Loudspeakers-Engineering/dp/113892136X/ref... (I recommend the Third Edition; the writing style is a bit easier read for the non-technical audiophile, and the author includes personal experiences which are imo very informative.)

Duke
Erik says “The idea of modeling multiple non-linear systems at once to derive a master model of behavior could probably be its’ own thing.” It certainly is. I recently finished a little project where I applied a machine-learning neural network model to classify 1000 clips of music according to a ‘harmonic signature’ (mostly “live content” case #1, or mostly “synthesized”, case #2). Interestingly, although FFT, spectral centroids, RMS energy etc. were important in defining the signature, the most compelling predictors were the Mel-Frequency Cepstral Coefficients. Why is this important?

Because the purpose of these coefficients is try to capture exactly what audiokineses refers to as “the transfer function of a device (how that device alters the input signal) through a psychoacoustic (i.e. perception-based) lens.” Two examples will explain: ‘the sound of a baby crying.’ Why is the baby crying? Hungry? Lonely? Needs diaper changed? We could do technical analysis forever and not know – but the mother knows instantly. Someone singing, “I don’t know what to do.” Why? Boredom? Lost love? We know from the voice; not just the context.

I think that psychoacoustic perception is exactly where we need to look to understand that last 10% or 20% beyond the point where purely technical/engineering analysis stops reliably explaining what we know to be true in our ears.

The science is not dead; it’s getting more and more interesting.
"Linear Time Invariant systems are important because we could solve them" Richard Feynman.

Once we get into the non-linear things, it can get complicated real fast.  


"Once we get into the non-linear things, it can get complicated real fast." - Richard Feynman

Very true!!

Fortunately as long as loudspeakers are not pushed past their linear excursion limits, non-linear distortion is highly unlikely.

What is NOT unlikely is this: Linear distortions to which the EAR has a NON-LINEAR perception response. So it can sound like a system distorts as the SPL goes up, when what is really happening is, the ear is having a non-linear perception of what is actually a linear distortion.

This is one of the reasons why listening tests should include some high-SPL listening, because there are linear distortions which are inaudible at low SPLs but which can set your teeth on edge at high SPLs.

Duke
So what do room measurements and treatments (or old fashioned equalizers) bring to the table? Does it not take flawed sound, badly measured sound if you will, and adjust it to measure better for the individual consumer in a specific locale. This would seem a better solution as it incorporates unique parameters; room, my equipment, and my ears. I understand the desire to eliminate sow's ears but you assume objective, standardized, and constantly improving measurement techniques  by audio press or manufacturers. Not sure about that.  Joe