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
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


I don’t get it. Why would you assume phase or compression changes? 
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
The point is to measure some quantifiable objective - before and after. As for equipment, I would need:
1. A really good analog oscilloscope that can measure jitter in time domain.

2. A really good phase noise analyzer to measure in frequency domain

3. A really good vector network analyzer to measure the freq. and phase response. The excitation signal can be varied in amplitude if you want to see how the cable response with different input amplitude.

These would measure the before and after and then comparing the result. Here is a link from Troels, in which he modified a small woofer freq. response and listen for the affect on soundstage and detail.
http://www.troelsgravesen.dk/W12CY003.htm

I suspect the pre-break-in measurement will be relatively "peaky" vs. the after break-in.  

But at the end, as I said above, one still has to listen.
You are applying the "audio complexity fallacy"
Who define this as "fallacy" and why?


For sure no measuring apparatus can rival ears....This is not a fallacy to say that...