[Danny Richie]

Okay, I am going to respond to these statements.

The one thing I have not seen in this discussion is a quantitative or qualitative definition of what I could expect to see or hear by making the changes I made.

I haven't heard what kind of issues doing this will cause from a sonic point of view.

I am interested as I stated earlier in what artifacts or anomalies I can expect by deviating from the norm. If I know what I might expect, I will look & listen for them.

The only way anyone can answer this is to replicate what you have done and then measure and test it to give you the objective results.  This isn't possible.

You are going to have to be the one to determine the effect. And that is only going to happen when you take the time to make comparisons yourself.

Mike, I may be way off base here but at the wave lengths that we are talking about when dealing with < 100 Hz, having something that close blocking part of the opening would have little impact.

No, you are blocking about 3/4's of the area behind the woofer. It will not have "little impact". That is significant.

Nothing I am doing is changing them from open baffle. I am restricting the airflow some from one side, I don't see what all the hubbub is about.

No you are changing them from being a true open baffle to semi open baffle/semi vented box design.

You are changing radiation patterns, driver loading, how the servo system sees and controls the drivers, linearity of output, box pressures, output levels, etc.

Sometime real soon I will pull the amps off, which I need to do to move them up to the middle anyway. When I do I will run an REW analysis with them off. I will then put the amps in the middle, and run the same analysis. Will this make everyone happy?

I think there are a lot of good people here doing their best to try to help you so that you will be happy.

[aceinc]

Danny, I am willing to test with my limited test equipment the effect of having the amplifier in the middle of the two woofers to visually see the changes. I am willing to provide the results to the group, and I will do so.

Do you have anything that you would like me to test that I can easily accomplish?

I am willing to acquire additional equipment (within reason), if you are willing to guide me through the use of the new equipment.

In my career, I have been successful partly because I do not accept "...because that's the way it is..." types of answers. Since I am trying to do something that no one else has tried, I am willing to take the responsibility to determine what effect it has. All I ask is that people are open minded and try to help, not closed minded and shut down discussion.

[Danny Richie]

You don't need a bunch of test equipment. You don't need to try and measure the frequency response either. That won't even allow you to really determine the differences of various combinations.

Measuring the frequency response in the room tells you an average of on axis response, first reflections, second reflections, third reflections, fourth reflections.... and at one given point in space. And all of that will change if you move your mic one way or another.

It will be further compounded when you try to measure both speakers playing at the same time. When measuring a room response you never measure the output of both speakers at the same time.

All you need to do aceinc, is listen. Listen to it anyway you want. And listen to it with the amp away from the back of the woofers. Go with what sounds best to you.

And the good folks here are not close minded or trying to shut down the discussion. Some of these guys are expert level hobbyist. And I am a professional in this field. We know that it will have an significant impact.

I can run an impedance sweep with and without the amp in place and it will clearly show a significant difference. And the impedance sweep is made with very low level signals. The woofer will barely move and the impedance curve will still show a significant difference in how the driver is loaded. Just imagine what the difference would be when playing it hard.   

[S Clark]

Our nearly universal advice is:  Listen to the designer! 

...or just do you own thing.  Seriously, hope it works for you.  The servo bass system is amazingly good, probably even when not well implemented.

[Early B.]

Why limit your measurements to the woofer? You should also measure the effect of hanging the amps a few inches in front of the tweeters and midrange.   

[sledwards]

I'd like to help out this discussion in a more pictorial manner:

[aceinc]

Danny, have you done the impedance test that you indicate, or some facsimile, blocking a little less than 1/2 the back of a W enclosure? Were the results significant? Is this a test I can easily perform?

If the difference is small  wouldn't the servo feature ameliorate that difference?

I was under the impression that one of the features of the servo was to compensate for fluctuating impedance changes due to voice coil heat changes.

If I need to test in a pseudo anechoic environment, I have a field that I can use where the nearest building would be 100-150' away. Or I could test nearfield, with a short window to eliminate reflections.

Having said all of that I have listened with the amps in the bottom position, and the sound is pleasant. I have listened to some Miles Davis acoustic bass, Eric Clapton-BB King Electric Bass, Sheffield Drum Corp various drums, and Symphonic music, heavy on the Tymapnis, and everything sounds as I think it should. I will also listen with the amps removed. I will ask a few others to listen as well so my personal bias does not impact what is a subjective test.

[aceinc]

Early, why the antipathy?

[S Clark]

Because you aren't the first Guy to refuse to listen to universal advice. 
Seriously, why are you proposing these measures?  If you want to keep the amps where they are, do it.  Just live with the effects, which you have been told will be measurable.  But if for aesthetic reasons you want to keep it as is, it is entirely your decision.  Or go take your measurements and report back to us.  We will be eagerly awaiting your results 

[Danny Richie]

Danny, have you done the impedance test that you indicate, or some facsimile, blocking a little less than 1/2 the back of a W enclosure? Were the results significant? Is this a test I can easily perform?

Yes, I have made many similar measurements. You will not be able to run an impedance sweep unless you have a specific measurement system that allows it.

Just changing the insulation or damping material can have a notable effect in a boxed speaker (minor compared to what you are doing).

What you are doing is the equivalent of me designing a ported cabinet for you that requires a 3" diameter port that is 24" long for proper tuning, and then you deciding that 24" is just too long and you shorten it to 1". Then reason that the difference will have little impact. 

[aceinc]

I will run impedance sweeps on one of the cabinets with both woofers in parallel, with & without the amplifier centered.

BTW, I purchased the No-Rez, but have not installed it. Is there something that shows where/how to install it?

What affect will the no-rez have on the sound? or if you prefer, what is its purpose?

[Danny Richie]

BTW, I purchased the No-Rez, but have not installed it. Is there something that shows where/how to install it?

What affect will the no-rez have on the sound? or if you prefer, what is its purpose?

The No Rez should be applied to each wall of the cabinet and staying about an inch off of the baffle so that it is not right up on the drivers.

It does have a very notable effect on the sound as well. The side panels are under a significant amount of pressure when the woofers are playing hard. This is especially true for the W frame version as there is a large un-braced panel on the front side. The pressure can excite and resonate those panels. The No Rez damps out the resonances. The result is a tighter and cleaner bottom end.

[genjamon]

What's the harm in just trying it with the amps on the floor behind the speakers, and then with them mounted?  Yes, testing would be ideal, but seems like you could get a pretty good subjective sense of the difference with a relatively easy experiment on your own.  And since you're a hobbyist, why not give it a try?  You're clearly curious.

[aceinc]

Genjamon, I will try it, but I prefer empirical objective information, as I believe that we and by we I mean first & foremost me, are heavily influenced by biases, prejudices and other psychological baggage we  carry around.

So I think I have the means to do some impedance sweeps, as well as listening. If I knew how to use REW better, I could probably get some useful data from there as well.

[aceinc]

I got around to doing the impedance sweeps this morning. I used "Woofer Tester 3" which I had received a few years back but had never opened. I fired up an old note book running Windows XP and has a CD drive to install the software. I calibrated the leads then calibrated using the supplied resistor.

Here are the first two plots;

The upper driver with the amp in the lowest position.



The lower driver with the amp in the lowest position.



I don't see much difference. What should I be looking for?

Or, how should I improve my testing?

[Davey]

You're not going to see big differences doing small-signal impedance sweeps with a simple tool like that.
Although I do see a noticeable difference in those plots, you'd need to re-scale and zoom the view for a much better look at the area of interest.  (Near the 200Hz range is the area of interest.)

Regardless, this is all irrelevant.  You've already said the amplifier is mounted in that location and that's where it's going to stay.  Case closed.

Dave.

[aceinc]

I said I was unwilling to remove it from the back of the cabinet. I am willing to center it in the back of the cabinet.

I am also willing to run tests to determine the effect having it in different places on the back of the cabinet, and off of the cabinet, publishing them here in case someone else wants to do what I want to do.

So Davey, how should I improve my tests to provide useful data?

What of significance do you see in the plots I posted?

Keep in mind I do not have the impedance plots from the drivers in free air so there is no way of knowing what they tested like in a controlled environment and the differences could be manufacturing differences.

[aceinc]

Per Davey's suggestion here are the plots zoomed and limited to 200 hz.

Upper driver with Amp on bottom.





Lower driver with amp on bottom;

[Danny Richie]

If you want to know what the effect is of the box then measure it with it in place then measure it with it removed.

As Davey said, you're not going to see big differences doing small-signal impedance sweeps. If you see any difference in a low level measurement like this though then it would suggest that the differences at high levels will be significantly more.

And again, to really know what the differences are then just listen to it with the amp as you have it and then with it out of the way.

[aceinc]

Danny, I do not trust my ears to that level of granularity. Having said that I do like what I hear.

I guess what I'm saying is, I am of the camp that says "If it can't be measured, it can't be heard." I will be the first to admit that my measuring skills ain't great, but I am willing to learn.

In running REW (I now have a calibration file for my microphone) I see very specific differences between the frequency response of the two cabinets, which for the moment I attribute to placement. One cabinet has solid wall beside it (about 30") and an opening into another room behind it, the other has a solid wall behind it (about 30"), and an opening to a hallway on the side.