The ForA-11 A Vertical Variation on the AR-11, by Pete B.

[Pete B]

This is a very rough prototype of a vertical variation on the AR-11 with a rebuilt crossover. This was just a first step to see if the stock drivers had the potential to compare with modern systems.

Best sound was obtained by replacing the stock .75" tweeter with a Dynaudio D21AF (not shown) and the crossover modified to match.

A novel method, patent soon pending, for matching the 6/8 ohm Dynaudio tweeter to the crossover for both impedance and sensitivity was used. Crossover mods make this system give some of the best modern designs a run for their money, the stock AR-11 does not even come close.

Let me say to anyone thinking of jumping to conclusions to hold your comments until you hear it.

http://members.aol.com/miscpubacnt/AR11/FORA11V2.jpg

[Steve F]

I remember asking AR about the side-by-side driver orientation of the 11 and 10Pi, which was even worse than the 3a's angled mid-tweet orientation. At least the 3a was roughly the same whether laying on its side or standing straight up.

They answered, with spin worthy of a seasoned politician, that the 11 and 10Pi were intended as BOOKSHELF speakers, and when used that way, the midrange-tweeter would be vertically-oriented.

I asked someone else at AR again a few months later, and got an even more "inventive" answer: "The woofer and midrange, which cover the first 8 audible octaves of the musical spectrum and are therefore responsible for what we interpret as 'imaging,' are essentially vertically-oriented when the speaker is standing straight up. The tweeter only handles the top two octaves--from 5-10kHz and 10-20kHz, and therefore doesn't play a major role in imaging."

Hmmm..somewhat plausible (since the human ear does respond in octaves and imaging is determined primarily from 500Hz-3000Hz), but accidental, that's for sure. AR had no clue about vertical orientation until the AR-9 and its successors, the 90, 91, and 92.

The 91 was essentially the vertical version of the AR 12" bookshelf (3, 3a, 11). I had both the 91 and the 11 at the same time in my home for a long time, and I A-B'd them extensively, and found their similarities to be much stronger than their differences, listening as I did from about 10 feet away, well into the reverberant field.

However, I'm sure the vertical AR-11 does sound very good. I always wondered about one.

Steve F.

[soundminded]

"The 91 was essentially the vertical version of the AR 12" bookshelf (3, 3a, 11). I had both the 91 and the 11 at the same time in my home for a long time, and I A-B'd them extensively, and found their similarities to be much stronger than their differences, listening as I did from about 10 feet away, well into the reverberant field."

Multiple driver speaker systems can affect perception of sound because when two drivers operate over the same frequency band, there are interference patterns which cause localized frequency response rises and dips everywhere in the field they create. The exact occurance of these response irregularities depends on the geometric relationship (especially the distance) between the points of propagation, the location in the field, the effective dispersion of the drivers, and the frequency. Differences in time delay between applied voltage and mechanical response in the drivers also has an effect. It is impossible to eliminate these unless both the time delay difference is eliminated and the path length difference to the ear is identical, ie a coaxial arrangement and then only directly on axis. Another method is to use very sharp cutoff filters such as in an active digital crossover network to minimisze the overlap region. I think NHT is doing something like this. Given that the typical room has around ten million resonance points in the audible passband, I'm not sure this is particularly important. Directional perception in humans is usually associated with high frequencies where small path length differences translate into large phase differences and significant loudness differences if the source is to one side or the other.

[Guest Bret]

>Let me say to anyone thinking of jumping to conclusions to hold your comments until you hear it.<

Why? You've never compared a North Creek cap to a Solen in a crossover and you know how must sound. Consistency is a good thing.

What you've built there is something I suspected was a very good idea. I think you'll find it's been done, though. This is essentially what Ken Kantor did when designing and building the AR-303. Like you, he used a different tweeter, presumably one that was better than the original AR tweeters were. He also avoided a problem by offsetting, very slightly, the tweeters from the vertical. He also changed the tweeter's capacitor type from NPE to mylar.

It's interesting to note that although the 303 was designed after the AR-91 which included a new, presumably improved, midrange that Ken went back to the old, to use his term, "bee's butt" driver. He made a comment in this forum that one could do some interesting things with that diffraction ring. (by which I assume he meant sonically interesting and not "interesting" in some obscene way connected with the loss of a bet) Unfortunately, he did not elaborate.

I've said on many ocassions that I wanted to try just almost exactly what you have done. I wish things were such that I could hear the results of your experiment without having to duplicate it.

Have you had the opportunity to compare it arrayed this way to the original side-by-side configuration? I'm curious if there is a large tonal quality change (due to changing the interference patterns per soundminded's comments) in addition to the improved imaging.

Bret

[Pete B]

I should probably add that on a casual listen the system is subjectively close to equal to my reference in the bass with regard to frequency response, however it does go crack when the woofer bottoms, only happened once and very gently since I was raising the level slowly and expecting it. I've never heard the reference bottom since I believe it has a raised back plate and the motor runs out of drive before mechanical limits are reached.

I'd want 2 woofers on each side as a minimum if I were running these and 4 would be even better. Allowing for modern design, I'd just use better woofers.

I thought, based on what I read here that the AR-11 would have bass equal to the AR-3a family, however now that we know that there was a significant change in moving mass (about 110g versus 68g) it seems that this is unlikely:

http://www.classicspeakerpages.net/dc/dcbo...7431&page=#5243

I expect the AR-11 to have less deep bass in the 30 to 35 Hz range and I'd like to hear the two side by side or even better measure them.

Pete B.