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An AR-90 question - stuffing?


johnfalc

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A quick question for those who know.

Early today I got a lovely, consecutively numbered, pair of AR-90s, all grills intact, cabinets' finish in very fine condition, and six totally shot surrounds.

I've taken care of the surrounds but was surprised by two things:

1) There is no stuffing in the enclosure holding the woofers.

2) The stuffing in the chamber for the lower midrange is VERY tightly packed, so much so that one has to push down on the driver (lightly) to mount it.

They sound great, far superior to my AR-3as, AR-92s and AR-2axs, more articulate vocals, solid imaging, and, of course a bit more extended bass. From all appearances they were "factory fresh," I.e., I was the first one to remove the drivers.

I remain curious about the absence of stuffing in the main enclosure since all my other ARs have had this. Any insight on the engineering/production decision(s) that led to this?

Thanks, John

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I just finished resotring a pair of 90's and saw the same thing. The final assembly dwg. of the 90's in the library shows the location of the stuffing is as you described. It's all located in the area of the 8 inch mid bass and above. A few years back, I saw a similar stuffing pattern in a pair of TSW 910's. It's seems this is common practice with tower speakers.

Not sure exacty what the acoustic reason is for this but I suspect it has something to do with mitigating standing waves.

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I can't speak for AR90 but in AR9 the subwoofer stuffing is in the top half of the enclosure. I forget how it is kept up there. This effectively creates two resonant chambers, one undamped above the cutoff frequency of the woofers and the other correctly damped at the intended resonant frequency of the system. IMO it was very clever and works very well. The ability to do this is because the high end of the woofers are cut off at a very low frequency of 200 hz. It would not be possible to successfully use this configuration with a 3 way design where the woofer had to operate at higher frequencies. The woofer/enclosure/stuffing works on exactly the same principle as all other AS speakers. The damping is created by forcing the woofer to work to push and pull air between the stuffing fibers. This creates an aerodynamic drag due to the very high total surface area of the fibers and their close packing. The math for this is explained exactly by Newton's second law of motion applied to forced oscillation. Thiel and Small used it and the electrical properties of woofers to turn it into an easy to use cookbook recipe so that you don't have to understand it to correctly design speakers and enclosures to get the F3 and Q you want if you know the T-S parameters.

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One more point about the stuffing is it reduces the Qts in the box. As I remember the Qts of AR 9 is about 0.5 and AR 90 about 0.7. More stuffing may make the bass sound leaner. In contrast, AR 2ax, 5 have Qts of over 1(same as some Allison speakers) Advent Loudspeaker is close to 1.

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Thank you gentlemen for your quick and authoritative replies..

I had not seen the stuffing in the upper section of the main cabinets but on reviewing some photos I took with my cell phone I clearly see the material is, indeed, held up there - apparently only by virtue of position and self-adherence.

I am listening again this morning and, in my not ideal listening area, I am most impressed by the pleasureable sound at low listening levels ... a real treat!

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Hi John

Congrats on some super speakers. I have their little brothers--the 91--and they are really nice.

There is an article here: http://www.audioasylum.com/audio/speakers/messages/248085.html

I don't buy some of the recommendations, but from my own experience there are two tweaks I'd endorse and one I "kinda" endorse:

  1. Adding some damping material to the metal basket may help by eliminating some spurious vibrations. The article suggests Liquid nails. I've used self-adhesive speaker gasket (like weatherstripping) or try Michael's craft store--they sell sheets of adhesive-backed foam. Here's a link to the non-adhesive but they have the sticky kind in the stores: http://www.michaels.com/Creatology%E2%84%A2-Foam-Sheets/gc1328,default,pd.html
  2. Bracing couldn't hurt.
  3. New capacitors. The caps are 3 decades old and electrolytics don't last forever. I disagree with the recommendation to buy the most expensive ones you can afford. Any film cap will be fine. Madisound has good deals on Carli caps and some "surplus" caps. Dayton or Solen won't break the bank. Erse http://www.erseaudio.com/ sells good caps at reasonable prices.

OR... Now that you've re-foamed the surrounds, just enjoy them!

Kent

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Hi Kent, Yes, I read the same post earlier and may delve into such things in the future. I feel good about just having got all the surrounds done yesterday and I did check ESR of the caps and they seem fine - later, when time allows I'll do electrical and acoustical checks of the speakers but, to my ears, they're working absolutely beautifully just as they are!

Here are a few pictures I took after picking them up yesterday and as they are now sitting in front of me being very much appreciated!

http://falcons.dyndns.org/links/open/ar-90/index.htm

John

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These are beautiful speakers! I noticed that there is a dimple in one of your UMR. I have some success in pulling out the dimple using a vacuum cleaner hose attachment. Actually I used a small funnel, attach the wider end to the vacuum hose and the narrower end(1/2 inch) to the dimple and it would suck the dimple out. This has worked on soft dust covers and soft domes and is not destructive.

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@ligs - Yes, that dimple (it is much smaller than it appears in the photo ... light does funny things) is the only real defect in this pair and I will try to deal with it, although perhaps not until I pick up a spare UMR. I don't hear any consequences but it is a bit annoying.

I took some "in the room" measurements today since I felt the LMR seemed a bit too pronounced. The measurements from my main listening position confirmed my appraisal and I've cut the LMR back to the -3DB position and think piano solos sound more natural. Actually these are the first speakers I've had (at least in this room) that let me enjoy sustained listening to solo piano - Chopin for the past half hour has been VERY nice.

Interestingly (a bit off topic) I was switching between the AR-90s and the AR-92s and there is a very interesting difference, which I did not anticipate, in the smoothness in the frequency response above about 3KHz. The AR-90s are remarkably smooth, with no pronounced deviations, whereas the AR-92s show a pronounced notch where the MR crosses to the HF driver. This looks the same for left and right speakers so I don't think it is a measurement artifact. This was just a quick FFT on Pink Noise.

I think I'm likely to own these AR-90s for a good long time ... and as a proactive step I've ordered (only) the caps for the upper crossover board from Erse.

John

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I took a measurement of both AR 90's pointed towards my mic at about 2M distance. The measurement is gated so only 200 hz and above shows. Nice and smooth as well. The slowly declining SPL as freq. increases is much like that measured by Allison many years ago. I've also included a couple of polars in another test of 1 speaker. 0 deg. 30 deg and 45 deg.

post-100237-0-58879800-1365866383_thumb.

post-100237-0-97525800-1365866391_thumb.

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Regarding stuffing the opposite half of a tower encloser, see Beranek's

"Acoustics" Chapter 8, p221, Figure 8.9.

It is described as a "Special type of cabinet for minimizing the shift of

resonance frequency of a loudspeaker mounted in a closed-box baffle".

It shows a glass fiber wedge with a stuffing density of 2.5 to 4 lb/cuft.

The stuffing is used in the end opposite the driver with it full for the

first 1/4 height of the box, then a pointed wedge for the next 1/4, the

remaining half is unstuffed. All walls are lined with .5" of damping

material.

Something similar is done in Allison tower speakers as you can see here

in this thread:

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=7052

The box appears much larger than actual and in simulation that assumes

a simple compliance for the box I had to use a much larger value for box

volume than actual to get the correct closed box resonance frequency. I'm

surprised that it is not discussed more since reduced box volume is a major

advantage.

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Thanks Pete ... Gives me inspiration to pull Beranek down from the shelf! That was my frequent "lunchtime reading" some forty+ years ago.

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Hope you enjoy it!

It is a closed end resonant pipe and Beranek shows that the input

impedance is actually a mass element rather than compliant over a

specific frequency range - this is expected with a resonant line.

No stuffing at the driver end provides a higher system Qtc than if stuffing

were used.

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I remain curious about the absence of stuffing in the main enclosure since all my other ARs have had this. Any insight on the engineering/production decision(s) that led to this?

Thanks, John

Below is the explanation provided by Tim Holl regarding the placement of damping material in the AR-9 cabinet. The full document "Engineering the AR-9" by Tim Holl (who was Director of Engineering at "Teledyne Acoustic Research" when he wrote the document in January, 1978) can be found in the forum Library. The damping discussion appears to be applicable to the tall AR-90 cabinet as well.

Roy

post-101150-0-58630600-1365910732_thumb.

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Thanks Roy. I found those after my initial post; should have researched mor thoroughly first but have learned from the dialog.

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Below is the explanation provided by Tim Holl regarding the placement of damping material in the AR-9 cabinet. The full document "Engineering the AR-9" by Tim Holl (who was Director of Engineering at "Teledyne Acoustic Research" when he wrote the document in January, 1978) can be found in the forum Library. The damping discussion appears to be applicable to the tall AR-90 cabinet as well.

Roy

I also want to thank you Roy for validating the supposition I posited in the last sentence of post #2 above! :-)

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  • 1 year later...

hello, was hoping someone could help me in restoring some ar90's. both speaker cabinets have all their proper speakers and such but no polyfill behind 8" mid range which seems to be a separate small enclosure, when I removed the 10" woofers I see no polyfill either.. It looks like someone may have started a restoration and gave up and just put the speakers back in. can anyone tell me how much fill to put in behind the 8" and seperately in the cabinet by the 10"s? I have heard that it is by weight but I was curious how that would be done. I am a beginner at this and have done capacitor replacement and foam surrounds, but soundwaves, Q, and all the other stuff are way out of my league currently. thanks.

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The archives drawing shows 2.5 ounces of polyfiber for the 8" LMR. I was unable to upload the drawing from my computer hard drive. I will see what else I can do to post it.

The woofer section should not have any polyfiber.

I believe its a type of isobaric speaker set up for performance. Stuffing would defeat that design.

isobaric.gif

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Here's a link to the drawings located in the Library.

http://www.classicspeakerpages.net/library/acoustic_research/special_sections/drawings/SpeakerDrawings.swf

Item #6 is the poly for the 'basket' the 8" LMR sits in. That's where the 2.5 oz goes. No polyfiber in the woofer section. The upper section has 4 rolled blankets. The weight for this is also given in the dwg in the link below.

http://i138.photobucket.com/albums/q261/newt_album/AR90-Drwg1of2_zpse5a2c092.jpg

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