Advice needed for AR2ax cabinets with air leaks

[mrbruce4]

Just obtained a pair of AR2ax from the original owner. Excellent condition other than rotted foam woofer surrounds. I've read about everything I can find here and at Audiokarma about them, and knew I wanted to replace the capacitors as well as surrounds. I settled on the JBL125a surrounds from Looneytune2001 and 5% Dayton capacitors. Cleaned the pots, installed the surrounds using Aleene's tacky glue and new gasket foam from Parts Express. Everything goes back together perfectly - except pushing on the woofers doesn't give me any return delay. Listening to them, driven by a Pioneer SX-1080, reveals some great sound in a 5,000 cf room. If I didn't know about acoustic suspension boxes needing to be airtight, I would be 100% pleased with the result; however...

So here's the dilemma/question: Could I expect significant improvement if I get these airtight? If so, what should I try? If not, I'll be inclined to leave well enough alone.

Background: I've refoamed six sets of Advent woofers, with 100% success in getting them airtight the first time. The ARs gave me no problems or hint of future problems. I've tightened the 4 bolts tighter than usual when I couldn't get the return delay, and I've also put putty around the perimeter of the baffle at the mid and tweeter of one speaker and over the bolt heads - just in case they might be the problem. Unrelated but of interest- both speakers show a date 6/74 on the paper on the backs, but S/N 257568 has a countersunk baffle for the mid and tweeter, while S/N 257482 has flush mounted tweeter and mid. Both have countersunk woofers. I have brushed a thin layer of water on one of the surrounds to see if the new surround might be too porous. No change. Also - when I say "airtight", I know it just means a return delay of 1-2 seconds.

All comments will be welcome and appreciated.

Thanks

Bruce

[Carlspeak]

I wouldn't be too concerned about passing the 1-2 second return delay. If your happy with the bass your getting (as you indicate) then everything should be okay as long as you used a good sealant under the woofer when you re-installed it. Double check the connections and pots at the back to be sure no leaks there as well.

[soundminded]

Just obtained a pair of AR2ax from the original owner. Excellent condition other than rotted foam woofer surrounds. I've read about everything I can find here and at Audiokarma about them, and knew I wanted to replace the capacitors as well as surrounds. I settled on the JBL125a surrounds from Looneytune2001 and 5% Dayton capacitors. Cleaned the pots, installed the surrounds using Aleene's tacky glue and new gasket foam from Parts Express. Everything goes back together perfectly - except pushing on the woofers doesn't give me any return delay. Listening to them, driven by a Pioneer SX-1080, reveals some great sound in a 5,000 cf room. If I didn't know about acoustic suspension boxes needing to be airtight, I would be 100% pleased with the result; however...

So here's the dilemma/question: Could I expect significant improvement if I get these airtight? If so, what should I try? If not, I'll be inclined to leave well enough alone.

Background: I've refoamed six sets of Advent woofers, with 100% success in getting them airtight the first time. The ARs gave me no problems or hint of future problems. I've tightened the 4 bolts tighter than usual when I couldn't get the return delay, and I've also put putty around the perimeter of the baffle at the mid and tweeter of one speaker and over the bolt heads - just in case they might be the problem. Unrelated but of interest- both speakers show a date 6/74 on the paper on the backs, but S/N 257568 has a countersunk baffle for the mid and tweeter, while S/N 257482 has flush mounted tweeter and mid. Both have countersunk woofers. I have brushed a thin layer of water on one of the surrounds to see if the new surround might be too porous. No change. Also - when I say "airtight", I know it just means a return delay of 1-2 seconds.

All comments will be welcome and appreciated.

Thanks

Bruce

I had a pair of AR2ax woofers refoamed by Bill Degall at Millersound last fall and the results were outstanding. He applied a "dressing" to the cones because he said the paper may have become porous. I also used the foam surround gaskets from Dayton to seal the drivers to the cabinet.

When air tight and operating properly, carefully equalized, and within their power handling capacity, I have found that AR2a and AR2ax can produce remarkably deep bass without audible distortion down to the very lowest frequencies. In this regard it performed well beyond my expectations. I also discovered that when the level controls are carefully adjusted (midrange slightly below indicated flat and tweeters full) and carefully equalized to compensate for their designed in rolloff, these speakers are also remarkably accurate, something I didn't expect either. Both tweeters had the same output and no buzzing so I assume they are both functioning properly. This pair I rescued virtually from curbside perform far better than any others I'd ever heard before.

This speaker can be damaged by your receiver. I accidentally badly damaged an AR2a woofer with a Pioneer SX950 which is not as powerful as yours. Bill repaired that perfectly too.

[genek]

My 1975 model 2ax's do not have anywhere near a 1-2 second return. I'd make a wild guess at maybe 1/2 second max. Their woofers are early 90's service replacements, which did not appear to be significantly different from the mid-70's originals, but otherwise I have not changed anything since they were new except the fabric on the grills.

My 3a's, OTOH, do take about a second to return, The surrounds on the 11/12" 3a woofer are wider than the 10" 2ax/5 woofer and the cone has more travel.

As Carl said, if you like the way they sound now, don't sweat it. If you're using them on a 100WPC+ amp and like to turn it up loud, fusing would probably be a good idea. It's not a guarantee against damage, though.

[mrbruce4]

I appreciate all of the comments so far.

I'm wondering if it would be worthwhile to pull the mid and tweeter out and run some putty around the perimeters (since I'm now out of foam gasket.) I retightened all the bolts on both mids and tweeters, but otherwise left them alone.

In the past, I've used Aleene's Flexible & Stretchable glue on cloth surrounds, with indeterminate success.

The surrounds I used this time seemed identical in composition to the Advent surrounds, so it's still a question of leave well enough alone or try to improve.

[Carlspeak]

In the past, I've used Aleene's Flexible & Stretchable glue on cloth surrounds, with indeterminate success.

so it's still a question of leave well enough alone or try to improve.

My answer is leave well enough alone and don't try to coat the new surrounds if they are indeed foam. You'll just risk stiffining them up.

[RoyC]

Another vote here for not losing sleep over the "slow woofer return" if the speakers sound OK.

The source of leaks I have found in order of frequency:

1) Driver-to-cabinet gasket seal (make sure to check around the tweeter and midrange as well).

2) The pots, if they are not fully seated. The tab on the pot should be firmly seated in the corresponding notch in the cabinet.

3) The small terminal board on the baffle of front wired cabinets.

4) The glue seal of the crossover board.

5) Porous replacement foam surrounds. (The JBL surround mentioned above is an excellent replacement, as it is very compliant, AND non-porous compared to other replacement surrounds...so I doubt it is your woofer. I have never seen the woofer cone alone allow enough leakage to be an issue.) DO NOT apply anything to the foam surrounds! Aleene's is not a good choice for cloth surrounds either.

6) VERY rarely, the edge of the front baffle of the cabinet.

If you are curious, get a roll of masking tape and completely tape these areas, one at a time, to see if it changes the woofer movement. Unless you have multiple leaks, it should be fairly easy to identify.

Roy

[genek]

I appreciate all of the comments so far.

I'm wondering if it would be worthwhile to pull the mid and tweeter out and run some putty around the perimeters (since I'm now out of foam gasket.) I retightened all the bolts on both mids and tweeters, but otherwise left them alone.

In the past, I've used Aleene's Flexible & Stretchable glue on cloth surrounds, with indeterminate success.

The surrounds I used this time seemed identical in composition to the Advent surrounds, so it's still a question of leave well enough alone or try to improve.

AR tweeters and mids used to be sealed with putty under their flanges until the switch to foam gaskets in the 70's. Either should do fine, but when I replaced the tweeters in my 3a's I used 1/4" wide foam tape on the cabinets to make it easier to swap drivers for future testing.

My '75 2ax woofers had a slightly shiny appearance and could become a bit tacky in hot weather, which suggests that a dope of some sort was used, but the service units I installed in the early 90's are matte finished and bone dry. Foam surrounds were changed to a different, more long-lived material sometime around 1990 that is usually not doped and who knows what doping will do to them. As Carl and Roy said, just leave them be.

[mrbruce4]

Thanks to all.

I will leave the woofer surrounds alone and will try the masking tape at various points. If I don't find a leak to cover, I will relax and enjoy the music!

Bruce

[Zilch]

http://www.audiokarma.org/forums/showthread.php?t=232380

[mrbruce4]

Thanks Zilch.

I had 1/4 inch plastic tubing and a Denon Audio Technical CD (using 21.5 HZ), so I tried this right away.

Going over each speaker's drivers and the rear, including connections, I can't find anything. I was moving the tubing 1/2 to 1 inch per second and had the other end in my ear.

I'll keep after this approach because there must be a leak somewhere. Frustrating at the moment but I've got the tools to find it. The question will be whether I have the patience.

[Zilch]

The Looneytune2001 30 Hz surround alignment disc works, too.

Sounds like a motorboat where it's leaking, if it is....

[genek]

I'll keep after this approach because there must be a leak somewhere. Frustrating at the moment but I've got the tools to find it. The question will be whether I have the patience.

Based on how you've described your speakers, I would say the question is still whether or not you actually have a significant leak.

[mrbruce4]

Thanks again Zilch. Good to know exactly what to listen for. I'm sure I didn't hear that during my first pass, but I'll do another check.

I bought only the surrounds, and not the "kit" from Looneytune since I had glue and didn't need instructions. For centering I use a C cell battery for about 20 minutes, to pull the surround down and keep it centered.

As others have advised, I'm not going to lose sleep over this return time thing; it's simply that I've gotten used to using it as a good check - with Advents, KLH 5, AR4x & KLH 17s. (Only the Advents were refoamed but I recapped all the speakers so I needed to be sure I resealed the cabinet.)

Bruce

[Pete B]

Thanks again Zilch. Good to know exactly what to listen for. I'm sure I didn't hear that during my first pass, but I'll do another check.

I bought only the surrounds, and not the "kit" from Looneytune since I had glue and didn't need instructions. For centering I use a C cell battery for about 20 minutes, to pull the surround down and keep it centered.

As others have advised, I'm not going to lose sleep over this return time thing; it's simply that I've gotten used to using it as a good check - with Advents, KLH 5, AR4x & KLH 17s. (Only the Advents were refoamed but I recapped all the speakers so I needed to be sure I resealed the cabinet.)

Bruce

You should hear a chuffing sound if you drive it moderately hard at 20 Hz.

I would think that 5W should be enough and not harm the speaker. You

can also pass your hand around obvious potential leak areas and you should

feel the air flow. You may not find it this way if the leak is widely distributed,

over the full paper cone for example, and not a small area where the velocity

would be high.

[mrbruce4]

I appreciate the thoughtful responses from members that I have trusted for a good while. While I only know you by the member names, I follow and respect your opinions. Many thanks.

I will be back in a couple of days after I've done a little more checking.

Bruce

[Zilch]

Thanks again Zilch. Good to know exactly what to listen for. I'm sure I didn't hear that during my first pass, but I'll do another check.

Somewhere in these pages, you'll likely discover that's how Villchur checked them....

[soundminded]

Two points.

First, you DO want some air leakage. If you do not have this barometric pressure changes will cause the woofer to move in or out and be off centered. OK, few boxes are sealed that tight.

Second, generally, if you do not hear chuffing or other weird sounds from the box area when playing deep bass the seal, assuming reasonable quality, is probably adequate. Above 100 Hz or so the sealing tightness will not matter much. Below that frequency sealing obviously matters, but it need not be perfect. It takes a pretty big leak for a 10-inch cone to be bothered enough to lose suspension support to a sonically degrading degree, and when that happens you usually hear chuffing noises. Remember, even with deep-bass music the cone moving a whole lot faster than the 1-2 second rate you noted.

Howard Ferstler

Understanding why it takes time for a properly functioning acoustic suspension woofer/enclosure system to return when the cone is pushed in goes to the heart of how it works. In a true and pure acoustic suspension system, if there were an air leak, the cone would not return at all. Only the air trapped in the box when compressed would cause the cone to return. If there's a leak, there's no compression. But AR woofers are not purely acoustic suspension speakers, their sourrounds and spiders produce some mechanical restoring force. If they didn't, the cone would have to be manually centered in its range of travel at the moment the cabinet were sealed and there would be no way to overcome the manometric behavior of the cone. In fact IMO the refoams provide much more restoring force (stiffness) than the original cloth suspensions. Even my AR9 woofers and LMRs seemed to have been originally manufactured with a thin rubbery highly compliant film like substance, not foam and the replacement foam seemed to me at least initially much stiffer. This stiffness will cause the cone to return even in free air. But even if the cabinet is perfectly sealed, the cone can still return instantly. The reason it wouldn't is due to the aerodynamic frictional drag created by the stuffing. (If you remove the stuffing and then seal it tight you should see it return instantly.) It takes time for the air pressure in the box to push its way through the spaces between the fibers to restore the cone to its original position by equalizing the internal pressure. This is analogous to the time it takes when you push down on the bumper of a car with hydraulic shock absorbers for the bumper to return to its original height. The piston takes time to overcome the frictional force of the oil it travels through as the bumper is being pushed up by the compressed spring. This viscousity in oil is exactly analagous to frictional drag of air the fibers create partially obstructing its flow. The compressed air is the spring. This is why for example, terminal velocity exists, that is when even an object like a lead anvil is dropped from an airplane it will not exceed about 600 mph, the force of frictional aerodynamic drag at that speed will be exactly equal to the force of gravity pulling it down and it will not continue to accelerate. If the stuffing fibers have been compacted over time for any reason such as settling, only the air trapped between the back of the cone and the stuffing will be compressed and this drag coefficient will be much lower. BTW, this number is often represented by the letter "b" in Newton's second law of motion which accurately describes both AS and non AS woofer/enclosure systems in a way that can they can be readily understood. (Villchur's thermodynamic explanation while not wrong does not give us a real understanding of what is happening, it is not the right model to use.) Newton's second law of motion is the souce for the mechanical coefficients in the Thiel Small analysis which reduces it to a cookbook recipe. This loss of drag will cause the natural system resonance frequency to increase and for an underdamped FR peak to develop. The only way to determine this is to measure the FR in an anechoic chamber. Overshoot upon rebound of the cone is too small and fast to see with the naked eye.

I wouldn't worry about not having enough leakage. If the aggregate cross sectional area of air leakage of the cabnet/speaker is the size of the area of the head of a pin, that should be sufficient to equalize the pressure considering that atmospheric pressue does not change rapidly or drastically compared to the mechanical force the mechanical suspension of the driver will provide in overcoming it.

[kimberley]

I had always thought that with 'air suspension' or sealed box 'speakers like the AR's or Ls3/5a's (I have both) that the push-wait-slow-return check actually depended in ther BEING a small leak: the slowness of the return depending on the small size of the leak through which air had been expelled and through which it is returning.

Surely, with a perfectly air-tight cabinet the return of the diaphragm would be near-instantaneous, as the trapped air at increased pressure would provide a returning force impeded only by the suspension of the cone of the driver?

[mrbruce4]

Well, to wrap up my part of this story- I spent time today listening for a leak in one of the speakers. I couldn't hear a leak anywhere. Also spent some time listening to music. A recommended test for bass is Bela Fleck's Amazing Grace. This sounded fine with deep bass, so I'm calling myself happy with the speakers.

As several knowledgeable members have suggested, I'm not going to worry this one any more.

Thanks again for the advice!