Sealing drivers in older AR cabinets

[johnieo]

Sometime ago I posted a note saying that I felt putty was a cheaper and better alternative to the AR replacement materials. Finally, the photos we wanted for illustration are done. I feel the rope caulk form of putty is superior to the harder, wider materials mentioned. To form a seal one needs both a sealing force and a restoring force. The mounting screws provide the sealing force; the restoring force is normally the elasticity of the gasket. However, putty is not elastic, it is plastic; it has no long-term restoring force and cannot make an airtight seal. It requires a lot of force to flatten, but with time, this force dissipates. This makes it quite usable as a driver gasket, because the cabinet cannot be absolutely leak tight anyway.

Each screw on an AR 12-in woofer has to seal a 4-1/2-in. perimeter. If the sealing surface is completely filled with putty, as one usually finds, it covers a width of 7/8 in. The force on the basket frame near the screws could be high enough to warp the frame, especially if one becomes overzealous with the application of putty- real or synthetic. Even if it did not warp the frame, it most certainly produces enormous strain on the Tee nuts imbedded in no longer so strong, thirty-five-year-old plywood. A single strand of rope caulk works admirably well in this situation, as it provides enough material to fill the gap and flatten with a low force. The difference is illustrated in the first sketch.

http://www.classicspeakerpages.net/dc/user_files/722.jpg

A second difficulty with the use of massive amounts of putty or AR putty-substitute is illustrated in the left-hand side of the second picture, below. (sorry for the poor color, it is really black, not blue.) When removing the black hardened gasket mass AR used in these 1968 AR-3 cabinets, the surface layer of plywood delaminated in many areas. To prevent this from recurring, we filled the voids with plastic wood, sanded smooth, painted black, and brushed a coat or two of varnish on just the flat surface of the back-bore. The right-hand side of the picture shows the restored sealing surface with new gasket. Should the driver need removing, the putty will separate much more easily than as originally installed. We have also found it useful to drill a 3/8” flat-bottomed recess near the edge of the drivers with a Forstner bit (easier in a drill press). Now one can lift the woofer with the little paint can opener (shown on the side) without nicking the baffle board.

http://www.classicspeakerpages.net/dc/user_files/723.jpg

[tysontom]

John's post, "Sealing drivers in older AR cabinets" is excellent! This points to the necessity of thinking one's way through a restoration effort in order to maximize the quality of restoration. A small detail such as the woofer baffle-board sealing technique is very important. You want a good "acoustic seal," but not a hermetic seal, and John's technique exemplifies the correct way to do it. Insofar as the later mid-70s open- and closed-foam AR gaskets are long gone, Mortite or plumber's putty, placed exactly as John describes it, is probably the best way to finish a restoration job on an AR speaker. It also allows re-entry, if necessary, without damaging the routed-out speaker hole.

With AR-3-style cabinets (all AR cabinets up through the late 1980s), we are fortunate to have eight machine-screw fasteners rather than the usual four or six wood screws tapped into particle board -- the usual practice in the loudspeaker industry. Open up an Advent, KLH or other similar speaker system and you can begin to appreciate AR's attention-to-detail in manufacturing.

http://www.classicspeakerpages.net/dc/user_files/724.jpg

AR-1 SN0006: even the earliest AR speakers used machine screws and Tee-Nuts to mount drivers. Note the thickness of the baffle (1-1/8-inch) and the glued and screwed cabinet braces.

--Tom Tyson

[soundminded]

I had a similar problem about a year and a half ago when I started my Bose 901 modification project. The original design which was the model I had used 9 acoustic suspension drivers per enclosure. The seal between the speaker baskets and wood enclosure seemed to be sealed with a material similar to plumber's putty which had hardened and cracked no longer forming an air tight seal. The prospect of removing eighteen small drivers, scraping off the old material from them and the cabinets and filling in obvious cracks with wood filler was not something I cared to do especially since replacement drivers are not readily available if I damaged one. After checking with the factory's repair department, I deceided to run a bead of clear GE silicone caulking around the perimeter of each one and over the screw heads being very careful not to get any on the suspensions or cones. It is very easy material to work with and you can even use your fingers. If you miss a spot you can just add another dab or two. It's cheap and only takes a few hours to cure (24 recommended.) In subsequent experience, I found out the hard way that this material will never cure if it has passed its "use by" expiration date because volitale components which catylize the cure will no longer be there so get a new tube if you want to try it. It seems to have worked well for me, gently pressing in on one driver immediately getting a response from all of the others. Jist thought I'd offer this suggestion as an alternative.

[frankmarsi]

Dear Tom Tyson, you appear to be quite a collector, and that's a good thing, I wish I was as involved as you are. My question is why haven't you ever responded to any of my posts? I've been an avid AR listener since 1967, it wasn't until 1972 when I was able to some how afford a pair or 3a's. No doubt you have seen my posts on this site and I've never heard from you, why? Is my dialog too mondane, or do I come across as a braggard because I revel in telling the world that I have stacked LST's, and soon the same with 3a's. Well perhaps I may never know your thoughts, however I can tell you that I appreciate your creditials and opinions and look for any of your responses to others questions.

Sincerely,and respectfully from a distance, Frank Marsi

[Guest postjob62]

Frank,

I'm sure Tom can and will speak for himself if he is so moved, but just a respectful hint: go back over your posts, and read them for tone, not content.

You're dealing here with a bunch of folks who have a tremendous collective body of knowledge and life experience; I myself ask far more questions here than I'll ever answer and I'm awfully appreciative for the help I get from the group. But not all my posts, or anyone's for that matter, are of enough interest to necessarily generate a response.

Perhaps just a little less enthusiastic approach would be helpful. But then, I'm reminded of the old Harley-Davidson T-shirt which proclaims, "If I have to explain, you wouldn't understand".

Regards,

Ed

[tysontom]

>Dear Tom Tyson, you appear to be quite a collector, and

>that's a good thing, I wish I was as involved as you are. My

>question is why haven't you ever responded to any of my posts?

>I've been an avid AR listener since 1967, it wasn't until 1972

>when I was able to some how afford a pair or 3a's. No doubt

>you have seen my posts on this site and I've never heard from

>you, why? Is my dialog too mondane, or do I come across as a

>braggard because I revel in telling the world that I have

>stacked LST's, and soon the same with 3a's. Well perhaps I may

>never know your thoughts, however I can tell you that I

>appreciate your creditials and opinions and look for any of

>your responses to others questions.

>Sincerely,and respectfully from a distance, Frank Marsi

Frank,

I am so sorry that I have neglected to answer your posts; it certainly wasn't intentional, and since I don't read each post I may have simply overlooked any specific questions you might have asked me. And remember: I am only one voice within a body of very knowledgeable AR collectors and experts. I had a head-start in this area a few years ago, but now there are many people on this site that are truly experts in AR speakers!

So, send any questions again and please accept my apology!

--Tom Tyson

[Guest daveshel]

Thanks, all. This will come in useful as I set about rehabilitating a pair of AR-4xs (project A).

Does the AR-7 (project require this same sort of seal? Seems to me I removed the woofers years ago for the sake of my curiosity and put them back in without resealing at all. And then I wonder if this might have had anything to do with the voice coil damage I eventually experienced on one of the woofers.

[RoyC]

Sometime in the early to mid 70's AR started using foam gaskets (the material actually held up better than the old putty, from what I've seen)...and I'm guessing the AR-7 had foam gaskets.

You could probably get away with foam again if the woofer seat is in good repair, but you can never go wrong with putty for a tight seal.

Roy

[Guest daveshel]

Ace Rope Caulk Weatherstrip was easy to work with and seems to do the trick. My renewed AR-4xs sound great. Thanks, guys.

[tysontom]

>Thanks, all. This will come in useful as I set about

>rehabilitating a pair of AR-4xs (project A).

>

>Does the AR-7 (project require this same sort of seal?

>Seems to me I removed the woofers years ago for the sake of my

>curiosity and put them back in without resealing at all. And

>then I wonder if this might have had anything to do with the

>voice coil damage I eventually experienced on one of the

>woofers.

Most likely yes. Serious air leaks would allow the "floppy" suspension of the AR-7 woofer to over-extend on heavy bass passages without the protection of the cushion of air in the enclosure. Over-extension would likely come in the form of the voice coil banging against the back plate and flattening the end of the coil, or perhaps outright damage to the suspension parts.

The AR-7 is just like all the other "classic" AR speakers (the Recoton and Audiovox ported ARs notwithstanding) in the use of the acoustic-suspension method of "loading" (for lack of a better word) the woofer. A/s requires an acoustic seal to perform properly, and this usually dictates the use of a gasket of some description for the drivers in the cabinet. One of the advantages of acoustic suspension, along with low harmonic distortion and relatively flat response, is the natural "roll-off" that occurs below system resonance, or fc (resonance of a closed-box system). This roll-off is approximately 12 dB/octave and, as such, the response begins to taper off below resonance. Another way of looking at it is that, below system resonance, the excursion of the woofer (for a given power level) remains relatively constant as the frequency is lowered. To a *large* degree this roll-off reduces over-excursion of the acoustic-suspension woofer under normal circumstances, a sort of built-in safety device. On the other hand, air leaks -- if bad enough -- could cause unrestrained woofer motion at sub-sonic frequencies with the resultant eventual damage that you have described.

http://www.classicspeakerpages.net/dc/user_files/876.jpg

Fig.1 Example of voice coil "strife-tested" at high power causing bottoming against magnet back-plate polepiece

--Tom Tyson