Use of foam/cloth in front of mid range drivers, WHY?

[Tom H.]

Hello again,

I have one burning question about 70's vintage AR Speaker design.

Why did they go to the trouble of installing what appears to be foam (Or treated cloth?) behind a metal mesh on the LST, AR-2ax, AR-5, AR-3a, AR-10PI, AR-11 mid-range drivers through the 1970’s? What was the theory behind the use of these materials in front of the driver?

What if anything can or should be done to restore this material? Given that the foam surrounds on woofers are usually long gone you’d think mid range foam would suffer the same deterioration.

Both my AR-10PI's and project AR-2AX mid ranges appear to have materials in front of the mid range drivers.

Thanks!

Tom

[michiganpat]

not sure what was used in the dome midranges, but I think the 2ax used fiberglass, which wouldn't really deteriorate.

they are used as mechanical, rather than electrical (such as caps and inductors) filters to shape the driver's response. my guess is that they mechanically attenuate high frequency response

[rjr513]

the dome mids use also "fiberglass" for damping and also to offset pressure to counteract the cavity damping from the fiberglass also used under the dome.

[Tom H.]

Thanks!

It's good to know that they are fiberglass not foam. I figured they were for some kind of dampening. AR is the only speaker manufacturer that I know of that did this to their drivers so I thought it was kind of odd.

[dynaco_dan]

the dome mids use also "fiberglass" for damping and also to offset pressure to counteract the cavity damping from the fiberglass also used under the dome.

Hi there

I have always been curious why they used such a large radius bumper rather than say an o-ring sized piece.

Obviously it must not have had any effect on the mid domes dispersion characteristics.

I have also seen a 5 cent size piece of tape? dead center for some reason.

[tysontom]

Hello again,

I have one burning question about 70's vintage AR Speaker design.

Why did they go to the trouble of installing what appears to be foam (Or treated cloth?) behind a metal mesh on the LST, AR-2ax, AR-5, AR-3a, AR-10PI, AR-11 mid-range drivers through the 1970’s? What was the theory behind the use of these materials in front of the driver?

What if anything can or should be done to restore this material? Given that the foam surrounds on woofers are usually long gone you’d think mid range foam would suffer the same deterioration.

Both my AR-10PI's and project AR-2AX mid ranges appear to have materials in front of the mid range drivers.

Thanks!

Tom

Tom,

As others have stated here, the material was fiberglass placed under the protective grill on the midrange drivers. This all started with the AR-3's 2-inch midrange driver back around 1961 or 1962. A fiberglass "pad" was fitted under the dome for the 1⅜-inch tweeter and the 2-inch midrange to improve damping within the "cavity" under the domes themselves. In addition, more fiberglass was placed further down in the magnet cavity between the ring magnet and back-plate/pole piece. It was found that the fiberglass under the dome tended to push against the dome, and in some cases the dome would "pop" out of the gap -- especially under high-power input -- thus basically ruining the driver. AR quickly recognized the problem and decided to put fiberglass on the outside of the dome, under the grill, to offset the pressure under the dome. The tweeter, in the absence of a protective outside screen and fiberglass, continued to have the problem, but it became a minor issue once some adjustments were made. Coincidentally, it was found that the fiberglass smoothed the measured on-axis response a small amount (perhaps ½ dB), so the fiberglass was ever-so-slightly beneficial.

In 1967, AR changed the midrange driver with the introduction of the AR-3a, and this new soft-dome 1½" driver also used the fiberglass for slight, but measureable acoustical improvements in this driver as well. Dispersion was improved somewhat in this driver. However, I also think the new midrange, introduced around the same time as Bill Hecht's patented soft-dome midrange driver for Avery Fisher, remained hidden from view and this may have by happenstance avoided any possible infringement litigation. This is strictly a wild hunch, but in a conversation with Hecht a few years ago, he indicated that he didn't realize that the AR midrange was indeed a treated-cloth soft-dome driver. In any case, Hecht's patent was indefensible -- as many companies had introduced soft-dome drivers around this time -- and the AR product was introduced about the time the patent was granted, so AR sort of co-invented that technology as well. Prior to that in 1958, Edgar Villchur tried soft-dome tweeters for the original prototype AR-3, but he decided against that design due to dispersion problems.

--Tom Tyson

[soundminded]

Hi Tom

Are you sure the material is fiberglass? I took the screen off an AR2ax midrange and the material seemed to me like it was probably lamb's wool.

[tysontom]

Hi Tom

Are you sure the material is fiberglass? I took the screen off an AR2ax midrange and the material seemed to me like it was probably lamb's wool.

Hi Soundminded,

I'm not sure of the material in the AR-2ax midrange -- it might be lambswool -- but fiberglass was used on the AR-3 domes (it was a fine texture, for sure) and I believe on the AR-3a domes.

--Tom Tyson

[RoyC]

Hi Soundminded,

I'm not sure of the material in the AR-2ax midrange -- it might be lambswool -- but fiberglass was used on the AR-3 domes (it was a fine texture, for sure) and I believe on the AR-3a domes.

--Tom Tyson

The material under the AR-2ax midrange grille is the same yellow fiberglass. The later AR-11/10pi midrange had pink fiberglass under the grille, but was functionally the same as all the other AR-3a mids.

Roy