AR-3a Acoustic Cabinet Sealing

[Guest mrhagerty]

In the Restoring the AR-3a article done so well by the authors, there is one confusion point. On page 5, the authors explain that if surrounds have disappeared due to deterioration the cabinet is no longer "air tight" which can result in voice coil damage due to motorboating on high signals.

Then on pg 22 for testing a newly replaced woofer, regarding tightening screws, the authors state, "Don't worry about a "air-tight seal or hemetical seal, it's not necessary".

So should the cabinet be air-tight or not. I would think that to get the test on the cone dust cover to spring back with delayed movement you would need an airtight cabinet without leaks.

Can someone clarify this?

Thanks,

Mike

[genek]

So should the cabinet be air-tight or not. I would think that to get the test on the cone dust cover to spring back with delayed movement you would need an airtight cabinet without leaks.

It's impossible to achieve an "airtight, hermetic seal" on a box made of porous material. Even with everything gasketed and the screws tightened by the incredible hulk, air will still infiltrate throigh microscopic gaps. The required seal is an *acoustic* seal, which means that air doesn't pass fast enough and in large enough volumes to enable the woofer cone to move unrestricted. If the surrounds are intact and the drivers are gasketed with caulking or other soft material and screwed down tight enough so they compress the gaskets and don't move around in their mounts, that's all that's required to achieve an acoustic seal.

[soundminded]

In the Restoring the AR-3a article done so well by the authors, there is one confusion point. On page 5, the authors explain that if surrounds have disappeared due to deterioration the cabinet is no longer "air tight" which can result in voice coil damage due to motorboating on high signals.

Then on pg 22 for testing a newly replaced woofer, regarding tightening screws, the authors state, "Don't worry about a "air-tight seal or hemetical seal, it's not necessary".

So should the cabinet be air-tight or not. I would think that to get the test on the cone dust cover to spring back with delayed movement you would need an airtight cabinet without leaks.

Can someone clarify this?

Thanks,

Mike

It would be possible to seal the air trapped inside to a far greater degree than is necessary to achieve the acoustic suspension effect by for example coating the inside of the box with a plastic lacquer and applying lacquer to the cone or making it out of plastic as is often the case now. Not only isn't this necessary, it is not desirable. Tom Tyson has correctly pointed out that doing so would create a "manometer" effect. This means that any changes to the barometric pressure of the space outside of the box, of the room for example due to changes in the weather would offset the cone from its desired nominal "neutral" position halfway betweens excursion extremes. A minute air leak allows the inside and outside pressure to equalize without compromising performance to any measurable degree.

The procedure specified in the restoration document will seal the cabinet to the degree necessary to allow differences between air inside and outside the enclosure to exert the desired restoring force in operation.

[johnieo]

So should the cabinet be air-tight or not. I would think that to get the test on the cone dust cover to spring back with delayed movement you would need an airtight cabinet without leaks.

Can someone clarify this?

Hello Mike:

As Gene noted, the cabinet cannot be completely airtight. If it were, then the woofer would remain forever pushed outward after driving to the top of Pike's peak! Acousically tight means it is air tight to some frequency below its range. In a quarter cycle of 20 Hz (12.5 ms) the quantity of air escaping or entering should not cause the internal pressure to change significantly. Permeation leaks of this size are found in the foam surround, around potentiometer shafts, and under terminal connectors. One depresses the cone and holds it for a few seconds then releases. It should take of order 1-to-2 sec. to relax. Leaks smaller than this are of no significance and are required to prevent the cabinet from becoming an aneroid barometer!

We should fix that inconsistency and others one of these days.

Cheers,