AR3a Improved Foam Surround Question

[FrankC]

Hello,

I have a pair of AR 3a Improved speakers purchased from the original owner. The surrounds are gone and will be refoaming them shortly.

The local speaker shop in Raleigh has a good reputation and has 1/2 roll surrounds in stock that are correct except that my roll is 5/8.

Originality is important to me and I ordered a set of 5/8 roll from Msound. However, the shop insists their is no sonic difference at all

with the 1/2 roll they have been using for years.

So my question to the folks here -Originality aside, is there any difference in performance?

Thanks,

Frank

[soundminded]

Hello,

I have a pair of AR 3a Improved speakers purchased from the original owner. The surrounds are gone and will be refoaming them shortly.

The local speaker shop in Raleigh has a good reputation and has 1/2 roll surrounds in stock that are correct except that my roll is 5/8.

Originality is important to me and I ordered a set of 5/8 roll from Msound. However, the shop insists their is no sonic difference at all

with the 1/2 roll they have been using for years.

So my question to the folks here -Originality aside, is there any difference in performance?

Thanks,

Frank

The surround is one of the factors that could potentially limit the maximum excursion of the cone. If the diameter of curvature, that is the crosssectional diameter through the material is the same, then the 5/8 inch roll will allow for a slightly greater maximum excursion if it and not some other factor is the limiting element. Otherwise it won't matter.

Far more important besides making an airtight seal and not deteriorating over time is the compliance of the material. Not compliant enough and it will impose too tight a mechanical restraint on the cone's movement. This will increase the driver's inherent resonant frequency fc. Too low and the driver will have no restoring force at all except the trapped air to return the cone to its nominally neutral position. Somewhere on one of these threads, I think someone who worked at AR said that about 90% of the restoring force is due to air pressure, about 10% due to mechanical force of the suspension. The fc should be in the range of around 16 to 19 hz in free air. The f3 in the enclosure should be 42 hz with critical damping of 0.707. This results in the smoothest lowest extended bass response for this driver in this enclosure without any peak in its response (I assume it's the same as AR3a, AR3, etc.) and is what it was designed for. The other critical element is the stuffing being the right amount, the right consistency, and evenly distributed. Hope that helps.

[FrankC]

soundminded

Thank you for the in depth response. That helps greatly.

Frank

The surround is one of the factors that could potentially limit the maximum excursion of the cone. If the diameter of curvature, that is the crosssectional diameter through the material is the same, then the 5/8 inch roll will allow for a slightly greater maximum excursion if it and not some other factor is the limiting element. Otherwise it won't matter.

Far more important besides making an airtight seal and not deteriorating over time is the compliance of the material. Not compliant enough and it will impose too tight a mechanical restraint on the cone's movement. This will increase the driver's inherent resonant frequency fc. Too low and the driver will have no restoring force at all except the trapped air to return the cone to its nominally neutral position. Somewhere on one of these threads, I think someone who worked at AR said that about 90% of the restoring force is due to air pressure, about 10% due to mechanical force of the suspension. The fc should be in the range of around 16 to 19 hz in free air. The f3 in the enclosure should be 42 hz with critical damping of 0.707. This results in the smoothest lowest extended bass response for this driver in this enclosure without any peak in its response (I assume it's the same as AR3a, AR3, etc.) and is what it was designed for. The other critical element is the stuffing being the right amount, the right consistency, and evenly distributed. Hope that helps.

[JKent]

Soundminded gave you good information on "why".

I assume you're following the AR3a "bible":

http://www.classicspeakerpages.net/library...ring_the_ar-3a/

5/8 is authentic.

My experience with MSound is that the kits are absolutely complete. Everything you need is there and the instructions are perfectly clear. So why not do it yourself? It may seem daunting but it's not hard. Are you at all handy? Did you build models when you were a kid?

Nothing wrong with taking your valuable speakers to a pro but there is satisfaction with DIY.

And use the RIGHT surrounds.

Good luck.

Kent

[FrankC]

Hi Kent,

I have not done a refoam yet, but ordered the kit. I am more comfortable with electronic components - vac amps and vintage solid state equipment.

Will give it a try.

Thanks for the encouragement and will post the results.

Frank