Diff in AR sound of 80's and today?

[Guest]

Having been out of the scene for several years (12) I am wondering if there is a diff in the new line of AR spkrs vs the old. I have the 91's and and 18s's, both from early 80's.

If I use these for surround sound, what is a good unit for center, and will I have a problem matching sensitivity?

Thanks,

Bill

[ar_pro]

There's a substantial difference in sound, Bill. Current AR products are vented, bass-reflex type systems that bear no resemblance to their previous acoustic suspension designs. If you intend to keep your vintage AR speakers, you might want to check eBay for something that would be closer in both "voice" and sensitivity.

[Steve F]

>There's a substantial difference in sound, Bill. Current AR

>products are vented, bass-reflex type systems that bear no

>resemblance to their previous acoustic suspension designs.

>If you intend to keep your vintage AR speakers, you might

>want to check eBay for something that would be closer in

>both "voice" and sensitivity.

Bill,

ar_pro is right on the money here.

Also bear in mind that the 91's are not magnetically shielded, so if those are going to be your main front speakers, keep them at least 18"-24" away from your direct-view television to avoid magnetically-induced color distortion. The stray magnetic field of an unshielded speaker can ruin a picture tube if it's left in close proximity too long.

Same thing goes if you decide an older '80's-era AR speaker is going to be your center channel. Keep it away from the TV tube. If you have to place an unshielded center speaker 2 feet above the TV, you can angle the speaker down towards the listening area for proper sound coverage.

Steve F.

[Guest Nigel]

>I have the 91's and 18s's, both from early 80's.

>If I use these for surround sound, what is a good unit for

>center?

Hi Bill;

1st shielding, is not really shielding, but shunting, that is providing the magnetic flux a path that is preferrable to it compared with going thru your TV. A .030" thick piece of steel between the speaker and the TV will take care of it.

I would suggest you get an additional pair of 91, 91.5, 92, 93 or 94 for your rears, and use the 18s's on thier sides, tweeter towards the inside as your center channel.

There is a pair of 94 on ebay right now:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=800418911

Make sure your home theatre amplifiers are up to driving the AR91 and AR94. Many will not do well with classic AR speakers.

Nigel

[Steve F]

>1st shielding, is not really shielding, but shunting, that

>is providing the magnetic flux a path that is preferrable to

>it compared with going thru your TV. A .030" thick piece of

>steel between the speaker and the TV will take care of it.

>

We have to be careful here. Every circumstance and every different speaker presents a different challenge. There are no blanket answers that cover all situations.

Shielding an unshielded speaker can be a tricky task, at best. Magnetic fields tend to flow like water. Simply putting a flat steel sheet of arbitrary size near a speaker will cause the field to flow around it. If the sheet is not large enough, the magnetic energy will flow towards the edges of the sheet and present an increased energy field at the edges.

To draw a simple, but accurate analogy, think of the magnetic field as water and the steel sheet as if it were a heated sheet of metal—like a hot cookie sheet. The steel will prevent the water (magnetic field) from penetrating to the speaker directly beneath it. And the fact that the sheet is hot means that the “water” (magnetic field) will evaporate (weaken) as it runs towards the edge. The trick is, will the magnetic field “spill over” the edge and cause a problem before its strength dissipates?

Another problem is the crossover. Even if the speaker’s magnetic field is contained when the speaker is in the static mode, the inductors in a crossover create their own magnetic field (independent of the drivers’) when the signal passes through them. Many supposedly “shielded” speakers will exhibit marked interference when playing a high-level signal. You can check this easily by putting a blue screen on your TV (turn on your VCR and hit stop with the TV in “video” mode), and playing bass-heavy, high-level music when the speaker is on the TV. You’ll see the screen jumping with black bars of interference in perfect time to the bass line of the music. This is evidence of a CROSSOVER-induced shielding problem, not merely a driver/magnet-based problem. It’s quite common.

The upshot of all this is that a properly shielded speaker is one that has been designed from the start as a shielded system, not merely one with shielded individual drivers. It’s hard to do. It’s very hard for the end user to do after the fact. If you want to go the sheet steel route, you may find that you’ll have to use a larger sheet than looks cosmetically acceptable in your room. And be sure to check the performance of the shielding against the TV’s blue screen—you may have magnetic interference (and eventual picture tube damage) that is not immediately apparent when the TV is playing a moving image.

You’re best advised to simply move an unshielded speaker a foot or two away from the TV, and save the sheet steel for repairing the rusty wheel wells of your 1990 Accord (they always rust there).

Steve F.

[Guest dogmeninreno]

I agree with Steve F. Unpredictable at best. It is kind of like high voltage in an electrical system usually 5KV and higher, in which the magnetic fields tend to travel on the outside of the conducter and need to be shielded and stress cone relieved at both ends and then run to ground dissapating the magnetic energy generated on the outside. The termination is called a "stress cone". It uses a semi conductor tape in contact with the copper shield surrounding the conductor and is built up with insulating material forming a cone which has a ground braid that is in contact with the semi-conductor on the cable and attached to ground. This gets very tricky and expensive at 115KV or 240KV and higher. Next time you see some 7.2 KV utility wires going from a pole to a underground conduit you will most likely see a stress cone. This traps the stray magnetic fields and prevents cable failure. Very tricky stuff! For example: Take an old speaker and put some metal filings on a piece of cardboard. Place the cardboard and filings near the speaker magnet (we all did this in school I believe) Then lift the cardboard and place a metal sheet between it and the speaker magnet. See what it takes to shield the fillings and cardboard from magnetic influence. Good Luck!! Dale in Reno.....

[Guest Nigel]

Guys;

First, my qualifications: I design hard disk drives, which have voice coil motors in them, typically with 1 Tesla in the gap. The external challenge is to design the drive such that you can pull it out the box while the one right next to it is running, and cannot be affected. Stray flux is what it is all about. Just like speakers and TV's.

The hot baking sheet is NOT a good analogy. Magnetic flux would rather go through steel than air - until it reaches saturation, which is not a consideration in this case. It is a shunt, a preferential path, not a shield.

Magnetic fields are predictable. In the hard drive industry, we have demonstrated in the laboratory the ability to write and read data at over 100,000,000,000 bits per square inch on a magnetic layer with a coil on tiny piece of ceramic.

It does not take much steel at a couple inches from the source to shunt the field a speaker's driver creates. This is all "stray" flux, it is the flux that the design is not adequately capturing, and thus it is not very much.

Nigel

[Steve F]

Nigel,

No argument about the basic facts concerning magnetic behavior—science is science. And I’m certainly not going to get into an ego-driven knowledge “contest” with one of my fellow Forum members, especially since you seem like such a good guy, judging from your previous posts.

I was just trying to make a simple, easy-to-understand point about the challenges involved in shielding a speaker after the fact. (Other engineers I spoke to liked the cookie sheet analogy—go figure!) Without having the same access to the specific magnetic parameters of the driver and the crossover (like the original designer does), shielding is kind of a trial and error situation for the end user. And you really do need to watch those crossover chokes, because that’s a completely different consideration than the magnets on drivers.

Steve F.

[Guest dogmeninreno]

>Nigel,

>No argument about the basic facts concerning magnetic

>behavior—science is science. And I’m certainly not going to

>get into an ego-driven knowledge “contest” with one of my

>fellow Forum members, especially since you seem like such a

>good guy, judging from your previous posts.

>

>I was just trying to make a simple, easy-to-understand point

>about the challenges involved in shielding a speaker after

>the fact. (Other engineers I spoke to liked the cookie sheet

>analogy—go figure!) Without having the same access to the

>specific magnetic parameters of the driver and the crossover

>(like the original designer does), shielding is kind of a

>trial and error situation for the end user. And you really

>do need to watch those crossover chokes, because that’s a

>completely different consideration than the magnets on

>drivers.

>Steve, I agree. Nigel has a good point and we should consider his views too! I think he is saying that shielding is tricky at best...Trial and error if you are not in the lab. Dale in Reno.....

>Steve F.