811 Fins

[SOOTSHE]

Need some advice on removing the fins from a pair of 811's.....or rather, why I should remove them. I've been trying to come up with a valid reason why this would make them sound better, but my head keeps saying "why is this so".

I assume Altec fitted them to make them stronger for possible pro applications but wouldn't they also provide bracing from the top to the bottom of the flare? Much the same as you would brace a speaker cabinet from front to back or from side to side. In the speaker cabinet scenario this prevents resonances from occurring, so why doesn't the same apply to the 811? Wouldn't the fins brace the top & bottom together making the whole assembly more rigid.

I've read posts where the fins are broken at the weld with rubber wedges inserted or black silicone applied....wouldn't this make the horn less rigid?

MAybe I'm way off here but I just can't seem to make sense of this.

[Old Guy]

Opinions are like...

I have heard then with and without can't hear a difference

You will ruin any resale value:doh:

IMHO wasted effort

Make sure damped well and forget it

[bowtie427ss]

You will ruin any resale value:doh:

IMHO wasted effort

Make sure damped well and forget it

QFT = quoted for truth.

[Steve Schell]

J Henry, you have posed an interesting question. Just what function do the fins serve on the 811 (and 511) horn?

Certainly they are not for pattern control, and being obstacles they would degrade the pattern slightly if anything. Perhaps they were intended to stiffen the structure, though the horn is still plenty stiff without them.

I think there might be other reasons: styling and product differentiation. The first version of the 811, the H-811 or 1952 or so, was a stylish thing. It was a one piece casting with a curvy appearance to the mouth, and the fins gracefully arched outward. It must have been a bear to cast, as it was replaced within a year by the two piece welded 811A.

The RCA MI-9594 (60 degree) and MI-9595 (90 degree) cast aluminum theatre horns had been introduced in about 1950. RCA produced the first radial horns, as the radial horn had been invented by RCA engineer John Volkmann. The Altec modified radial horns were obviously inspired by Volkmann's work. Perhaps the fins were intended to differentiate the look of the Altec product from the RCA horns, which did not use fins.

Another reason may have been to maintain a visual continuity with the earlier Altec horns. Altec Lansing had produced only multicellular horns until the introduction of the H-811, and the fins served to visually divide the mouth area, providing an evolutionary appearance change rather than a revolutionary one.

[Old Guy]

Pretty much 100% with Steve here, just adding the fins make them resemble multicells, Altec's bread and butter.

Jim Lansing didn't like multicells, but JBL later was forced to buy Vitavox multicells thru DuKane in order to bid on jobs where there was a specific muticell spec.

[GM]

Certainly they are not for pattern control, and being obstacles they would degrade the pattern slightly if anything.

Have you measured or seen polar plots of them with/without? In theory they should affect it just as is shown in Cohen's book.

GM

[zelgall]

Have you measured or seen polar plots of them with/without? In theory they should affect it just as is shown in Cohen's book. GM

I don't have plots but I do have several 811's and I did cut the fins out of one. I also filled in the casting radii between the top and bottom halves on a pair. I found no audible difference between the fin / no fin versions within normal listening distances. I listened to both config's and really didn't notice much differnce between them. Checking by ear is always a crap shoot anyway.

[RonSSS]

I removed them on my 19's. I like the results. I sit nearfield, and it removed the sensitivity of the sweet spot. It's bigger, not so sensitive to head movement.
Think of it this way, the center vane is right in line with the center of the driver.
In a theatre....these are theater horns....no issue because the sound will recombine. Sit close and it's a different story.

I also remove the bug screens, a mod that others here poo poo. Huge diff in my experience.

I have a picture tutorial on how to do it if you like. It involves a sawsall, a die grinder, bondo, sanding and paint.

I also damp the horn dead with latex paint and sand....thanks Jim Dickenson.

Ron

[Steve Schell]

Hi Old Guy, thanks for your reinforcement of my 811 fin theories. Just a tiny comment... Jim Lansing did seem to like multicellular horns, as they were the only type he used. The last one he designed was the H-1000 which was used with the 175 driver in his postwar 1200Hz. crossover two way Lansing Sound systems:

http://www.lansingheritage.org/image...1948/page7.jpg

Jim died in late 1949 and within a year or so William Thomas had secured the services of Bart Locanthi, who designed the small potato masher "Koustical" horn (later 175DLH) and the large masher theatre horn for the 375. The folded and flat plate lense horns came soon after. His work was based on that of Kock and Harvey of Bell Labs who had published a paper on acoustical lenses in 1949. Mr. Locanthi's wife was an optics engineer, and likely provided guidance as well.

As I have heard the story, JBL added the Vitavox-built multicellulars to the line in the 1970s so their dealers could bid on contracts that were written to virtually specify Altec equipment. These horns were sourced through Dukane (I think), who also marketed these horns under their name.

[Steve Schell]

Hi GM, your post sent me scurrying for Abraham Cohen's book "Hi Fi Loudspeakers and Enclosures", always a good thing. For those who haven't read this book, try to find one at abebooks.com or somewhere, as it is essential reading.

Were you referring to the picture of the Altec 3000 tweeter and illustration of the pattern? I may be all wet, but it seems to me that at least on a radial midrange horn like the 811, the dispersion pattern has pretty much been determined by the time the sound energy reaches the fins. Being a radial horn with two straight sides, the horizontal pattern would be largely that of a conical horn with its excellent directivity.