811 Fins

[bfish]

...Just what function do the fins serve on the 811 (and 511) horn?

Perhaps they were intended to stiffen the structure...

I think there might be other reasons: styling and product differentiation... 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.

I guess Western Electric must have never gotten that memo.

"The horn is sectoral in form and integrally cast partitions are provided to reduce reflection effects within the sound passage."




courtesy HiFilit.com

I don't know the production dates of the pictured KS series, but W.E. did have fins in sectoral horns that pre-dated the 811.

[Steve Schell]

Scathingly brilliant, bfish! I had forgotten all about the W.E. horns. Altec had received the rights to manufacture them in 1949 along with the speaker and microphone designs. Altec built them through most of the 1950s, some continuing until much later.

[bfish]

Don't mention it Steve, always glad to be of service.

So, for the record, Western Electric included "partitions" (fins) in their sectoral horn designs (at significant added manufacturing expense) for the functional purpose of "reducing reflection effects in the sound passage".

Works for me.

[Mustang Marvin]

So, I presume the sand and latex paint will work equally as well on the 511 horns. I understand the 511's ring worst than the 811's. I have not had the ringing condition on my 511's but, I would like to know of a remedy should ringing occur.
Thanks...

[GM]

I would leave it. The reason is if Altec went through the trouble to put them in, there must be a reason. It is probably there as as a method of stiffening the horn.

While there's no such thing as too 'stiff' in a HF horn, it's still stiff enough to easily damp it without them as Altec proved with rubber inserts and Aquaplas coating, so not a prime consideration WRT tweaking its performance.

GM

[GM]

Hi GM, group,

I have to confess to being a dyed-in-the-wool conical horn enthusiast.........

I have seen some data for the on-axis hole in h.f. response.........

I am not sure what the mechanism of the problematic response would be........... What is neat about conical horns IMO is the degree to which the off axis response mirrors the on axis response.

It is freaky to listen to an LC-9A while standing waaaay off axis, virtually beside the speaker, and hearing very nearly the same response as on axis.

Greets!

Me too up to a point, but I found out early on that without a proper throat/mouth termination that it too left something to be desired. In my experiments I never went as far as Dr. Geddes did though, his and others ovoid shape was more than I wanted to mess with and not sure it's an audible or even perceived improvement over the same design as the round variant I empirically arrived at. Indeed, even with the best HF hearing person I've ever known could barely perceive a difference between my round and ~1.00:1.273 aspect ratio radial variant.

Right, the 'hole in the middle' on a conic expansion is due to its HF naturally falling on axis power response being distributed over a wider (flatter) arc than expo, etc. as you measured and why the 511/811 has more HF roll-off on axis with no vanes, i.e. they're more than just stiffeners.

WRT a conic expansion's intrinsic aberrations, look to telescope design for a ~complete tutorial on optimizing them along with Dr. Geddes's various papers, especially WRT compression driver filter chamber/initial expansion design.

I agree, it was a lone LC9A used for live entertainment that filled Hotlanta's first Shakey's Pizza Parlor in '67 that got me motivated to experiment outside the WE/Altec 'box' I'd been pursuing as the 'Holy Grail' of high performance sound reproduction. It sounded enough 'different' overall WRT my DIY A7-500s that were already somewhat optimized with what I'd already learned about cab construction, horn damping, polar response, etc. that it became an obsession to figure out empirically when I couldn't find much in the way of design theory. I've often wondered why it wasn't included in Cohen's book or any of its revisions.

GM

[GM]

........W.E. did have fins in sectoral horns that pre-dated the 811.

Actually, the few I've seen were true sectoral horns, i.e. the 'vanes' extended all the way back to the throat to create a cast multi-cell whereas the later 311/511/811 appear to be a manufacturing compromise to at least get some of the earlier design's HF gain, though over a narrower arc.

GM

[GM]

So, for the record, Western Electric included "partitions" (fins) in their sectoral horn designs (at significant added manufacturing expense) for the functional purpose of "reducing reflection effects in the sound passage".

Right, something I forgot to note is that they break up the mouth's eigenmodes to much higher frequency ones to further 'scatter' whatever HF output it has and reduce the amplitude of any reflections back to the throat, so if 'de-vaned', either a foam plug in the throat ala the Geddes speakers or plenty of damping around the mouth ala Peavy's Quadratic horn design seems the thing to do for smoothest overall performance, either of which will roll off the extreme HF a bit more.

Thanks for the reminder! I'm increasingly needing them these days.

GM

[GM]

So, I presume the sand and latex paint will work equally as well on the 511 horns. I understand the 511's ring worst than the 811's. I have not had the ringing condition on my 511's but, I would like to know of a remedy should ringing occur.
Thanks...

Yes, though being a larger horn its physical Fs is lower, ergo has a wider BW and higher amplitude, so requires more damping mass, so keep layering it on until a 250-300 Hz sine-wave is < ~-24 dB (and ideally > -35 dB) referenced to whatever your lowest average listening SPL is.

Mounting a horn to a rigid/damped baffle with an area at least = to its mouth area, effectively doubling the mouth's half space radiating area, goes a long ways towards damping it. Ditto using some form of vibration absorbing mounting at the driver end same as cutting out the vane welds and isolation damping them.

GM

[Steve Schell]

GM your explanation makes a lot of sense, as I was struggling to understand how fins in the last couple of inches of a horn flare could do much of anything. Mouth reflections are certainly detrimental, as the energy traveling back down the horn throat acts both constructively and destructively at various frequencies to alter the driver loading and thus create ripples in the output.

The LC-9a is certainly a unique and charming speaker, and the owners I know wouldn't give them up for anything. Alan Sides has revived the design recently for studio monitor use. Mr. May had intended to put passive radiators ("drone cones") in those three ports in the bottom front, but his manager at RCA approved the design for production and reassigned him to other projects before he had a chance to finish his work. RCA started to fall apart soon after, so the design never got the marketing push it deserved and it became an obscurity.

It strikes me that here in the 21st century, horn and driver design is still a young science and there is still much improvement possible. As a legacy of the early theatre developments, drivers and horns are often designed separately, and their integration is haphazard at best. It is fun to try and combine the best of both old and new thinking and see what can be done in service of good listening.