Building sleds for Altec multicell horns

[Elitopus1]

I went through my plywood scraps today and found two 28"x16" pieces. I used some L brackets I had, and mounted the horns. I'll paint them eventually.

I'll use these sleds until I find the wood and hardware to build a more original pair.

I am trying to decide which of the bracket holes I should use. There is about an inch difference between them. One setting would put the horns about 3" from the top of the speaker, and the other about 4" up.

I also have to figure out how far back the horns should go. Most pics I see have the front of the horn hanging over the front of the speaker cab. I'll start out with them like that and then do some listening.

[GM]

I have heard about a terminus in reference to a pipe. I never did understand what exactly it was though.

How can I determine the proper distance from the bottom of the 805 to the top of the 828?

I figured there was a reason Altec raised the multicells up so much. I like the idea of having the horns low, like Phil's are. With the parts I have right now, the horns will be about 2.5-3" above the 828 cabs. Do I need to raise this more?

My simple minded way of understanding it is that the end of a pipe/horn has a very thin 'film-like diaphragm' attached to it that loosely holds in the air mass 'slug' that vibrates within it and since it weighs much more, it bulges out the end at ~ its radius*0.613 [end correction] assuming it's not laying on a surface.

With a horn it gets a little more complicated since one has to factor in the flare frequency to find its effective perimeter, i.e. can't just use the horn mouth's perimeter.

When the horn sets on a surface, then there's a different end correction and if in a baffle, yet another, but don't recall these. Mostly it affects its polar response.

In general though, it's normally measured in a polar response plot or just use Altec's spacing. The sectoral horns OTOH terminate such that it can only be measured AFAIK, though again AFAIK, the wood 'L' mounting board plus spacer is the correct gap: http://www.lansingheritage.org/images/altec/plans/1960's-lf-design/page14.jpg

Altec A7 with cinema horn spacing.jpg

GM

















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I also have to figure out how far back the horns should go.

Historically, vertically aligning the VCs was sufficient: http://www.lansingheritage.org/image...ign/page16.jpg

Better though to use a test signal to do it. There's an instruction doc, but was surprised to not find it on any of the Forum's various 'libraries'.

GM

[mah]

FWIW: http://www2.physics.uiowa.edu/~fskif.../Impedance.pdf

[Elitopus1]

That makes sense. Thanks GM

[Elitopus1]

I have my eye out for some old rod end turnbuckles for the rear height adjustment. I need something beefy to handle the weight of the 288H drivers.

I'm looking for some old aircraft parts at work that are past their service life, or if I can find some new ones online. They need to be about 6" when totally collapsed.

[GM]

Yeah, nice to have, but me and most folks around here just used el cheapo ones at the local hardware emporium and drilled a hole in a scrap piece of 2x4 to wedge the bottom loop into; industrial 'look' for an industrial looking device : Galvanized Turnbuckle - Eye & Eye - 1/4" x 4

GM

[Elitopus1]

I found these turnbuckles with 1/4" rod ends. About 5"total length collapsed and 7" usable range. They are rated to support the 288 weight.

My next question is about how the throats mount to thd horn. The horn side is tapped, and the screws go in from the throat side. The throat is slightly cocked on the horn. The screws wil really only line up this way and there is no wiggle room.

Are they supposed to be slightly crooked? I can't see down into the throat to see if it is not aligned on the inside. Any way to fix this without enlarging the holes?

IMG_9527.jpg

Don't know why the pic is sideways. I need to find a new pic hosting site

[endeeinn]

FWIW, horns, pipes, etc., have a terminus [mouth] end correction, so ideally the horn spacing to the cab/whatever should take this into consideration to maintain its polar response, hence Altec's rather high spacing on its bigger horns.

GM

I have 311-90s w/288Gs on 825s and have positioned the 288 voice coils back from the front of the cab 19.3 inches (or what ever that measurement is) to line up with the voice coils of the 416s. The 311's are attached to a "T" sled that has the bottom of the horn only 1" above the 825. With this setup the 311s overhang the front of the 825 with the end of the horn on both sides about 1" back from the front of the cab. With this overhang should I still raise them up? I have a pair of Altec horn mounting brackets with and adjustable rear support that I could use if you think it would be beneficial. Thanks.

[GM]

I found these turnbuckles with 1/4" rod ends. About 5"total length collapsed and 7" usable range. They are rated to support the 288 weight.

Are they supposed to be slightly crooked?

Cool! Where? How much?

Is it a 30162? I presume the other one is strictly for use as a stand. Regardless, never seen/heard of this problem with the right part[s], but the goal is an ideally perfect throat match, otherwise its HF response will suffer: altec 30162 adapter - Google Search

TIA,

GM

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With this overhang should I still raise them up?

In theory, yes, but minimizing driver vertical c-t-c spacing is more important in this case, so if the entire mouth hangs off the end, then recommend dropping it flush with the 825 horn's internal top if it can be dropped this far and still have enough room to put a rubber or similar isolation/damping cushion between the two to rest on; otherwise just sit the horn mouth on a sufficiently large cushion to also damp early reflections.

In this scenario, the sled will be either notched or narrow enough to fit between the mounting tabs, so needs to be thick enough for screws going into the end grain without splitting and in all cases ideally screwed down to the 825's horn top to stiffen it for HIFI/HT apps. That, or mass load it like Jeff/lowohms advocates.

GM

[endeeinn]

In theory, yes, but minimizing driver vertical c-t-c spacing is more important in this case, so if the entire mouth hangs off the end, then recommend dropping it flush with the 825 horn's internal top if it can be dropped this far and still have enough room to put a rubber or similar isolation/damping cushion between the two to rest on; otherwise just sit the horn mouth on a sufficiently large cushion to also damp early reflections.

In this scenario, the sled will be either notched or narrow enough to fit between the mounting tabs, so needs to be thick enough for screws going into the end grain without splitting and in all cases ideally screwed down to the 825's horn top to stiffen it for HIFI/HT apps. That, or mass load it like Jeff/lowohms advocates.

GM

Thanks GM!