Whilst I'm developing a smaller Altec bass horn for my own system, I'm using part of that experience to do a CAD/CNC Altec VOTT A7 cabinet.
Attached are the initial drawing from my engineer CAD mate.
This will be built in Euro Birch Ply, 19mm.
A significant change over the original is the central baffle which is rebated into the sides, top and bottom of the cabinet.
The horn sections will be bent by CNC'ing the multiple grooves into the back of the ply. This creates an easy bend. CNC'd brace supports are used as the template for the 31 inch radius curve.
The back 2/3 of the cabinet will be fixed, while the top 1/3 will be removable.
As a decorative feature, we'll create an oval shape port at the front, which will sit in a removable panel, per the original design. This way we can add different size ports to suit the tuning of the cabinet. The initial sizing is smaller than the original A7…and the current pic shows the oval cut out too large. We’ll target a size of approx 30cm x 7cm (12 inches x 2.75 inches).
Altec 515's will be used in this version.
I suspect the cabinet will be heavy, with as little cabinet resonance as possible.
Andrew
Attached are the initial drawing from my engineer CAD mate.
This will be built in Euro Birch Ply, 19mm.
A significant change over the original is the central baffle which is rebated into the sides, top and bottom of the cabinet.
The horn sections will be bent by CNC'ing the multiple grooves into the back of the ply. This creates an easy bend. CNC'd brace supports are used as the template for the 31 inch radius curve.
The back 2/3 of the cabinet will be fixed, while the top 1/3 will be removable.
As a decorative feature, we'll create an oval shape port at the front, which will sit in a removable panel, per the original design. This way we can add different size ports to suit the tuning of the cabinet. The initial sizing is smaller than the original A7…and the current pic shows the oval cut out too large. We’ll target a size of approx 30cm x 7cm (12 inches x 2.75 inches).
Altec 515's will be used in this version.
I suspect the cabinet will be heavy, with as little cabinet resonance as possible.
Andrew
Interesting approach.
The holes through the braces might not act exactly like the open space in the original A7. I suspect they may behave like a "bandpass" type enclosure, in essence two volumes + the final port.
Also the holes that access the rear side of the horn flares likely ought to be sealed up as they too will act like some sort of tuned volume/space.
Fwiw, I converted an A7 for a client to 4 x 4" dia PVC ports (or was it 5 ports?). This improves the performance at LF and reduces substantially the rear lower mid radiation through the large open port of the original A7, and permits precise tuning.
I used a gypsum pour as a solid/stiff backer for the horn flares. The amount of energy impinging upon them should not be underestimated. The original flares, the way they were built, resonated badly - both from the front energy and the stuff inside the box going to the rear side.
The stiffer the mount/throat for the driver, the better the results too...
The holes through the braces might not act exactly like the open space in the original A7. I suspect they may behave like a "bandpass" type enclosure, in essence two volumes + the final port.
Also the holes that access the rear side of the horn flares likely ought to be sealed up as they too will act like some sort of tuned volume/space.
Fwiw, I converted an A7 for a client to 4 x 4" dia PVC ports (or was it 5 ports?). This improves the performance at LF and reduces substantially the rear lower mid radiation through the large open port of the original A7, and permits precise tuning.
I used a gypsum pour as a solid/stiff backer for the horn flares. The amount of energy impinging upon them should not be underestimated. The original flares, the way they were built, resonated badly - both from the front energy and the stuff inside the box going to the rear side.
The stiffer the mount/throat for the driver, the better the results too...
Appreciate the insights.
I chose to open up the smaller chamber either side of the horn flare to maintain the overall cabinet volume (as I've added quite a lot more bracing). I can always close these off later.
With bracing, I guess there'll always be a potential trade-off with bandpass odd tuning, vs lower/better cabinet resonances. I'm purposely choosing the latter to focus on.
Given the stiffness of the cabinet, would do you recommend for deading? I use a 2.5inch thick wool/fibre house insulation product on my other projects...seems to work very well.
The attached pics show Tannoy GRF-R, Altec '620,' and my current large bass bins for EV 18's...all with the same wool fibre.
Attachments
Everyone has their favorite methods and materials for internal wall damping.
For the A7 there is, imo, some yin/yang in the choice. On one hand absorbing the midbass and midrange reflections is a really good idea. The short rear wall distance behind the driver makes this important if you want to hear hi-fi. The problem, if there is one, is that too much "stuff" back there could reduce the effective air volume at LF... the density of that stuff vs. thickness of the stuff and how it behaves at LF will have *an* effect.
Think about making that front port removable - although I think you just glued it in place. Difficult to change or modify after it is glued in place.
For the A7 there is, imo, some yin/yang in the choice. On one hand absorbing the midbass and midrange reflections is a really good idea. The short rear wall distance behind the driver makes this important if you want to hear hi-fi. The problem, if there is one, is that too much "stuff" back there could reduce the effective air volume at LF... the density of that stuff vs. thickness of the stuff and how it behaves at LF will have *an* effect.
Think about making that front port removable - although I think you just glued it in place. Difficult to change or modify after it is glued in place.
Thanks...I'll go steady with it.
On the front port...actually I have three options available.
1. the existing open port (full size, as per the photo now).
2. a panel inserted with the oval cut out (the most likely option...and the best looking).
3. a panel inserted to fully block off the port.
Panels for options two and three have already been CNC'd, and fit nice and snug against the back supports.
Please refer to the drawings in the first post of this thread.
Regards.
Andrew
Correct, the original acts like a crude BLH without its vent baffle boards, ergo a ML-horn with them.
The ‘cheese grater’ bracing will add a resistive component, damping it, though no clue how much it will affect the driver, vent LF output, so recommend doing high resolution measurements before adding any damping experiments.
Right, the flare cavities create band stop filters that adds a deep notch in the response and why I always recommend ‘killing two birds with one stone’ by filling them with something massive such as kitty litter/oil dry, sterile sand or similar to mass load/brace the horn flares. Some folks tried expanding foam with mixed results plus one person all but blew the flares out from over filling/expansion, so Caveat Emptor. The three vent holes/side just lowers their ~identical tuning same as a reflex alignment: http://users.cms.caltech.edu/~ps/All.pdf
FWIW, four 4” vents had become a recommended conversion when I came on-line in ’96, though at this point I don’t remember the tuning [length] or who was championing it, though remember seeing it on the Thomas Dunker horn site, so maybe it was him.
Agreed, drivers should be mass loaded to the cab and it in turn should be mass loaded to the floor for max electro-mechanical-acoustical efficiency. Early on, I had a mechanical engineer poo-poo that this made any audible difference and to prove me wrong he put a block wedge between his little ~5” ‘FR’ driver and cab rear wall and stacked a tall pile of old vinyl albums on top of the cab and couldn't believe how much it ‘tightened up’ its response.
Of course, 825 cabs with properly filled horn cavities are already heavy enough to not need any additional floor coupling mass, but the bottom line for me is that if I can’t perch a ~square edge coin on end and it not obviously ‘walk’/fall over when the speaker is played at high SPL down low, then it’s not braced/damped well enough for any sort of high SQ app.
GM
As I implied with the coin test, the object is to move the cab’s Fs well outside its intended pass-band and since it’s easier, cheaper to raise a bass cab’s Fs with high MOE panels and bracing than to lower it far enough below it with stone, concrete, sand filled walls or similar, the choice is clear for me plus weighs at least an order of magnitude less.
Internal damping then is reduced to any reflections back through any driver, vent diaphragms. Critically damping the vent using the impulse response [‘click’] test takes care of it and to some extent any other drivers, so should ideally be done before adding any other damping other than at the closed end to quell any obvious LF eigenmode harmonics.
Ideally, the goal is to damp reflections back through any drivers down to > -25 dB, so adding pads to one side, behind them is usually the first step. Some folks find that taking a ‘shotgun’ approach by loosely covering the rear of drivers with an empirically arrived at damping factor [DF] acoustical ‘blanket’ is the best way overall, while others will diagonally hang an acoustical ‘curtain’ behind drivers to do similar.
FWIW, your material preference ‘sounds’ like extreme overkill for my ‘tastes’ except for ~aperiodically damping an alignment, but to each his own. I didn’t/don’t have any measuring equipment sufficient to do any but the crudest of measurements, so doubt any of my builds would meet the > -25 dB ideal whereas yours might. Hopefully you’ll do full testing to educate us enough to get a better ‘handle’ on all this as historically it’s all been very subjective.
GM
The other thing worth mentioning in general about the A7 cabinets is that the horn provides a rising response in the lower mids, starting just below 200Hz. This is fine for PA work, as the goal of the design was for speech intelligibility and also (I presume) to be able to meet the compression driver/horn above with a minimum of attenuation on that HF combo.
So, if ur goal is a "flat" hi-fi response you can't get it.
I used a brickwall filter and met the HF horn at ~200Hz. Obviously not the stock Altec horn, but a 203 or 1003 will do that job ok. With the right compression driver behind it.
The main advantage then to the A7 box & driver became relative efficiency AND the setback of 15" driver so it is about where the acoustic center of the horn assembly is.
Even so, the current owner of said modified A7 ended up doing a passive biamp with a high DF solid state amp on the bottom and a variety of DHT triodes and similar stuff on the HF side. It sounds very good.
Truthfully, after all that is said and actually done, we asked ourselves if it was worth the effort to modify (in your case, build) the A7 or would it have been better to go with some other sort of enclosure for the bottom end - given how we solved the issues, and that being in large measure the xover frequency to get "rid" of the rise due to the horn rise in the mids...
Heresy no doubt.
So, if ur goal is a "flat" hi-fi response you can't get it.
I used a brickwall filter and met the HF horn at ~200Hz. Obviously not the stock Altec horn, but a 203 or 1003 will do that job ok. With the right compression driver behind it.
The main advantage then to the A7 box & driver became relative efficiency AND the setback of 15" driver so it is about where the acoustic center of the horn assembly is.
Even so, the current owner of said modified A7 ended up doing a passive biamp with a high DF solid state amp on the bottom and a variety of DHT triodes and similar stuff on the HF side. It sounds very good.
Truthfully, after all that is said and actually done, we asked ourselves if it was worth the effort to modify (in your case, build) the A7 or would it have been better to go with some other sort of enclosure for the bottom end - given how we solved the issues, and that being in large measure the xover frequency to get "rid" of the rise due to the horn rise in the mids...
Heresy no doubt.
2nd order would work to some extent - you'd have to measure to get it to work properly. To extend the response properly you'll need to look at what is going on after the roll up is corrected, assuming you want the response extended... the other half of the idea is that one *might* maybe EQ the bottom end up... but that limits the headroom and requires a lot more power, and maybe a different driver in that case...
My present opinion is that there is not so much "magic" in the stock A7 arrangement for hi-fi/home use. Sorry to dissappoint, if that is what saying this does.
_-_-
My present opinion is that there is not so much "magic" in the stock A7 arrangement for hi-fi/home use. Sorry to dissappoint, if that is what saying this does.
_-_-
Not to me! I generally prefer Altec over similar components, so an advocate of sorts, though stop well short of ‘bleeding’ Hammertone Green, but when I got on-line, I was pretty adamant that these old horn systems were now museum pieces, having been designed for electronics, apps that were irrelevant in today’s high power, digitally EQ’d, THX ‘world’.
By the time an 825/828 is digitally EQ’d to match a period correct set-up’s frequency response, it’s efficiency is only ~93 dB and its F3 is around 60 Hz and ~12-18 kHz depending on the driver/horn combo, so why bother unless one has the appropriate electronics to properly match up to it.
More expensive, but in the ~14 ft^3 net of an 825/828’s ‘shell’, I much prefer a pair of 515Bs, though a pair of GPA 416C would probably be fine too and maybe have a bit more bottom end depending on whether or not EQ is used.
GM