Re-porting 828 enclosures

[GM]

>GM,

I got your response while working my way through the T/S calculations in Altec Technical Letter #245. What you have to say makes far more sense to me!

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Do you have #264A for non-Thiele alignments also?

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>It's good to hear someone say that the nomograms are not the last word. On the other hand, I've had a pair of 1202B clones for 34 years that were ported by using the 1968 Altec graph and I now have no idea if I'm even in the ballpark. Obviously, more experimentation is indicated. That means many more hours of listening - darn the luck ;-)

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Yeah, ain't life a b$#@%. The nomograms were 'close enough' back when records/radio rolled off below 50Hz and speakers were all driven with high output resistance tube amps, but it's a whole different audio environment with the wider BW material and vanishingly low output resistance amps of today.

Another thing that has a big effect on it is when the cab has a high aspect ratio, or in the case of the A7, an effective one due to it's stepped height dim, making it a horn of sorts, which loads the vent more than a golden or acoustic ratio reflex. T/S doesn't account for this, and until MJK developed his Mathcad worksheets, the DIYer could only derive its impact on vent tuning through much experimentation (been there, done that!), so if you're into speaker building of non-T/S designs then I highly recommend you get an older copy of Mathcad and get comfortable using his worksheets: http://www.quarter-wave.com/

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>Even before reading your post, I had decided to cut a new port board that would allow the attachment of movable "slats" to allow easy port adjustment.

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Just make sure it seals well, or the tests will be meaningless.

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>In my largish room, an initial port of about 100 sq inches sounded best to me. Again, more darn listening tests are in order!

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A lot depends on how close to a wall and/or corner they are. If well away from them, then the higher tuning is best, though a sub is needed.

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>Finally, re 825/828. I'm pretty sure mine are 828's. They are made of MDF, have 2 X 4 bracing in a ladder pattern on the one-piece back panel, and bear stamped serial numbers and dates of manufacture that are 7/89 and 8/89 respectively. However, the areas behind the horn curves ARE NOT sealed.

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Interesting! It's my understanding that this late a date build should be early 828Hs, which has the rear bracing on the inside only, sealed off flares, and made with 3/4" MDF. Also, it's only drilled to take the later 16" frames, so if the early 416s bolt in, it's either an earlier version or been drilled by a previous owner.

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>One of the big reasons I had for going through the T/S alignment calculations was to determine just how much volume is "ideal" for the 416-8A's I have. If it was large, I though it might be better to leave the horns open (perhaps fiberglassing the inside). If the "ideal" volume was smaller than 8.7 cu ft, I'd probably seal off those areas and fill them with urethane expanding foam.

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OK.

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>Now, I haven't had a chance to check my math -and given that I'm doing it at work the possibility of error is high! - My initial calculation is that the ideal volume for a 416 woofer should be about 6.8 cu ft. If this is correct, there seems to be plenty of room in an 828 to close off the horn areas.

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Well, if you mean 'ideal' as in a T/S max flat, then using the measured specs I have, a 416-8A needs 6.67ft^3/37.5Hz, -8B = 12.21ft^3/31.7Hz, -8C = 5.01ft^3/37.7Hz.

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>I haven't yet found a definitive figure for the volume of the 828 or the T/S parameters of the 416-8A (I'm using the published numbers for the 8B). If anyone knows those, please send me in the correct direction!

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Altec listed the 825 at 12.66ft^3, and ages ago I came up with 11.43ft^3 loaded with a 416-8A and the horn flares sealed off. I haven't calc'd the 828, just assumed it would be 'close enough' to my number, but Roland (who used to post here regularly, but haven't seen around for awhile) concluded that his were ~9.4ft^3 with sealed off flares and no driver, so I'm not sure what gives.

Here's an averaged set of measured specs for two 416-8A:

23.9 Hz Fs

29.67ft^3 Vas

3.55 Qms

0.27 Qes

6.58 ohms Re

12.9" eff. dia.

0.14" Xmax

1.26 mH Le

98.13 dB/W/m eff.

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>Again GM, I appreciate you input - and I'm with you. Let your ears decide!

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You're welcome! Lets us know what you all you did to them and your thoughts on the various tunings.

GM

[Cal Weldon]

Hello everyone,

I also reduced my port to a slot of 100 inches and heavily damped the cabinet with batt. The inside of the horns are open but are stuffed with batt as well.

I am using this woofer:
15" woofer

It sounds just great but it's hard to brace the cabinet enough. She tends to howl a bit when the volume goes up. These are pretty decent drivers and I'm running them on a Hafler 500. I can't really hear the howl too bad 'cause I've got the 511 cowbells on top and that helps to drown it out. I'm also running two A4's along with it to boost the bass.

Now, I am a little confused when people were mentioning that the radiating area of the driver was reduced due to it's placement in the horn. The radiating area doesn't change. The amount of air being pushed through the horn doesn't change so this is the source of my confusion.

I would think that because the driver is loaded in the horn and the the coupling to the air is better, that one should consider that the driver has been made "bigger" not "smaller".

I am wondering if that's what the Altec engineers were thinking when they designed what we are thinking is an oversized port?

One thing I love about this forum in particular is that the older ***** feel welcome and the design teams still use the best testing tools to be found: The Ears!

Please go easy on me about my choice of non-Altec drivers. These are an outdoor speakers that have to be stored outside also. Hence the QCF cone.

Cal

[taudo]

Good to hear another vote for the 100 sq in port! So far, my listening tests have born out this choice.

What is _really_ interesting to me is how bad my old 1202B clones sound when directly compared to my re-ported but largely stock 828's! Both low end and mid-bass are seriously diminished in comparison. This might have something to do with the guys who designed the 828's actually knowing what they were doing (as opposed to your humble narrator - who just worked from their 1968 pamphlet!) Clearly, I'm going to have to work on the SAF of the 828 cabs so I can bring them up from the basement (not to worry, its nice and dry!)

Seriously, though, I'd be quite appreciative of any thoughts on how to _approach_ tweaking my old cabs to bring them nearer to the performance of the 828's. I don't have access to real-time analyzers and such - except those attached to my head.

The old cabs are about 10 cu ft in volume - so I'd think it should be possible to improve them somewhat. After all, they actually sound pretty good when placed next to anything else but the 828's.

Thoughts on approach appreciated!

Taudo

[Fitts]

Hi All,

Interesting comment by Cal on why a 15 inch woofer with typically 133 square inches of radiating area would necessarily be reduced square inch for square inch of radiating area just because it was put behind a smaller opening. Perhaps the area of 100 square inches represents the effective reduction in radiating area when placing a 133 square inch driver behind a 75 square inch opening.

Bottom line, time for me to go make some new panels for the A7 and try a 100 square inch port.

Cal, your choice of drivers seems to check out fairly well for the volume of the A7 or 828 cabinet. If anything, the cabinet is probably a bit larger than your drivers would like so I say brace away and use up some of that internal volume. Also, I don't know what kind of batting you are using but if it is fiberglas, try switching to polyfill. From what I have read, Fiberglas will make the cabinet seem larger than it really is to the speaker. Polyfill, on the other hand, does not. (and it is a lot less itchy, too)

Regards,

[Fitts]

Hi All,

Interesting comment by Cal on why a 15 inch woofer with typically 133 square inches of radiating area would necessarily be reduced square inch for square inch of radiating area just because it was put behind a smaller opening. Perhaps the area of 100 square inches represents the effective reduction in radiating area when placing a 133 square inch driver behind a 75 square inch opening.

Bottom line, time for me to go make some new panels for the A7 and try a 100 square inch port.

Cal, your choice of drivers seems to check out fairly well for the volume of the A7 or 828 cabinet. If anything, the cabinet is probably a bit larger than your drivers would like so I say brace away and use up some of that internal volume. Also, I don't know what kind of batting you are using but if it is fiberglas, try switching to polyfill. From what I have read, Fiberglas will make the cabinet seem larger than it really is to the speaker. Polyfill, on the other hand, does not. (and it is a lot less itchy, too)

Regards,

[Fitts]

Sorry about the double post, klutzy fingers I guess.

[taudo]

Dave,

FWIW, on my 828's the woofer cutout is larger than 75 sq in - probably pretty close to 100, in fact. After reading your post (and, coincidentally, a previous one in which you gave the ideal port = woofer surface area theory) I grabbed a tape and found that the "rectangular" area of my woofer cutouts is 8" X 11". I haven't had time to estimate what the area of the round remaining areas is - but I'll bet it's close to the remaining 12 sq in. So, it looks like the theory is working pretty well.

BTW, after hearing how much better the 828's sounded - my wife told me to go get another set! I suspect this is a first in this Board's history! Obviously, if you pick the spouse correctly, 828's have sufficient SAF. That's my girl!!

taudo

[taudo]

GM,

Whoops - you caught me in a "brainographical error". The correct dates are in '79. They sure sound good for being so old!

taudo

[GM]

>I would think that because the driver is loaded in the horn and the the coupling to the air is better, that one should consider that the driver has been made "bigger" not "smaller".

I am wondering if that's what the Altec engineers were thinking when they designed what we are thinking is an oversized port?

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The vent's BW is well below the horn's, so the WLs are long compared to the horn's dims, ergo the horn has no effect on the vent's portion of the BW beyond acting as a large resonant cavity, unless of course you tune it up around 100Hz to extend the horn's BW a bit.

WRT tuning, nothing's changed over the decades, they just picked a compromise between extension Vs gain with whatever driver/amp combo they were using for the basic design. It was left up to the installation contractors to fine tune them for the app. and the vent area wound up being whatever worked.

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>FWIW, on my 828's the woofer cutout is larger than 75 sq in - probably pretty close to 100, in fact. After reading your post (and, coincidentally, a previous one in which you gave the ideal port = woofer surface area theory) I grabbed a tape and found that the "rectangular" area of my woofer cutouts is 8" X 11". I haven't had time to estimate what the area of the round remaining areas is - but I'll bet it's close to the remaining 12 sq in. So, it looks like the theory is working pretty well.

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The throat area isn't the driver cutout, but the rectangular area, or 8"*16" = 128"^2. Anyway, per the above, it has no bearing on vent area. Indeed, Sd has no bearing on vent calcs at all, it's just a frame of reference Altec chose to use as a very simplistic way for DIYers to calculate a ballpark area for other than an 'optimum' large cab where it would take a vent area = Sd to tune it to ~Fs.

If you look at their nomogram in the '68 booklet, an 825 with a 25Hz Fs driver should be sealed, and for an 828 with sealed off flares, ~50"^2. An earlier chart is just plain wrong. It uses 133"^2 for a 15" and graphs a ~53ft^3 cab as optimum where the vent area = Sd. Using the simple formula listed to reduce it for 9ft^3: (9/53)^2*133 = ~3.835"^2, which will tune it somewhat lower, and if energized will audibly 'whistle'. Might as well just leave it sealed.

GM

[GM]

>GM,

Oh, this is all so much fun - especially so when in the virtual company of others who enjoy the same subject and have alot more experience!

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Agreed, except for a few brief encounters with some real pros back in the '60s, I didn't really know anyone who had a clue, or cared, until I found the old basslist when I got on the net.

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>Thanks a million for those measured T/S parameters! I really hadn't wanted to buy a frequency generator, etc.

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You're welcome! I actually got them from someone here IIRC, but I don't keep whole messages and don't recall who it was.

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>Even more interesting, the 8/79 cabinet was drilled for the 416 and the 7/79 cabinet was drilled for the 16" frame. No owner mods visible. Also, the wood used on the horn flares is different on the two

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Hmm, you listed '89 before. '79 would be the original 828. No idea why they are built different with only a one month different build date.

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>I find this the most interesting of all. I'd think that this volume would be known with certainty, but it's clearly not.

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One would think so, but I've never seen it published.

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>When I used a tape measure to break up the interior volume into several parts, I came up with figures almost identical to Russell's. I'm just about tempted to bring home about 300 pounds of rice and see if I can answer this one definitively!

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Just don't get it wet.

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>I also wonder about the volumes given fot the 811B and driver. One Altec source I have says the volume behind the flange is 0.19 cu ft and another source says the volume with driver attached is 1.2 cu ft - almost 10 times as much! I'll have to give some thought to refining these measurements as well.

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Yeah, just looking at a 511/811 in the flesh you know 1.2/1.5ft^3 is so far off as to be obviously wrong by a wide margin.

GM