Getting more bass out of 825-828 cabs

[Roland N R]

Can 825-828 cabs be made to produce reasonably efficient below 30s Hz output without the need for separate troublesome subs?

Bass output traditionally has been boosted with the use of the wings which although awkward was effective. Unfortunately it does not lower the output to the 20s Hz so other methods have to be designed.

The internal volume of these cabinets with the horn flares open are overly large for the 515/416 driver. Perhaps a sub driver can be added where the vent is located while at the same time keeping the efficient and highly resolving horn and driver. The 515/416 needs close to 10 cubic feet. The subdriver will need from 2 to 10 cubic feet depending on size etc. Fiberglass stuffing of separate sub space will increase apparent volume only 20-25%.

It becomes a matter of knowing the actual internal volume of the different areas of these cabs. The tough volume are the horn flares? Does anybody have these numbers handy? Suggestions appeeciated.

[Roland N R]

Obviously a separate sub driver using the vented lower part of the 825-828 cab will never match the efficiency of the horn. But the idea would be to biamp with big SS amps and use the brute force approach which is OK for sub use anyway.

As GM said it becomes a matter of using the 515/416 only for the horn output close to 180 Hz and adjusting whatever rear space it needs to allow that. Then using the remaining volume for the sub as tuned port but driver needs to reach XO point of approx 180Hz. If horn flares are needed and could be used the better. Granted much reinforcement will be needed not only of the flare but entire cab...but this needed to be done already. I would add 3/4" fancy plywood all around.

The sub driver should have Fs somewhere in the high teens or 20s Hz and be able to reach it in a tuned cab of the space we have available + a fiberglass boost = somewhere about 9+ cubic feet. Additionally needs to reach 180 Hz with the most efficiency possible. Apparently GM has done something similar using the Altec 411 driver as the sub in the past. Maybe he can educate us further as to the details. I wonder what other drivers would also get the job done JBL 2245?, stryker AV 15? (relatively inefficient).

If this works out it would have been an EASY solution to keeping the 825-828 = A5-7 system in theatres with its intrinsically better MR clarity and projection due to its MR horn.

[GM]

>Obviously a separate sub driver using the vented lower part of the 825-828 cab will never match the efficiency of the horn. But the idea would be to biamp with big SS amps and use the brute force approach which is OK for sub use anyway.
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I thought this was implicit in my response, but it appears not to be the case.
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>. If horn flares are needed and could be used the better. Granted much reinforcement will be needed not only of the flare but entire cab...but this needed to be done already. I would add 3/4" fancy plywood all around.
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One more time, these need to be *****d, not just to add mass/stiffness to the horn, but more importantly to remove the cavity resonances.
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>Maybe he can educate us further as to the details.
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What details? It was nothing more than blocking the area off, bracing it, adding the driver and using sinewave sweeps to tune it to best blend in the room, critically damping it using Olson's 'click tester'.
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> I wonder what other drivers would also get the job done JBL 2245?, stryker AV 15? (relatively inefficient).
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I already gave my thoughts on the IB forum.
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>If this works out it would have been an EASY solution to keeping the 825-828 = A5-7 system in theatres with its intrinsically better MR clarity and projection due to its MR horn.
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Well, I can't comment about other areas of the country, but here in Hotlanta, Altec's attitude/marketing philosophy in the '70s/'80s/90's begged them to be removed from install bids or the expanding portable prosound market, and for the most part they got their wish. JBL, and Bose in particular, OTOH were interested in doing business here, offering what the buyers wanted, good value for the $$ with good service/tech support. The decades long Altec distributor finally gave up on them and switched to Bose, offering them as replacements when they got requests for Altec reconing, etc..

I imagine the new Altec is going to have a 'tough row to hoe' to overcome their previous incarnation's bad rep no matter how well they may perform.

GM

[Roland N R]

Looks like either Altec 411 or JBL 2245H will get the job done. Already have 2 JBL 2245H so like cars the best one is the one that is paid off and functioning. The JBLs were the designed for studio bass with above average transient response highest XO point recc 800 HZ, 600watts RMS 4" voice coil, Fs 20 Hz, 95dB/W at 100-500 Hz. In a box 8-10 cubic foot tuned to 26-28Hz will have flat response to 30Hz while also giving excellent transient response needed for XO point of 180 Hz. I imagine the Altec will perform similarly or even better especially transients. For the ports we need to calculate in detail but so far it looks like two 3 inch diameter ports depth to be adjusted. We still need to work on the back space for the 515/416 . Ideally should use the 515G for the ultimate MR performance. Further info to follow.

[Albert Allen]

Roland, I don't have the numbers but the volume for the flairs should be 1/2 the volume of a cylinder that has the same radius as the flair and the same height. I will have to dig out my 828-C plans which are "D" size right from Altec (they were feeling generous when I asked for them) I should be able to approximate what the diameter of the cylinder should be even if the flair is not a perfect arc. I will try and get it tonight or tomorrow.

[GM]

I guess you missed my post about sealing up the rear of the horn section and installing a 411-8A in the bottom half of the cab. Like I said on the IB forum though, the big problem if you're trying to match the horn's efficiency is finding a driver that can go high enough and still do the bottom, and told you the only one I know of that in theory can do it.

WRT the 416/515s VB requirements, for just driving the horn, the Vb behind it should be sufficient, but if need be can be tuned using an Aperiodic vent. This leaves you with at least 7.6ft^3 gross to play with.

GM

[GM]

Hmm, I see I overlooked the part about the volume behind the flares. Frankly, they should be *****d and not factored into the net Vb calc.

GM

[Roland N R]

To convert these cabs to subwoof plus separate horn you need to close off the connection between the horn and vented space and also close the vented opening in the front of the cab. Easy to do with just some 3/4" MDF. Actually this modification can be totally reversible since no actual cutting needs to be done to the cabinet itself. The port openings can be placed alongside the 18" driver with all openings made in the new board covering the front vent.

SUBWOOF

The lower vented space of 7+ cubic feet volume should be moderately stuffed with fiberglass with the paper backing off and keeping it away from the rear of the ports. This should bring it to about 9 cubic feet of total apparent volume.

The JBL 2245H has been extensively used as a subwoof in an 8-10ft^3 cab. The opinion varies wether to tune the cab to 20Hz via two 3" diameter 7" long ports or tuning it to 28Hz with same diameter ports but only 2" deep. The lower 20Hz cab tuning offers some degree of protection from the danger of overdriving the woofer below the cab resonance. But then there is less power tolerance and slightly decreased transient response. The 28Hz tuning gives more power tolerance and better transients. But is easy enough for one to adjust the depth of the ports to fit the acoustics of the room and preferances. You can also use a six inch diameter 14" long for 20 Hz or six inch diameter 4" inches long for the 28 Hz tuning. Only advantage would be less port breathing noise. JBL recommends a narrow 6db boost around 26Hz
(Q of 2) to make this cab fairly flat into the 20s Hz. Remember room acoustics also tends to boost at less than 50Hz


MR Horn

The 515G or 515B with their stronger magnets and better MR control can be used to drive the MR horn with better results than the 416. Interestingly in all literature the MR horn is rated to 125Hz yet the measured output rarely goes below 150-175Hz. What is the cause of this DISCREPANCY?? The NEW rear space of about 1.8 cubic feet is more than enough to allow the 515 to reach 125-180Hz. But would it be of additional benefit to make this space even smaller to mechanically bandwidth limit the 515 to less?..say no lower than 100-125Hz?? Obvious expected advantages are ease in XO design, improved MR clarity and increased efficiency. My freq response calc is out would appreciate somebody checking out smallest needed box volume for 100Hz response in 515-8G. The available rear space of 1.8 cubic feet measured grossly already has the driver volume subtracted.

T/S
Fs=37Hz
VAS=12.40
qts=.27
qms=5
qes=.28
vd=22.4 in^3
Xmax=.17
Re=6.20

Iconic is making an improved version of the Alted 411..will need to check that out also. It should give outstanding results.

Well at least on paper this looks like a winner and a piece of cake as far as construction is concerned.

The purpose of this exercise was to see how much bass can be obtained out of the 825-828 without a separate sub cab. Additionally comparison can be made to the other half of the experiment via Albert's version which increases bass only via
porting without additional sub driver. (his version on this page also)

Will compare versions once completed

[stpower]

Roland,
Please keep us posted on your results with this project.I planned a project of this type with the 2245, but never got around to it. I tried using 4 411 woofers in separate sub enclosures once but was not satisfied with the subjective blending. The bass in the upper range sounded more muddy with the subs and my A-7's at the time. If the 2245 blends well you will have a real winner here.

stpower

[Albert Allen]

There is a minus to the added sub driver in my 828 cabinets anyway..my cabinets with all the added material to make them inert has netted me a 250 pound (each) cabinet and I can't hardly move them now let alone with another 15" driver in them. I practically throw my back out if I only want to adjust the tow-in....oh to have a good, young and strong back!