604 Helper Woofer

[Altec 604 8H]

I have some 604 8Hs in some heavily braced and well sealed no-void baltic birch 612 (the old ones from the 40s and 50s) cabs I made that sound great. I'm using a Rane time align crossover but want more clarity so I'm upgrading the power supply caps and ICs in my Crown VFX2A and will be searching for a discrete Hi-Fi time alignment delay to run in between the xover and the low frequency amp(s) (suggestions welcome).

I'm using Crown amplification for now but will likely upgrade at some point. Highs were run with an ST70 at one point and it sounded pretty cool. My point is big SS amp on the lows.

I'm thinking about making some helper woofers in larger cabs to handle some of the heavy lifting below 70 Hz. I do not plan to crossover the very low end on the 604s but to filter the high end (active) on the helper woofers so they add to the 604/612 that is already there. I hope extend the response of the 604s below 70Hz with the helpers and extend F3 down at least an octave over what i have now...more if I can. A response hump between 30 and 60Hz wouldn't break my heart.

I'm think of building some plus size 612 style ~15^3 ft enclosures with extra bracing for a pair of 515s or 416s (recommendations please) with a vent location similar to the A7 so I can tune the whole setup depending upon which room they're in. We're talking about four cabs for stereo operation. Two 604/612 (A?) and two 515/612 plus size working together.

Here is my question: Is there any dis/advantage to going with a ducted port versus a straight vent in a cab this large? I've had great success with ducts in 3ft^3 guitar enclosures (single 12) but I'm not clear on their specific advantage in very large enclosures. My guess is that a high area low volume vent will provide greater efficiency and that the low area high volume port may enhance phase inversion but will not be as efficient.

If there is no advantage to ducting (mine would be all 3/4" birch and not the silly toilet paper rolls I see on some cabs) I can eliminate some stuff I'll have to build into the cabs that will allow me to be able to tune the area of the duct (pia).

Can someone who really knows this stuff tell me what I need to read to get smart about ducting a really large enclosure and how ducts couple to the transducer in >10ft^3 cabs. I read Badmaieff 35 years ago and consider myself an avid hobbyist.

Thanks.

[GM]

OK, so you want a 2.5 way system with the 0.5 in separate cabs. For this low an XO point, GPA's 3184 (or two/channel if you're so inclined to raise efficiency and/or pound your eyeballs flat) seems the 'no-brainer' choice. Using singles and spreading multiple subs around the room ala Dr. Geddes would be my choice though. Not knowing GPA's pricing, there may be better choices overall elsewhere though.

Normally, one doesn't choose the type of vent, the cab's alignment combined with the peak power handling required dictates the vent design. I can't think of a short answer to explain the 'why'/'how', so for now I'll just say that ducted port is probably what will be required if you plan to have much power on tap for EQing/whatever and the net is littered with tutorials, so you can read up on the subject at a much more in-depth level than the Davis/Badmaieff gloss over that doesn't take today's high power handling driver requirements into consideration.

GM

[RonSSS]

I use 2 real Altec 8184's with my 19's crossed at 70hz....or is it 80hz? Flattens my eyeballs and is pure Altec.
I have no EQ, just volume for the subs.
When I hear folks that use bass boost with the 19's, then I don't think they do real low bass all that well.
Listened to Frankie Goes to Hollywood last week, and that album has the thunder! Stuff you will never hear/feel with 19's alone.
BTW, many albums have the thunder!
The 19's are run with 35W PP300B tube amps and the subs with an old Kenwood integrated KA 9100.

Ron

[mah]

I wonder how 8182s would suit?

[Altec 604 8H]

GM,

I did some looking around on the net and didn't find any 'tutorials' beyond "insert tab A into slot B..." I'm wondering why Altec used what I refer to as a 'Vent' (0.75" deep port) on so many of their cabs. Also I couldn't find much on how increasing Vb increases efficiency and extension and how that changes suspension and tuning.

I found a mention of shelf vents and went searching and the only thing of interest I found was an old post on another forum from some guy that cut down his 30 ft^3 2 x 515 cabs to 20 ft^3 (GM). In that post you mention that Altec used shelf vents on some cabs. Where can I learn about those?

Here is a guitar cab that I cooked up that sounds better than anything I've ever heard. All who have heard it agree. No modeling here. I couldn't get to this from freeware modeling. Pic is from my cab builders site ( I make amps). He builds guitar cabs for a living. This is his personal cab (and his personal metal handle which is not part of my design). He could build anything he wants for himself and he built this. Port is pretty deep and works killer but I don't understand why. I just know what my ears tell me. I've tried this same cab in a more volumetrically efficient port of the same depth (4" x 6" and 3" x 8") and didn't get the same results. Works good with lots of speakers with different specs too. My intuition tells me that it has to do with the way it couples with the rear of the speaker and the enclosure's internal volume/mass of air.



I also found this file thanks to members here: http://alteclansingunofficial.nlenet...Enclosures.pdf

It talks about modifying Altec cabs which I think is really cool. I guess I was thinking of building something like the 416 in a 620 but making it bigger for improved efficiency and lower extension and then fiddling with the tuning of the port. If I use the 'vent' I can then change a panel on the front of the cab to fit the room like I did with my A7-500s.

If I just go out and buy some 8184s I don't learn anything.

[GM]

Greets!

Interesting, I found quite a lot of what I consider tutorials. For sure, there's enough info to educate someone on all aspects of bass reflex and ducted/shelf port alignments good enough to understand when to use what type and design them.

Your cab's slot shelf vent has such a high aspect ratio that an added resistive component has been added which increases its damping toward aperiodic and why more square vents of the same cross sectional area (CSA) and length don't sound the same due to having a lower F3-Fb with a faster roll off below Fb, i.e. yours is in-between a vented and sealed alignment WRT roll off slope/group delay. You could stretch layers of grill cloth over the more square versions to critically damp it to achieve at least a 'close enough' (and probably audibly identical) vent performance.

As such, it indeed has more tolerance to driver spec variance than a non resistively damped vented alignment same as sealed and why it's a popular alignment for high Qt drivers or ones that require a very large/long vent to keep vent mach low enough to not audibly 'choke' the vent's output.

It has nothing to do with how the driver couples to the cab's air mass though. The vent is an open pipe, ergo a 1/2 WL resonator and the added resistive component is altering its harmonic structure to a lower Q one (Qp), like stuffing a TL, which in turn alters its gain BW power response.

Note that it can be simmed in WinISD Pro alpha freeware or any other that allows adjusting Qp to add a resistive component to a ducted port of the same area, length.

Basic overview (follow all links): HyperPhysics

Basic vented alignment design considerations: Elsinore Box Alignment

Basic vent design: The Subwoofer DIY Page - Port Calculations
PORT DESIGN FORMULAs for SUB WOOFER ENCLOSUREs

This is just the 'tip of the tip of the iceberg' of what's been published on the net, but from these you should have a better idea about how/what to Google for to gain a more rounded view if desired.

Last, but not least, Mssrs Thiele's, Small's papers gives you all the theory/math and references to all the prior art that alone will keep you occupied for a month or three : Read Research - Articles

GM

[Altec 604 8H]

GM,

Thanks for your guidance. I had a sneaking suspicion that I would have to do some homework before I could participate in an intelligent discussion of vents. I look forward to understanding this more fully. I'll be sure to check out those links.

When I started experimenting with what you refer to as a slotted shelf vent my intuition told me that volumetric inefficiencies would have an impact on fluid dynamics as the air moves past more surface. Is the resistance that you mention something to do with how the volume of air reacts with the surface of the vent? And because I have more surface area for my volume as compared to a round port I have more of the resistance you mention? Or does the actual shape of the port irrespective of surface make the difference because of the way fluids behave in different shapes/configurations? And is this accurately modeled simply by specifying the port dimensions and material (surface shape)?

If this sounds like I'm guessing it's because I am. I'm imagining that I have to learn more first. I'm looking for any short cuts to understanding slotted shelf vent dynamics versus round column shaped ports. Are the dynamics of length the same or similar for both types?

I really appreciate your guidance.