Altec 604 cabinet design considerations

[rogerh113]

Hello,

In starting the process to chose a cabinet for my 604s, and then optimize the features of it, a private thread was started with some useful information. I am continuing to resolve some of the design issues, so I have started this thread to post that earlier information, and to allow others to contribute, access the information, and learn (depending on what you are looking for).

regards -- Roger


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Quote Originally Posted by GM
Greets!


Funny you should ask! I've gone from doing no design for quite awhile now to working on several different custom MLTLs [including 604s] for others ATM.


FWIW, Olson referred to his tall vented cab [MLTL] as just a reflex, though being fully aware of TL, horn design theory it seems reasonable to assume that he understood its additional 1/4 WL [TL] loading of the vent and like most designs is a compromise between height, size, driver/floor height, so while it will perform well, taller with an offset driver is better overall as it will allow much less internal damping, preserving more of its bass efficiency. The drawing is in a magazine article:

Gearslutz Pro Audio Community


WRT the Stonehenge V, my main complaints is driver offset, ditto vent location that ~defeats the point of a tall cab other than to get the driver near/at seated ear height and also adds a pretty big notch in its mid-bass response to get the traditional Altec/JBL 'West Coast Sound' [college party central's 'thumping' bass]. Great vintage vinyl playback/party speakers, but HIFI, not so much.

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Re: Olson's MLTL for 604

Thanks for the reply, GM. I appreciate the comment and the link to the original design. With regards to the Stonehenge, I had already decided to go with a vent like that of the Billfort design (split at the bottom of the cab). Don't know how much that will mitigate the vent concerns though.

I am certainly no speaker expert, or speaker designer - just like to listen to well presented music. Thumping bass is not high on my list - it should be appropriately presented like the rest of the spectrum. I will search for other posts of yours to find out what there is on MLTL. I know there is a quarter wave group on Yahoo, but imagine it is beyond where I want to go. Hopefully I can locate a design of interest. The design in the paper you sent is scaled about right, so I will spend some time trying to understand that.

best regards -- Roger

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Quote Originally Posted by GM
Greets!

You're welcome!


Huh! Wonder why part of it is in much smaller type as it's fine in the doc I copied it from? He asks rhetorically.


His vent doesn't allow any fine tuning.


Based on what you say, you'll theoretically want more of an offset driver and vent than Olson's and Billfort's, i.e. a ~56-61.5" o.d. height [depends on seated ear height] and at least a 30" wide baffle x 24" deep cab tuned to ~27 Hz or actual driver Fs if driven with a high output impedance.

Regardless, long sob story short, all my reference tech library is currently unavailable and with a bit of 'oldtimer's' disease I'm not remembering a lot anymore, so factor in only having a few hours/week to indulge in audio related stuff about all I can say is you can either wait till I get the others done [probably Christmas at my 'snail's pace'] or search 'gm' or 'gpm' or 'GregM' and 'altec' or 'mltl' and hopefully Google/whatever can find enough of what I've published over the last ~20 yrs to make it worth the effort.

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Thanks, but I think my wife might have something to say about a pair of substantially larger cabinets... the doghouse is way too small for all of my equipment.

I have been looking at the original 50's design, and actually like it a lot. Seems quite similar to Billfort's actually. I don't think I can get away with any larger a cab. You figure I will have issues with the 604 in there?? Any reference to designing in vent tuning, or am I pushing my luck.....

regards -- Roger


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Quote Originally Posted by GM
Greets!


Sorry, me and the folks I've known with large systems can't relate, none of the women in our lives seem to care how big they were once they got to enjoy their music through them and for some, we made a reasonable attempt to blend them into the decor, but it is what it is............


Anyway, as it turns out, Olson's cab just barely meets the MLTL performance criteria, i.e. its 1/4 WL action is just 'strong' enough to damp the vent a bit, so the vent will either be tuned a little higher or need to be a little larger for the same tuning, but pretty sure you don't want its ~58 Hz tuning [Fb]. Net volume [Vb] is ~7.5 ft^3 loaded with a 604.


Instead, an optimum vent [by the pioneer's standards] would be too long [deeper than the cab], so considering these will only be able to handle ~15 W max below ~90 Hz at low distortion, a single 4" i.d. tube/pipe x 2" long should be large enough for a low vent mach and get you to ~28 Hz once damped with 1" acoustic fiberglass insulation [AKA OC 703 ductboard] on the back, one side and top.


Don't have any program loaded [or otherwise for that matter] that will convert this to billfort's dual triangle vents, but if you want to experiment, then both vents combined must have at least as much cross sectional area [CSA] as the single 4" = 12.566"^2, so if round would be 4" x 0.707 = 2.828" dia./6.283"^2 each. Note that they will be >2" long since the combined internal pipe area will be greater, ergo will have more frictional 'drag' [stiction].


GM

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I actually feel pretty fortunate that the wife has bought into the Billfort size cabs (9+ cu ft) - hopefully with good construction and proper venting, they will allow the 604s to demonstrate their stuff. I am planning to inside mount the driver for aesthetic reasons, and assume that will not have any substantial impact.

I have been doing some port research, and found some useful starting information. The flared ports seem like a good solution compared to the straight, and with the variable pipe length, allows for tuning. Hopefully the affect will be fairly audible so I can tune by ear.

Have you an opinion on side vs front venting? I have heard that music/sound does come out of the vent, and should be directed into the room (recommending front venting). Also heard that various chuffing and breating sounds emanate from the vent, so the vents are better placed on the sides. I certainly don't want to punch punch a bunch of holes in the cabs to experiment......

regards -- Roger

[rogerh113]

Hello,

I have two questions that I am currently trying to resolve. Input would be appreciated.

1 ) Have you an opinion on side vs front venting? I have heard that music/sound does come out of the vent, and should be directed into the room (recommending front venting). Also heard that various chuffing and breating sounds emanate from the vent, so the vents are better placed on the sides. I certainly don't want to punch a bunch of holes in the cabs to experiment......


2) I have read about materials choices for the cabinets, and understand baltic birch plywood is highly favored (along with MDF). I also understand the issues with solid wood. However, I have spotted a local place that sells 'recycled' or 'reclaimed' wood. It would seen that this product, if in good condition (physical and appearance), would not have most of the concerns that have been raised about wood in general. It should have stabilized and dried out over the decades, and one might assume that the density of the vintage wood would be greater than the current product. If they have 3/4 or 1" thick product that could be used to build a cabinet, might that be a suitable choice, or would baltic birch ply still be the winner ??

regards -- Roger

[Phil-G]

as far as plywood goes I found a product called OSB 3/4" subflooring. I tried it on the last pair. it works great. it's very strong, water resistant, has a sound deadening quality, and cost less the 30$ a sheet 4x8. it's a lot like the environboard Altec used.

[Elitopus1]

When I have the choice I prefer plywood over MDF. Plywood handles more abuse and still sounds good. I do have two pairs of Altec MDF cabs that sound nice. They are no where near as durable as plywood. A few of the corners are missing on my MDF 817a cabs.

Speakers can be built out of solid wood. Even if it is old it will still expand and contract though. And much worse so if you live in a climate where it gets cold and snows. My father has a pair of speakers he built in the late 60's or early 70's. They are made from regular boards. During the summer they look nice. During the winter the corner joints separate slightly

[bowtie427ss]

If you build a cabinet out of plywood, that was properly designed and braced for MDF, it will most likely ring(probably why the master engineer above is selling his brand new model 17/18). In fact it will likely have areas that ring at different frequencies. If you change the engineering of the materials, you need to appropriately change the engineering of the construction.

as far as plywood goes I found a product called OSB 3/4" subflooring.

In my neck of the woods we have a similar product called "Advantech". It's very heavy/dense and has somehow been plasticized. It looks like OSB, you can see the oriented strands. It is said to be waterproof, being aimed at roof decking and subflooring. One drawback is that the sheets are tongue and groove on the long sides so you lose that bit.

[Altec Best]

as far as plywood goes I found a product called OSB 3/4" subflooring.

In my neck of the woods we have a similar product called "Advantech". It's very heavy/dense and has somehow been plasticized. It looks like OSB, you can see the oriented strands. It is said to be waterproof, being aimed at roof decking and subflooring. One drawback is that the sheets are tongue and groove on the long sides so you lose that bit.

3/4" Oriented Strand Board (OSB) has lots of voids.. It is not the best material for cabinet construction IMHO... I agree with Bowtie it also does have a tongue and groove edge.. I Just put some down not long ago.. If someone is going to go through all the trouble of building their own cabinets they shouls use Baltic Birch plywood.. It is the best material to use IMO ! It comes in 5' sheets in 3/4" or 4x8 sheets in 1" thicknesses in my area.The 1" stuff is extremely heavy,so bring a friend... Lol

[Audio_by_Goodwill]

as far as plywood goes I found a product called OSB 3/4" subflooring. I tried it on the last pair. it works great. it's very strong, water resistant, has a sound deadening quality, and cost less the 30$ a sheet 4x8. it's a lot like the environboard Altec used.

I've thought for some time, that if I build a cabinet it will be made of OSB....... and yes, I too see the Altec Enviroboard as being very OSB like, although maybe not as coarse.

[VolvoHeretic]

FYI, you can by 3/4 OSB without the T&G and it is substantially cheaper-$18 at Menards not on sale.

[GM]

I am planning to inside mount the driver for aesthetic reasons, and assume that will not have any substantial impact.

The flared ports seem like a good solution compared to the straight..........

Have you an opinion on side vs front venting? I have heard that music/sound does come out of the vent, and should be directed into the room (recommending front venting). Also heard that various chuffing and breating sounds emanate from the vent, so the vents are better placed on the sides.

Well, at ~7.5 ft^3, it can start demonstrating a bit better than Altec's ~same net Vb cab and of course the room will help, so just a matter of finding the best overall tuning, which may not be the same for both speakers.


While in theory all driver cutouts should be chamfered when rear mounted and many on the backside when front mounted, it's obvious that most manufacturers, DIYers don't bother, but when it's a wide BW one, and especially if there's a coaxial tweeter, then it really should be; and not just a simple cosmetic roundover, but one that knocks it back at least 60 deg. relative to the baffle, then do the cosmetic blending of this waveguide [WG] 'mouth' [terminus] to the baffle and this radius should be at whatever the WG's flare frequency is.


In the scheme of things though, making a 30 deg cut relative to the baffle beginning at .707x the 60 deg's axial length is sufficient. Since even this only gets one a subjective improvement over just making it look smoothed over, virtually no one does it unless the baffle's quite thick.


In kind of short, doing just the 3/4" router bit round over to the cutout serves no useful purpose acoustically, just cosmetically as it still has a theoretically infinite number of the same eigenmodes back where this round 'wall' is still parallel, so at the least, knock it back 90 degs and smooth the outer edge for the cosmetics.


Note that these are included angles, so divide by 2 for bit angle.


How audible this is depends a lot on the person's hearing acuity, SPL, sound source, but me and some others liken it to 'lifting a [diaphanous] veil' from in front of the speaker, which in a way it actually is.


For max effect though, it requires seriously rounding over the cab's corners or at least sufficient foam/whatever to damp its much larger [stronger] eigenmodes and no doubt why Olson horizontally canted the baffle [vertically is less important due to our ears being on the sides of our head].


Note that my vent length suggestions are too short [will tune higher] for flared vents, so will need to take that into consideration.


Contrary to popular opinion, 'music' per se doesn't come out a vent, just its own pipe harmonics and a narrow gain bandwidth [BW] centered at its tuning [Fb] that falls in and out of phase, ergo when tuned down low, we want the vent to have a <0.05 vent mach [< ~17 m/s, though some claim <0.035% = ~12 m/s] to ensure a high acoustic impedance and no vent 'chuffing' from being overloaded. Done this way, the vent can be put anywhere on a typical BR, though with a MLTL it's a bit more critical for best overall performance.


In all cases though, one has to consider any local vertical boundary impact, so in most cases where there's a corner nearby [like < 3x the vent's effective diameter] and the cab/corner's toe'd in included angle is < ~9.5 deg, put them on the front; or if up against a wall, then lower rear corners of the cab to get some free vent corner loading.


GM