Altec 604 cabinet design considerations

[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

[Altec Best]

after cutting through several sheets I have yet to find a void.

that's more than I can say for some of the BB I have come across.

Well I don't know what kind of BB ply you are getting in your neck of the woods, but up here it is super heavy and solid as a rock (Void-Free)..The 5 sheets I bought for my project were all void free where all the cuts where made...MDF has to be glued 100% as mechanical fasteners are all but useless in MDF..And MDF doesn't like moisture at all, even high humidity will make it swell if not properly sealed..Personally I don't feel that a product that can swell like MDF would make a good choice for cabinet construction...Has to be sealed or painted completely.

But the biggest and most important aspect of the MDF vs.Baltic Birch Plywood is how they sound IMO&E.... Baltic Birch cabs sound a lot better to my ears.. But that is my personal preference.But as usual YMMV.

[Phil-G]

at 50$ a sheet you can have all the BB you want.
aside from the wood grain and the exotic name, I don't see any great benefits over OSB.
I have no use for mdf especially here in florida where it starts expanding as soon as it leaves the air conditioned store.

[GM]

It's a lot stiffer [higher MOE], so the cab will have a higher Fs for a given panel thickness even before bracing is added, though one can increase their thickness to get up to the BB's:

Modulus of elasticity:


MDF = ~527,000 psi


BB. etc., no void plywood = ~1,800,000 psi


Grade O-2 [OSB plywood equiv.] = 800,000 psi


Grade O-1 [general purpose] = 650,000 psi


Grade R-1 [waferboard] = 450,000 psi

GM

[rogerh113]

Hello GM,

Thanks for the info. Just to clarify my understanding of your terminology - you are suggesting running a 30 degree chamfer bit around the outside face of the speaker cut out, and then easing the edges. The start of the chamfer would be at .707 of the depth, so for example on a 1" thick baffle, the chamfer would start at approximately .3" in from the inside face of the front baffle, and extend outward at 30 degrees to the outer face.

regards -- Roger

[Phil-G]

OSB/2 is not what I use

[GM]

OK, what's the one you use? O-2 is the stiffest I could find?

GM

[Phil-G]

#3 is the the least I see here. #2 is not rated for flooring or humid conditions. the flooring material sold in florida is guaranteed not to go to crap when it gets rained on. it's not unheard of for a rainstorm or storms to come thru before the roof get finished. im not finding PSI specs but I will say the material I've played with is very stiff.

[VolvoHeretic]

At that angle, you will never be able to install inserts without blowing out the chamfered hole. You will have to mount it like GM earlier mentioned by front mounting the speaker in a dadoed panel and then mount that onto a chamfered hole baffle.

I will upload a new diagram as soon as I can log into stupid Photobucket.
(I had to directly upload the picture into Altec User's Board so it is a little LARGE.)
(Also, the drawing is of a 15" speaker that I stretched out to 16", so some dimensions might not be accurate.)

604-8G Speaker Mounting-2.jpg

[GM]

Hello GM,

Thanks for the info.

Greets!

You're welcome!

Not quite, I posted the theoretical best overall flair [AFAIK], but then I basically said it's not worth the effort unless the baffle's quite thick. Guess I should have added > 2".

At the end, I basically said to just angle it at 90 deg/2 = 45 deg its whole depth, then just sand/smooth the sharp edge on the baffle face, but if you want to router another little flair in lieu of sanding, then it's ideally at 22.5 deg..

But back to the ideal if you want to do it.......... [wish I had at least a scanner set up to post a sketch]

The 30 deg [60 deg total] initial flare is full depth, then measured axially out to 0.7 - 0.71" [0.3 - 0.31" measured from the baffle face] begin the 15 deg [30 deg total] 'flip out' flare end correction out to the baffle face.

GM

- - - Updated - - -

#2 is not rated for flooring or humid conditions.

Must be a Fla.code as all that I listed is subfloor rated nationally unless it's changed since I last had a need to know a few years ago.

Regardless, no OSB is going to be anywhere near as stiff as BB, apple or marine no void plywood unless it's anywhere from relatively a little to a lot thicker or braced a lot more.

The price differential though makes O-2 with plenty of bracing from scrap a good substitute if a fine finish isn't required.

GM

- - - Updated - - -

604-8G Speaker Mounting-2.jpg

Cool! Got us a draftsman! Join Flickr, it's a lot more user friendly or at least has been for me, but haven't loaded anything new in years, so YMMV. I hate Photobucket, what with all the pop-ups, slow to load.

Yeah, cosmetic baffles look good, though normally only used to 'flush' mount the driver, which negates the need for a flare.

For rear mounting, I long ago switched to clamps, [2] locating pins for big, heavy drivers, beveled cutouts mounted on baffles just tall enough to do it.

GM