Altec 604 D Enclosures

[GM]

Hi GM,
I dont know where you are located in the US but it seems to me that you are on the site either very early in the morning or very late at night!

Actually GPA told me that they are not used to measure the speakers as a standard service .Since i insisted to get my 604 measured after the reconing(i was advised by Jeff Markwart that it would be useful for cabinet and crossover design)they accepted to do it but then had no time to actually perform the measurements.I guess they had other priorities and actually i was not ready to wait for weeks.

For now and without measured elements to guide me ,i guess i have multiple options

1.To copy my friend's enclosure and to change(reduce) the dimensions of the vent and assess the results by listening.

2.To keep the same design (bottom vent boxes)but change the dimensions of the boxes and of the vent while keeping the total volume.In this option i noticed that for the width vs depth they are a lot of acoustic ratios published and none of them correspond to the golden ratio.So i am a little bit at loss when it comes to select one of those.Any recommendation?

Regarding the height, my ear are at about 3 feet from the floor.So what could be the ideal dimensions of the boxes(without the stands).And what distance should i keep between the upper part of the speakers(rear mounted)and the upper part of the front baffle?

Last but not least ,what should be in this set up, the dimensions of the bottom vent to get to an around 40 hz tuning?By the way,Altec Best suggested to go for circular vent rather than rectangular one.

3. to change the design and go for an MLTL( or a OB if the dimensions are reasonable in this relatively small room -12x16 feet).

What would you advise me to do .

GM,if i take too much of your time,let me know,i will understand.

Thanks,
Charles

Greets!

I live in a suburb of Atlanta, Ga. [EST] and being a life long insomniac, am up all hours, typically just ?cat napping? when I can since retirement.

Not surprised with GPA?s ?attitude? towards your request.

Yes, there are a lot of acceptable ratios and since the wider the baffle, the better overall, it really boils down to how wide [or deep] you can tolerate. I have a nomograph that makes it easy to see if ?w? x ?d? preferred is an acceptable ratio and how close I can match it, so not a problem.

For a ~36? floor/ear height it would be around 54.25? i.d. [inside dimension] for ~11.3 ft^3 using 19 mm thick construction and assuming setting on the floor with the vent on one of the sides near/at the bottom. Width x depth would normally be an acoustic or golden ratio to minimize internal damping.

Using ~24? x 15? i.d., makes it ~the same o.d. width as your friend?s cabs and an acceptable acoustic ratio. Driver offset from the top would be ~18.93? i.d..

Since you?re driving them with 300Bs, tuning them to Fs will need to be found empirically if you want the most efficient power transfer, so a removable vent baffle is a good plan. In this case, square or rectangular is easier to adjust tuning as you start with a probably too large a hole and progressively block it off until Fs tuning is found, then make a permanent vent baffle with the correct area as opposed to trying many different diameter ones.

The 604D had a 40+ Hz Fs, though it seems reasonable that the re-cone Fs will be closer to 30 Hz like later models, so allowing for up to a ~42 Hz tuning [Fb] just in case, a baffle thickness ~12? diameter [~113.1?^2 area] reflex vent is probably the largest and ~4? [~12.57?^2] the smallest needed if Fs should be really low like some E series. If the vent turns out needing to be < ~6? [~28.27?^2] though, then it needs to be tuned with a ~6? diameter pipe of the correct length.

Note that even tiny air leaks will raise Fb.

Internal damping would be Altec?s recommended top, one side and back with 1? acoustic fiberglass insulation or similar, though being a MLTL it?s typically not necessary to damp below ~ the cab?s midpoint. Some folks find even this amount too much due to the cab?s naturally high vent damping, so you may find that just a pad at the top, one side and back opposite the driver is sufficient. One person even wound up with no internal damping for a similar alignment S.S. driven 515B MLTL, only ?critically? damping the vent.

WRT bracing high aspect ratio cabs, what matters most is to run panel type stiffeners down the long panels, adding at least one set of interlocking stiffeners to tie all six sides together, positioned to also support the driver. Some folks use ?window? bracing, but these may negate some of the ? WL TL action, so don?t recommend them. Since the highest pressure is at the closed end, you can?t hardly make the top too rigid/massive, so making it double thickness or at least well braced is a good plan. If the bottom is setting on the floor, it normally doesn?t need any bracing other than any required to make the cab stable. Spaced up on legs, it ideally should be as rigid/massive as the top.

WRT OBs, it should be extremely rigid/massive to compensate for the lack of tie bracing, with sand filled double plywood panels proven to be an acceptable tradeoff between performance, weight for < ~0.7 Qtb systems. As Qtb increases, the panels need to become increasingly absorptive to damp system ?ringing? to the point where some folks have made excellent performing OBs and even TLs from braced Styrofoam or insulation board for very high Qts drivers.

WRT panel size/shape, this is so room, placement and system Qtb dependent that most folks just make it as large as they can accept in the desired location and then EQ it to get the best overall tonal balance. Again, using golden or acoustic ratios for the panel and even driver location is desirable.

As for what type of cab loading is best overall in your room, OB, IB or ideal compression horn is the theoretical ideal, but to go low with any authority with the first two requires a considerable amount of acoustic efficiency and the MLTL adds ~6 dB, so to match it with an OB would take at least one closely coupled 'helper' 15" woofer and 2x times the power, a significant price adder.

GM