Free air resonance

@unreceivedogma 

I could not afford them either. Just happened to be at the right place and time.

I read an Altec spec sheet on the Duplex and the 3dB down point was 60 hz which would be about right for a woofer of that type of construction. 

T-S parameters (Thiele/Small) are a set of parameters that define a driver's electromechanical behavior. From them you can calculate the volume of the enclosure you need to obtain a given type of performance. They help getting you in the ballpark when designing a loudspeaker.  https://en.wikipedia.org/wiki/Thiele/Small_parameters

In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF. The type of enclosure, sealed or ported and the dimensions of the port or ports. In today's world of loudspeakers, with the advent of DSP you can get away with murder. I design and build my own subwoofers and rely on digital signal processing to get them under 20 hz at + 6 dB. I am about to shoot the finish on my new ones and will publish pictures of the entire process on Imgur when I am finished. The best way to handle subwoofers is with digital bass management which is antithetical to the type of system you have or rather the type of attitude you have. It certainly is more of a challenge and I understand the attraction. You can push the envelope a lot further. You can high pass your  Duplexes by putting the right value capacitor in series with the input of your amps. You have to know the input impedance to calculate the value. Then low pass the sub with the filter it has, the highest it will go without calling attention to itself. With two subs I would definitely plug the ports but I have to warn you that I really dislike ported speakers. I would rather push the speaker down with digital bass management and use subwoofers designed specifically to make low bass. What you have in the Duplex woofer is really a very large midrange driver. Middle C is 254 Hz. In it's day it was a very fine driver and still is. Back then subwoofers were nonexistent.  

@mijostyn 

"....In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF."

I am certain that they are plywood but I can tell you tomorrow. Altec specs from their brochure below: I will check to see if mine in actuality do match exactly.

SPECIFICATIONS

Type: Bass reflex enclosure

Dimensions:  40" (1 01 .6 em) H  26" (66.0 em) W  18" (45.7 em) D

Pressure Sensitivity: 100 dB SPL at 4' with ALTEC 604-8G loudspeaker when measured on axis with 1 watt input of band-limited pink noise from 100 to 10,000 Hz and calculated to 4' equivalent (Ref.: 0 dB == 0.0002 dyne/cm2)

Speaker Accommodations: 15", front-mounted

Internal Volume 9.0 cubic  ft

Weight: 104 pounds (47.2 kg), enclosure only

Finish: Hand-rubbed oil on rift-cut oak, charcoal brown grille cloth

Recommended ALTEC Speaker: 604-8G Duplex Loudspeaker

ARCHITECT'S AND ENGINEER'S SPECIFICATIONS

The speaker cabinet shall be the bass reflex type, and shall provide front mounting for 15" speakers only. The cabinet shall meet the following criteria. Grille assembly,snap-on. Grille cloth, charcoal brown. Internal volume, 9.0 fe. Dimensions, 40" H x 26"W x 18"0. Weight, 104 pounds. Finish, hand-rubbed oil on rift-cut oak.

The speaker cabinet shall be the ALTEC Model 620A.

https://forum.audiogon.com/discussions/ported-versus-sealed-speakers-is-one-type-better/post?highlight=QTS&postid=1951168#1951168

@unreceivedogma 

Great. I'll bet they are plywood. The older manufacturers used plywood. MDF did not become available until the late 60s. Plywood is stiffer than MDF and will resonate at a higher frequency. It is also lighter and not as well damped. It is also way more expensive. Look at the back of the cabinet. If the rear panel is held on by screws it is for certain plywood.  J Frum is correct in what he says from a modeling perspective but the equations assume an infinitely stiff structure in an anechoic chamber. They can not tell you what any given speaker is going to sound like in a real environment.

If I had those drivers I would build 10 cubic foot sealed enclosures 20" X 20" X 40". I would mount the driver in the 20" X 20" face and create a stand that held that face at ear level. I would cross them to subwoofers a 80-120 Hz and control the whole show with a digital preamp. Making it look good would be a challenge

In your case I would block the ports from the rear with plywood and cross to TWO 15" or FOUR 12" subwoofers using an active 2 way crossover. If you really wanted to get the absolute best out of the system and be able to tune it exactly to your taste get a digital preamp like the new DEQX Pre 4, Trinnov Amethyst or Anthem STR.

If you are asking why the strange shape (my enclosure), mounting a driver like that increases the stiffness of the baffle and places the mass of the enclosure in line with the driver minimizing the effect of Newtonian forces created by the driver. The small face minimizes enclosure effect which should improve the imaging. 

Good morning @mijostyn

- plywood

- the “port” is just a rectangular hole about 2.5” or 3” x 12” cut into the baffle in front, below the driver. A wooden block is affixed inside the cabinet that I slide up and down to open or close the port, or place in between.

- the Altec crossover is 1600

- The Mastering Lab crossover that I use is 2000. It’s shelved (I’m not sure what the term means, but I know what they are for, 😆) at 8000 also.

- the novel cabinet shape that you use sounds like an interesting approach but it begs the question: all that weight being supported so high off the floor must be supported by something that will have its own resonance issues. ? Might be a bridge too far for me 😬

- I would send pictures but this forum does not seem to allow for attachments.

- I have noticed that some cabinets have a piece of wood at an angle inside the cabinet. This does not. ?

@timlub  thanks for that link. I see that this topic has a history here!

As always, thanks again. I’m really learning something here.