Free air resonance

@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.

@unreceivedogma 

Actually, I would modify that design to make the baffle 18 X 18" decreasing the size of the face. A tube would be perfect. You can buy aluminum pipe in that diameter. This assumes a cutoff at 100 Hz. It would be like the driver suspended in space. No enclosure effects. An appropriate stand could hold that at ear level no problem. 

I suspect the crossover is second order. They are shelving it to keep higher frequencies from dominating the bass, a high shelf filter. This is probably due to baffle step, a problem my design above will not have. 

You would close the port and epoxy the panel in place to keep it from resonating, only if you are dedicated to subwoofers. 

My goal would not be to maintain a period presentation, but to take the driver to its maximum performance in term of sound quality and image. It is the very rare system that can cast the best possible image. Images of all instruments and voices should be well delineated in space with blackness between. They are hung in space as if standing there. In most cases, particularly high frequency instruments like cymbals, items are bloated and run into each other. Close your eyes and listen. The analogy would be with light. Shine a naked lightbulb is a room with white walls (reflective) and the whole room and everything in it will light up. Shine the same light in a flat black room and only the the objects within will light up. Theoretically you could get the same effect in a darkened white room with individual lights focused exactly on the objects only. Hard to do for sure, same for sound. It is very difficult to get the sound focused on the individual instruments. This is why the speakers and room have to be considered as one transducer. All this assumes the best recording and mastering. The other problem is that the recording and mastering are being done by people who are listening to their own problems. In any case you would be surprised how good a system/room can image. Line source dipoles are so good at this because if you absorb the sound coming from the rear they are "shining light" only on the instruments and not the room. The problem with omnidirectional speakers is they are the naked light bulb in the white room.