Free air resonance

@unreceivedogma , Boy are you making like complicated.

The free air resonance of a driver is determined mostly by the mass of the construct (cone, suspension, voice coil and former) and the stiffness of the entire suspension. I doubt you will hear any difference between the drivers. Try them both. 

As for the enclosures, each surface has a resonance frequency. Every enclosure is a symphony of resonances. If you stiffen the walls by doubling up on the plywood you will raise the frequency of those resonances. There is no way to know if that will be better or worse than what you have now. Certainly a very stiff, heavy baffle (the surface to which the woofer is attached) is beneficial. It will limit the transfer of energy to the rest of the enclosure. I think I would leave everything else alone. 

If you really want to make them sing get another subwoofer and a two way crossover. Cross at 100 Hz or even higher. The woofers in that speaker carry a significant portion of the midrange which is being Doppler distorted by bass below 100 Hz. Taking those frequencies away from the Altecs will clean them up  noticeably, very noticeably at higher volumes. 

I was right about you  ... 😊

Your answer make sense for me and help me thanks... I hope it will helpful fot the OP...

Hah, thanks @mahgister - the free air resonance is not, by itself, the most important thing in determining the use of a ported enclosure or not, and certainly nearly useless in determining in-room performance.

The questions of whether this is best as a ported or sealed cabinet is a different question than "what would work well in my room."

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

I think this is not the right way to think about it. The question is driver to cabinet matching and then matching the total speaker (driver in a specific cabinet) to the room. There’s no best answer based on resonant frequency alone.

 

These questions are best asked in DIYaudio but you can also read the last paragraph here:

 

https://eminence.com/blogs/blog/sealed-vs-ported-enclosures

@erik_squires

thank you Eric.

I am working on finishing the room.
All the walls are 5” rock wool.
The gable ceiling is 12” rock wool.
All covered with fire resistant burlap instead of sheet rock.
The floor is hard wood.

Essentially, a semi-anechoic room. Very dead: the difference in sound quality is immediately perceptible as you climb the stairs into the room.

I was told by the guy who built my Futterman OTL3s that stiffening lowers the resonant frequency, but he was also quick to offer that speaker cabinet design is not his area of expertise. What you said about stiffening the baffle only makes good sense, as I would like to decrease the energy flowing through the cabinet walls. 

I think you are really getting confused about how the resonant frequency of a speaker driver (a good thing) works and mixing it up with how cabinet panels resonate (which is completely undesirable). The resonant frequency of a driver is used to model the bass performance in a cabinet in terms of the -3dB point as well as the slope and optimal flatness (Q) of the low end. It has nothing to do with distortion introduced due to imperfect cabinet materials. Along with Qts, and equivalent air mass and other parameters the resonant frequency is something you plug into your cabinet design software.

Driver resonance frequency has zero to do with the materials used or bracing methods used to construct the cabinet itself. The driver resonant frequency also has zero to do with driver value or quality but it is an indicator of the type of driver you have on your hands. A driver with a resonant frequency of 300 Hz is probably not going to go lower than a midrange for instance.

Also, you want diffusion between and directly to the sides and often behind the speakers and listening area. Anything truly anechoic is going to sound too dead, and lack imaging and space.

@erik_squires I agree entirely up until the last comment. This argument between wide dispersion and limited dispersion has been going on since I was knee high to a grasshopper.

Obviously, wide dispersion bounces sound all over the room creating more and louder reflections. Very limited dispersion, like a flat panel ESL is very annoying but sounds terrific if you lock your head in a vice. Controlled dispersion on the order of 45 degrees like you might get from a horn system or curves panel ESLs is best in terms of image and comfort. Dipole line sources are even better because of  strong attenuation to the sides, floor and ceiling.

I think it helps to think of sound like light. If you shine a flashlight at an object only things in line with the object will light up but switch on a naked bulb and the entire room lights up.  Omnidirectional speakers "light up the entire room." This usually makes a system very bright which some people like especially at low volumes. You may note that cymbals are poorly localized and there is a tendency towards sibilance with female voices and violins. A system with controlled dispersion usually sounds dull at first and people will think there is no high end. if you pay attention you will note that the cymbals are well defined and the high frequencies are there but now they are coming from the cymbal and not the entire room. Such a system is smooth and effortless without any sibilance. With loads of sound deadening you can make an omnidirectional system sound more like a controlled dispersion one at some expense and nail holes. 

@unreceivedogma , What is rock wool? I have not heard that term. Sounds like you have a nicely dampened room which is great because Altecs  have a tendency to get shouty. 

I think you are overthinking things. Use the pair of drivers you think are best constructed and listen to the results. Stiffening the front baffle of the cabinets makes sense but doing it right would mean removing the old one or using screws every 6 inches. You can not place clamps over such a large structure. My approach to the problem would be to make entirely new enclosures. I would remove the ports, decrease the volume depending on the math and cross to subwoofers at 80 to 100 Hz. That would be killer!