Using 4612 JBL drivers in new cabinet. Firing one driver rearward

[weltersys]

Art you mentioned the Crowe horn being rounded over for a narrower baffle than I will need (about 15mm per side) Is that likely to be audible and could some chamfering of the baffle to blend into the horn overcome this?

If you don't notice the funky dispersion patterns resulting from the side by side woofers in your 4612 cabinet, more subtle effects like edge diffraction are not likely to be audible to you.

Conversely if I decide to build a wide cabinet and mount the drivers side by side like the original 4612 cabinet can I mount such a horn on a flat wide baffle above the woofer pair?

Yes, you can, but the horizontal response will be narrow and peaky in the midrange due to the comb filtering imparted from the off axis path length difference between the cone (and horn) center to center distance.

The 4612 cabinet is 40litres.

40L, 20 L per driver sounds about right if it the size of the 4612B, built from 19mm (3/4") plywood:

Funny, they list the 4612B as only 28L.

The 4612OK (Oak grain) is near the same dimensions other than lacking the Cabaret corner protectors:

I don't know if the 4612OK crossover runs one of the woofers "full range" like the 4612B.

I cannot understand why JBL used that size but don't publish it and how it performs in their literature. How is the smaller recommended size enclosure ever going to deliver the bass of the original 4612 vented enclosure?

I have seen no response graphs of the "original 4612 vented enclosure" or the 4612B.
The response in post #28 is a 14L box tuned to 70Hz, taken from the 2118H/J driver spec sheet.

The 4612OK Fb is 80Hz, as can be seen by the 8 ohm impedance minima between the two lower 32 ohm peaks.
Response is more than -10dB down (useless..) at 60Hz. These cabinets were not designed to "deliver the bass", even in the 1970's.
The (near) equal impedance peaks and flatter low frequency response than the 14L box are a result from the higher tuning, and perhaps somewhat from the 4612 larger baffle size.

The horizontal cone orientation shows the -6dB beamwidth (dispersion) dropping to ~25 degrees at 3kHz, compared to 100 degrees with the cones vertically oriented.
The midrange is "cancelled" off axis with the cones horizontal.

Using pink noise as a source, the cancellations should be very obvious when walking from side to side in the coverage area.
Room EQ Wizard has a pink noise generator.
Using a microphone and the software's RTA also would allow you to see the changes you hear in real time.
https://www.roomeqwizard.com/

 

[Audio>X]

https://audio-database.com/JBL/speaker/4612b.html

These cabinets were not designed to "deliver the bass", even in the 1970's.

It was my understanding that you intended to use the JBL 8"s for low mids upwards (?) This is where smaller box volume is perfectly ideal.
PS.
Don't forget about their rather moderate power handling.

 

[Onslo]

I thank you guys for your information. My plan now is to house the units in 28l enclosures, vertically oriented as originally planned but I think I will forward face both drivers (not locked in on that) in a 2.5 way arrangement with the Celestion tweeter on the Cowan flare. I will supplement it with a woofer/subwoofer for each channel. Not yet sure what I will use yet but will start with the HiVi D10Gs that I have but replacing those with something that might go a bit deeper and hopefully more sensitive or I might horn load something. I will use the Marchand 3 way electronic crossover and Holton amps I have in my collection. I can feel a mini DSP in my future.