I'm just going to jump in and suggest two active drivers like UM22-12s in a push push, wired phase correct to 2
or 8 ohms and an 18" PR either in a slot or on one of the two remaining right angle sides. I used and modeled
at 4 and a 6cf box with great success. I used 2 disk and MorTite putty until I had BOOM and removed putty at 3/4" balls
to begin with and wound up with putty pinching the size of large peppercorns. Add 1/2" size ball of putty in certain
music passages and the boom comes back.
You can tune to a given frequency, the problem is the ROOM/amp/cables (in a passive cabinet) may not go as deep
as it can or to far and boom the room. One is to a given tuning frequency one is to the actual room your in.
There is a huge difference in what you have to EQ from that point. I prefer to start with the box (cabinet and drivers)
matched to the environment they are in.
IB or ported rooms are just as much of a consideration on the type and cubic feet of air you want to move in a room
too. I've never been a fan of sealed or non vented rooms. Larger rooms over 400sf with 12" lids sealed work pretty
well with OB servos like GRs design. They pressure the room differently (at least by my ears standards).
I've tinkered with a LOT of different PRs and the old 2X rule is just not the case with 90% of music. I've seen and
built several cabinets that a single 15" passive will tune 12/15" active mechanically to 16 hz in a 6cf box and still
hit 120db. VMPS did it for years with their passive subs. They had columns that were 4 12 and a single 15" active
with a single HE 15" passive in a lower slot loaded position. The slot is for vibration control (no open bottoms to
excite the floor plane) or another huge passive driver that is also out of time.
The floor will alway arrive at your feet and bottom before it hits your ears at 80hz <. Depending on the room and
floor is can get pretty muddy (for the lack of a better term) once you experience a room that's decoupled with any
effort at all.
Regards
or 8 ohms and an 18" PR either in a slot or on one of the two remaining right angle sides. I used and modeled
at 4 and a 6cf box with great success. I used 2 disk and MorTite putty until I had BOOM and removed putty at 3/4" balls
to begin with and wound up with putty pinching the size of large peppercorns. Add 1/2" size ball of putty in certain
music passages and the boom comes back.
You can tune to a given frequency, the problem is the ROOM/amp/cables (in a passive cabinet) may not go as deep
as it can or to far and boom the room. One is to a given tuning frequency one is to the actual room your in.
There is a huge difference in what you have to EQ from that point. I prefer to start with the box (cabinet and drivers)
matched to the environment they are in.
IB or ported rooms are just as much of a consideration on the type and cubic feet of air you want to move in a room
too. I've never been a fan of sealed or non vented rooms. Larger rooms over 400sf with 12" lids sealed work pretty
well with OB servos like GRs design. They pressure the room differently (at least by my ears standards).
I've tinkered with a LOT of different PRs and the old 2X rule is just not the case with 90% of music. I've seen and
built several cabinets that a single 15" passive will tune 12/15" active mechanically to 16 hz in a 6cf box and still
hit 120db. VMPS did it for years with their passive subs. They had columns that were 4 12 and a single 15" active
with a single HE 15" passive in a lower slot loaded position. The slot is for vibration control (no open bottoms to
excite the floor plane) or another huge passive driver that is also out of time.
The floor will alway arrive at your feet and bottom before it hits your ears at 80hz <. Depending on the room and
floor is can get pretty muddy (for the lack of a better term) once you experience a room that's decoupled with any
effort at all.
Regards
Whoa, let's back up a bit here.
First off, a single 12" driver cone is approximately 4x the size of a 6.5" driver cone, not twice.
Second, don't try and use smaller drones than the active cone, no matter how many.
For example The Xmax of your average 6.5" is about half what a comparable 12" has, so that means you need:
8x the drivers to have the desired double cone area and a further 8x the drivers to compensate for the lack of travel, meaning you need 16 of the 6.5" drivers for a proper match to the single 12" Now if that doesn't sound ridiculous, then have fun. If it makes you shake your head then consider that a single 15" is what you match to your 12". Just like the rest of the world has been doing for more than 50 years. You want around 1.5x to 2.0x effective piston area and approximately 2x, potential travel. There is also mass, cove rigidity and surface damping to consider.
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Here is the ratio between the effective piston areas of your average drivers.
4.5" - 1
5.0 - 1.3
6.5 - 2.6
8.0 - 4.3
10 - 6.8
12 - 9.9
15 - 16.0
Meaning that a single 15" is 16x larger than a 4.5".
12" vs 6.5" is 9.9 / 2.6 = 3.8x larger.
Here are the average piston areas of the drivers:
4.5" = 8.3 in^2
5.0 = 11.0
6.5 = 21.6
8.0 = 35.7
10 = 56.7
12 = 82.5
15 = 132.7
4.5" - 1
5.0 - 1.3
6.5 - 2.6
8.0 - 4.3
10 - 6.8
12 - 9.9
15 - 16.0
Meaning that a single 15" is 16x larger than a 4.5".
12" vs 6.5" is 9.9 / 2.6 = 3.8x larger.
Here are the average piston areas of the drivers:
4.5" = 8.3 in^2
5.0 = 11.0
6.5 = 21.6
8.0 = 35.7
10 = 56.7
12 = 82.5
15 = 132.7
Is its Art? Noted man. So inertia is not the problem? I also don't understand about the linear requirement for a passive?
I dont like the sound of sealed either. Never have
Thanks for the input, trying to digest that
I live in the part of Canada whose weather in not that much different from San Fransisco. The real Canada is on the east side of the Rocky Mountains.
Inertia of the PR mass should be near the same as the equivalent air mass of a port the same diameter as the PR Sd that produces an equal Fb.
If the PR Vas varies with excursion (is non-linear) the Fb will be affected.
If the PR "rocks", (upper portion of the cone not traveling the same direction as the lower portion) at high excursions, it's output will be reduced compared to lower levels, as well as changing the Fb.
If i'm not mistaken:
Fb = tuning frequency
Sd = diaphragm area
An optimal reflex port has no flow resistance
The optimal bass reflex port for a small enclosure must be much longer than in a large enclosure, and requires more air mass to achieve a flow rate low enough to eliminate turbulence and net air flow.
The PR emulates the equivalent air mass of a port the same diameter as the PR Sd that produces an equal Fb, but does not allow any net air "flow".
This patent goes into some detail about approaches to reducing rocking modes and increase linearity:
https://patents.google.com/patent/US6176345B1/en
"An important consideration for proper functional operation of a passive radiator is that it exhibits true pistonic motion over its entire design frequency range, and accommodate a very large linear excursion. Pistonic motion means that the entire diaphragm and suspension (compliance) move back and forth to displace air substantially the same distance, in the same direction, at the same time. This movement replicates the reciprocal movement of a piston. Linear excursion refers to the length of travel of the radiator assembly. Both effective pistonic motion and relatively large linear excursion are very difficult to achieve with conventional passive radiator technology.
Another serious design challenge in passive radiators is minimizing the many different breakup modes of the outer compliance, which produces undesirable audible effects.
FIG. 8B shows the effects of compliance breakup in a rim resonance mode of operation, during large excursions. In some cases, the compliance will actually move out of phase for in the opposite direction), relative to the radiator's diaphragm."
There are lots of different approaches depicted in many different patents to reduce the problems...
https://www.earthquakesound.dk/wp-content/uploads/2021/04/US10349166.pdf
"Proper funtional operation of a bass reflex port" also only exists in theory
A passive radiator seals the cabinet, there is no net airflow.
I'd think any PR Vas would vary with excursion, though in well designed and executed models, that variation should be minimal.
Compared to a wheezing undersized port, a decent PR would be far preferable in every aspect but cost.
Cheers,
Art
A passive radiator seals the cabinet, there is no net airflow.
I'd think any PR Vas would vary with excursion, though in well designed and executed models, that variation should be minimal.
Compared to a wheezing undersized port, a decent PR would be far preferable in every aspect but cost.
Cheers,
Art