This weeks harebrained and possibly costly dalliance is trying to fit an AER BD5 driver into the ‘Pan’ of a Lowther PM3 to see what they sound like in some TP1 Cabs - In several respects this is most likely dumb.
The original PM3 pan + Driver went back to Lowther years ago and came back bastardized. Thankfully they kept the magnet but some weird later version cone and frame all tampered with my Chi of the time. - They succeeded in this bodge by drilling holes into the side of the pan x3 (It’s a chamber for back-loading) to clamp the frame to the Lowther driver - the pan then locates to the TPI enclosure which has an 8 odd ft folded design for the bass and a kind of jipsom semi horn for the front firing side of things.
The issue - The BD5 doesn’t have holes in the side or indeed any way of fixing it into the pan - It will sit into it but there’s no obvious way to secure it in place.
Dumb question no: 445558886 will this necessitate @murrayjohnson glue and heaven forfend @stu ???
I could screw this BD5 directly into the cab (OK it will make a few holes but so what) - Then I can screw the pan to it’s original fixings of the enclosure (Over the top if this makes sense - Like a big metal helmet. Potentially (supposing everything is straight) the pressure of clamping would be sufficient to get things almost air tight -
The biggest concern initially was the BD5 driver has a rather powerful magnet and this could foul or generally not be compatible with the pan, the upshot here is it fits and takes up less room than the bulbous PM3 magnet.
Perhaps I could use some type of sealant if required? I guess no one has actually done this here but your thoughts on this would be appreciated (sort of) Please tell me there is a 3D printing solution so I can get my spaghetti on.
I’m happy try and knock up a model for 3d printing. I do only have a 175mm cube area printer. If l understand things correctly a rectangular flange needs to be made so the aer driver has a perpendicular flat on the base to screw to a shelf? I’m wondering if the easy way is to have a slot thro the 3d base so a couple of large tiwraps could secure driver to base
Thank you Andy - there may be a spaghetti requirement after all - The foam of the AER is raised above the frame - In short screwing it down as it is may cause damage -To get around this I was thinking of a 190mm I/D o ring which would give it a bit of height (Clearance) and assist with getting things airtight - A 3D printed Shim running around the frame profile surrounding the foam would be a nicer solution
Edit:
It looks like the white casing in your photos is effectively the compression chamber for the back part of the horn. It’s therefore likely that you could change the response by changing the size of this chamber. Normally you wouldn’t want to do this, but since you’re changing the driver, it might be exactly the way to make a new driver work within this cabinet.
I’ve never modelled a back loaded horn in hornresp, but if you can, it might be a good way to make the new driver fit, both mechanically and acoustically. And cause less damage to everything!
Maybe @edd9000 or @coco could help with the horn side? I could do the 3D modelling and printing if you want.
In the bottom-right picture, you’ll notice the rectangular vent (surrounded by green felt) for the back-loading horn, which connects to the pan/chamber exhaust. It’s critical that the height of this section is correct; otherwise flow gets obstructed.
This is the problem with the O-ring idea. The mounting frame of the BD5 driver adds around 6 mm in height, and if you add another ~3 mm from the O-ring under compression, the total increase means the pan vent no longer aligns with the cabinet vent.
OK now I’m at a computer and can type, and am consuming coffee. I’m going to put my understanding of the situation, please correct me if anything is wrong!
It looks to me like the Lowther is screwed into the pan not using its 4 mounting holes but using the holes drilled into the sides of the magnet (dafuq?!?), and then the pan is mounted into the cabinet using 2 bolts sticking up from the horizontal baffle and the 2 screws into the side of the pan.
How good is the interface between the flat part of the pan and the green felt? How do you get in to do up the screws? (I’m presuming that you screw from the far side in the bottom right photo using machine screws into the pan that is tapped?)
The AER driver seems to stand proud of the pan when placed into it. Does the Lowther do the same? Any change in the vertical alignment would be a disaster, as the pan wouldn’t line up with the vent or the screw holes. Could you show a photo of the Lowther in the pan?
It looks to me like the AER will be much better to mount well, as it is flat both front and back. The Lowther being irregular on its front has necessitated that rubbish foam stuff.
What I would do, if my above understanding is correct, would be as follows:
Mount the AER driver directly on the horizontal wood of the cabinet. Use a TPU gasket maybe 1mm thick between driver and wood. You would need to drill 4 holes in the wood for the mounting system (maybe use winged t-nuts?)
3D print a new pan, that fitted over the driver completely. Have a TPU gasket between this and the horizontal wood baffle, and another TPU gasket in place of the green felt.
This would have the advantage of separating the driver and pan, so you could use any driver of the same basic mounting. You could use this with the Lowther, maybe adding a TPU gasket more customised to the shape of the front of the driver.
I think that I would make the new pan much larger than the old one - flat against the green felt bit and the same size, but otherwise closer to the shape of the cabinet rather than round. I would use the two vertical bolts to mount it, plus a couple more for a good seal. The inside dimensions of the pan could be whatever we want - should start off with ones the same as the Lowther one, but the good thing about 3D printing is that you can easily run off variants with more or less volume.
The PM3 I got back from Lowther looks like this (Note the plastic bullshit that changes the height and in turn how the vents couple (Or not in this case and likely that of the AER) - it’s not sat fully in but you can see the side screws mount into the outside of the frame - Their idea not mine…
The pan is the chamber, according to Lowther it was specific (You could not buy the PM3 from Lowther they were only fitted to the TP1 enclosures) to the driver. The BD5 has a much more powerful magnet, the original PM3 chamber therefore may not be optimal (might even be dangerous for the driver it runs a bit hotter and kicks out more. I guess if it were possible to create the ideal space (optimized chamber) for the BD5 this would be the ultimate solution - The BD5’s are on loan and somewhat expensive for ‘lets see how it goes’ plans I have conjured so far…
The inter-phase between the pan and the folded horn is pressure fit by screws pulling it into spac (I’m not sure felt is the most air tight thing in the world either.
Ugh, I can see why you were so disappointed by what they did - they took the PM3, which was dedicated to that cabinet, and turned it into a Frankendriver, hacking on their current housing to the pan. And what an awful job they did of it.
The one advantage that metal has here over anything 3D printed is that it conducts heat. There is a risk that you allude to that changing the heat dissipation characteristics might make the driver overheat. Again get advice from the likes of Pete and Guy, but I think it’s pretty unlikely - my mid bass horns don’t have anything to remove the heat, and they are worked harder than this driver would be and are less efficient (i.e. would produce more heat). I think that the risk of damage is very small, but I would recommend a second opinion.
Anyway do you want to do this? I can produce an outline design if you like, and then we can start to get exact measurements to fine tune it.
It is intended to be a gasket for the AER driver, fitting outside the black foam. I’ll need a photo of how well it fits, so I can see the exact outline of the driver compared to my model!
Mostly agree with Adam , who has done some good work here. Obviously getting the front rear loading balance is both critical and extremely difficult to achieve.
My observations : 1; To protect the AER driver foam surround wouldn’t a 1mm thick stable plastic spacer be safer? Perhaps a flexible seal could locate to the outer perpendicular faces?
2 The Pan (Rear chamber) - Almost certainly a new one will be needed - Embrace this and make it with a removable top so that the volume (ideally on the fly)can be easily adjusted.
3/ 3d Print the unit in modules : Starting with the pan outside to felt adaptor. - Maybe the back chamber walls could be from suiably sized rainwater pipe?
Also - When I had my Hedlund horns - I tried a bulbus polepiece insert near field listening helped a lot to ease treble issues. In this design could be a physical part of the chamber spacer
AER’s don’t go that route - When I’ve heard them in other horns they do very much the better end of what lowther does without doing what Lowther fall down on (Shouty avec poor bass)
