Something Stirs in the Workshop

[Ant]

When i cut the aperiodic vents in the fane boxes i found some bits of info, i 'think' it was on rod elliots site. Pulled this quote from the fane thread when id just done it, not sure if its too relevant to what you are doing

"Bashed some big holes in the cabs this afternoon.
Found some info on aperiodic loading on diy audio projects, then followed some links on the page. Read a 50s paper by e jordan and a few others. A basic guide was to cut a vent in the cab that is 10 square inches for every cubic foot of cabinet volume, mine are just short of 5 cu ft, so a hole 5" x 10" was hacked out of the cab."

I have faffed about with the restrictive felt in the vent since

[vinylnvalves]

Thanks for your inputs, hopefully get a chance to experiment tomorrow. Think I have around 40 ltrs, so my vents would be too big on that rule.

Having seen numerous examples to managing the back wave whilst searching for info.

This one peaked my interest, unclear whether it’s a second chamber or a long vent. I was wondering if it offered the opportunity to have a longer path length, ie more less dense resistance material, bit like the Perkins aperiodic speaker.

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Can a second tuned chamber- remove the impedance peak. As a Helmholtz chamber tuned to 70 hz is doable? I know Dynanco A35 was a double chambered aperiodic arrangement.

Then the question of whether I need to reduce the impedance peak needs to be asked, I really want have the cleanest lower midrange, so in my mind minimising the reflected back wave was key. Couldn’t get the TL to sim anything sensible for the Beyma so ended up with a leaky cabinet…. Front wall to close for OB.

.

[Scottmoose]

It's an attempt at creating a semi-cardioid polar in the lower midband / upper bass (note caveat). From the dimensions, it may reduce internal reflections or kill an upper mode, but it's almost impossible to wipe out a box primary that way.

The reflected back wave has nothing to do with the system resonant frequency, where the driver is loaded by the air-spring of the enclosure, so if you're worried on that score, rest easy. Look at the free air impedance of the drive unit, then its impedance in the box. Ignoring the shift in the basic system resonance frequency, any smaller deviations above that in the impedance curve of the driver mounted in the box, as compared to the driver in free air, are internal modes (usually standing waves). Those you can damp out, or re-shape the internals to break them up & make them easier for a lighter quantity of damping to absorb.

[vinylnvalves]

After a little experimentation with the slot area, I came to the conclusion that 25 sq inches is about optimal. This is down from the 68 sq inches I started with. Green trace, v others. It offers about a 5 ohm improvement, as attached. I did get a flat response, but couldn’t replicate it. I can now design and built the proper boxes, which will be slightly smaller, as using 30mm birch ply, not the 18mm the test cabinet was built in. I was wondering if I should go for a single aperture on the back, instead of two on the side. As with 30mm thickness could end up designing tuned ports if I am not careful.

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Slow progress on finishing the sub cabinets, after the staining ended up patchy on the birch ply, even though I had used a prestain to avoid this. Shame as it was interesting grain. Should have assumed I was veneering from the start, as birch is a crap wood to finish, only really any good for the fire….

[Ant]

I put one big vent in because i had the same thought, the top is 28mm thick in total with an 18mm cabinet top and a 10mm black top cap. I found the resistive element of the vent needed some tweakery, initially i had it too restrictive so the box was acting more like a sealed cabinet than a leaky box. I did find that it snapped in, once i got it almost right i knew straight away, although that might be more because the vent is in an odd place rather than out of the way

[vinylnvalves]

How did you tune it, by ear? I have found listening to the test cabinet, I needed to tweak the cross over, as 5 ohms less resistance means more output.

[Ant]

Partly by ear but partly with the rudimentary measurement setup i have, rew on the laptop, dac, behringer ecm8000 mic and interface. Tweaked it how i liked it then measured to see what it was doing and then tweaked the filling in the vent again until i was happy with what i could get out of it with what i had to hand
Don't have the facilities to measure impedance though

[Scottmoose]

You do if you have REW. It just needs a couple of resistors & leads. Granted, that means a low voltage measurement, but [much] better than nothing.

[vinylnvalves]

Was wondering whether a vent underneath would work.. Will have an 8mm gap, generated by the spiked feet, thats there towihopefully decouple the midrange cabinet from the subs they sit on. Will have some additional resistance too, I know Hiraga, used a down firing vent onto a plinth, so it has pedigree.

[Ant]

I think thats where the vents mi dad put in the 12 inch fanes he had were, they werent restricted by space under them though because they had legs on them that were quite tall

I seem to remember the big castle speakers had slot loaded cabs which fired downward onto a plinth. So did all the ports on my various metronomes for that matter

[Scottmoose]

Was wondering whether a vent underneath would work.. Will have an 8mm gap, generated by the spiked feet, thats there towihopefully decouple the midrange cabinet from the subs they sit on. Will have some additional resistance too, I know Hiraga, used a down firing vent onto a plinth, so it has pedigree.

Yes. You're just bleeding off a bit of pressure.

[vinylnvalves]

Cabinets all now nearly finished, one more coat….

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Fronts currently are pacing me getting them finished. After the failed staining disaster. Decided to spray paint them. Working through the coats, cutting back with 320 grit now, won’t be piano grade finish. Varnishing so much easier, someone at work suggested wrapping them, might have been easier.

Harnesses all ready to go in.

[steve s]

I wrapped my red speaker's when we decorated, used the 3m stuff, it's OK... Nice gloss, now in white...

[vinylnvalves]

I tried wrapping a test piece, it wouldn’t shrink enough to be able to do the horn profile. I have gone for a deep red, so we won’t clash now at Owston

[vinylnvalves]

Maybe I am going mad…. Well that’s what SWMBO thinks. Been laying out the acoustic treatment for the midrange cabinet. Interesting discussion on DIYAudio about using melamine foam for absorption. With a hack of using magic erasers, as they are an order of magnitude cheaper than buying it in sheet form. Bought 50 to play with, lots of options for different pattens..

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