A Monster Construction Methods Shootout Thread

[lcwin]

I think the B&W spherical inverse horn is a perfect enclosure. It hv almost no diffraction due to the spherical front. A sphere is inherently very rigid so is a inverse horn. The single major resonance mode can be overcome by tapering part of the sphere which destroys the single resonance mode.
Recently I built a small 2.5in speaker using a semi spherical plastic enclosure with a inverse horn. I lined the 2mm thick plastic enclosure with 2.5mm butyl Aluminium sheet ( its usually call CLD) and stuff using polyester fibre. I tapped with a hammer and measure the sound emitted from the enclosure and surprised to find the FR to be smooth across a wide band around 500hz to 8000 hz. I also check my other normal enclosure box but its much more rugged in response.
Ok one caveat I measure using my phone Spectroid App so the result is not as accurate as should be. I think its still good as I am doing a comparative test.
I think its better to just go TL way as it almost completely and gently absorbs the rear sound from a speaker driver rear from 500 hz and above through the 3 to 5 ft bend and taper lines. Its also the reason why I personally prefer sound from a TL vs BR .
Theoritically BR give a better behave response and simple in construction. The main problem is sound from the rear of BR speaker can easily leak out of the vented port as the path length is very short. The absorption fibres does a fine job if density is high though.
Lastly I think its better to hv an enclosurewith a smooth noise signature which mimic ambience noise than a very rigid box but with ragged noise emmision.
I found out about this when I soundproof my car. I got all noise down a lot but unable to effectively do anything effectively below 200hz so now the car is very quiet but the low frequency become much more prominent....not a good idea that i can tell you.

[Sixto]

I'm building some speaker boxes (various sizes with removable 1-1/2" baffles) to test performance of a few 12" extended range woofers (roughly between 30hz to 1000hz)

The plan is to have 3/4" plywood and/or MDF on the exterior, with 1/4" plywood bonded using CLD on the inside, with CLD bracing, and also a butyl automotive sound attenuating barrier and finally 1" of foam on the inside. I'd like to use something cheaper than Weicon or Sikaflex.

I wonder if anyone has tested any silicone sealants in diy CLD speakers? I found this for $3 a tube at that large river retailer: GE Sealants & Adhesives-2709203 Window & Door Max Shield All Weather - Light Brown, 10.1 Ounce. It claims to have "800% stretch" whatever that means... does it make sense to use about a 1/4" bead, about 2" apart to bond the 1/4" material to the 3/4" material? Would adding a filler like aluminum oxide powder make it more viscous, and would that improve the damping action?
Thanks! Sixto

[augerpro]

Any reason for the 1/4" ply? Ply that thin is kinda hard to work with, doesn't have many plys, and magnifies the effects of voids in the wood.

Otherwise your plan looks real good to me. I've never used silicone since it is not great at dampening vibration. Use the Loctite PL300 if you need cheap. Have clamps ready though, it is not that tacky and will separate and cause air pockets in the constrained layer.

Speaking of air pockets, laying beads like you propose will cause a lot of cavities not only preventing the CLD from dampening properly, but could be worse than just solid wood since the two panels could vibrate against each other at those cavities. I use a method similar to laying tile. Use a fine tooth trowel, the one I use makes 1/16" square beads. Then, when attaching the panels push across the direction of the beads so they collapse which expels air. If you aren't able to do that movement I would still apply the PL300 with a trowel, but then use the smooth side to lay the beads down and attach the panels. Hopefully there won't be an air pocket.

[Sixto]

Thanks Brandon, I appreciate your input.

The main reason for the 1/4 was that it was the thinnest sheet I could find, premise being that it would be thick enough to create shear in the adhesive plane... but I can definitely go thicker. Your other point about reducing air pockets makes sense... the constraining action of a troweled finish would be more uniform across the surface.

The part about adding a filler was aimed at ways to improve the dampening action of silicone... not sure if that would do the trick, though? Others have discussed adding fillers to various rubbers, etc... so I thought, why not silicone?

Thanks! - Sixto.

[HeadShake]

Found "paid by the industry" paper on bamboo.
The Development Of Sound Absorbing Materials Using Natural Bamboo Fibers

Could be interesting to try. I see it has been mention a couple of times in the thread. Here is some data.

[hifijim]

I see you are getting closer... your multi-layer baffle cutouts look very clean.

[augerpro]

Where does one find bamboo fiberboard in the US?

[augerpro]

Started another round of testing. I hope to have them all done within a week. For now here are the port results. I tested two models: the my Augerport and another based on the Roozen paper attached earlier.

The Roozen port is similar to the Harman - it appears that Harman got the constant curve contour from the Roozen paper - but with a much shallower flare as he specifies an angle of <6 degrees from center of the pipe to the exit.

My Augerport is in 3 variants. 1 = taps are aligned, 2 = taps are offset, and 3 = taps offset and bigger. I used wool batting to damp the taps energy.





First the Roozen results:









Now the Augerport variants. Augerport 1:








Augerport 2:








Augerport 3:








Some comparisons:

[augerpro]

Couple observations so far...Augerport 3 really knocks down the port's 1/2 and 1/4 wavelength resonances. At this point I though it was a slam dunk - until I started harmonic distortion testing. Performance degraded to similar to the Precision port clone and worse than the short version Harman on which the Augerport is based. Chuffing is noticeable and comes on soon. I slightly chamfered the taps and I think this was the wrong thing to do. I should just leave a clean hole. Larger holes were obviously better for knocking down the resonance but it seemed like I heard chuffing sooner than the other two variants even though the HD plots look similar.



What do you guys think I should try for taps now? I'm temped to do a slot, maybe broke up into 3 segments. Maybe increase the void/wall ratio. Has to be something clean though. Also, I only put the one line of taps for the 1/4 wavelength from one end, I think this should damp the wave, but maybe I should try taps at 1/4 and 3/4 length?



The Roozen also excited me at first with its best-in-class sensitivity. I thought I might switch the Augerport from the Harman profile to this one. But harmonic distortion was really unimpressive.

[Sin Phi]

If you are having turbulence issues in your taps maybe you should explore low turbulence/drag inlet/outlets. A lot of solutions to this in aerospace. Probably the easiest to go with is is something like a "NACA duct". The other thing to keep in mind is that large discontinuities in flow area will cause issues. Just sketched something up to consider.

8 NACA ducts near the throat of the port with a smooth ducted transition to a large volume. You would want to avoid stuffing to closet to the duct inlet.

NACA duct - Wikipedia

I have a feeling tap location will be pretty critical, need to try before after and right at the throat at least.