BC horns from standard FLHs

All sims are in 2pi, 1.41v @ 2 ohm, measured from 10m with values scaled to represent a measurement from 1m.

Image of how the standard BG horns are simmed.

Image of how the BC horns are simmed.

Red=BC, Blue=standard BG

I think these results are interesting, solid 5hz more extension is nothing to ignore. I suspect with a larger hole, it may have slightly increased efficiency. Let me know what you think. If you want me to try anything let me know!

It's definitely interesting, some thoughts.

Do you take into account how the hole/ mouth area is radiating into 2,0 pi for some frequencies but 4,0 pi for other frequencies
Partially due to cabinet stack height, partially due to the irregular hole shape with it's variable distance to the edge of the boundary loading
The hole looks like an eye in a pyramid, Illuminati confirmed
Would increasing the hole not simply decrease average horn path length if it's indeed the limiting factor with regards to efficiency

I've measured quite a few horn extensions myself, some followed the sim accurately, some behaved really different somehow. I suspect that is because of the large steps in effective surface area and different behavior to what the mind thinks the air path will be like. I would measure it and then correlate the simulation to the measurement.

Long way of saying, simulate, build, measure, repeat.

The idea for the Boundary coupled woofers like the 415 is that the hemispheric mouth “bubble” is actually bounded by the 60 by 60 inch flat baffle but in order for that to couple to the interior horn, the interior horn has to have an end area large enough to provide something close to the needed expansion rate to mate up to the exterior expansion. At the low end, the exit hole is a minor effect compared to the radiation resistances. The up side is whatever the acoustic gain is by that coupling AND the acoustic energy because of forward directivity which reduces the spl going to the rear. Keep in mind your talking about a subwoofer where 4 or 6 would be used in a large football stadium and ones and two’s per side for loud EDM events, not a home speaker unless one wants to dry their hair with it;

The Mouth does not need to be as big as the last part of the throat, following traditional horn terminstion rules, as the real termination is the air bubble contained by the flat baffle, this comes fromt the Bc creator voice... at lower Wl the mouth is not seen and the important thing is continuity between throat and exterior. At higher and smaller Wls where the mouth is seen the size is enough from what I understand.

Interesting threads on diyaudio about Bc and its predecessor Bdeap from some years ago.

Nice threads lately, this place seems more lively thanks!

Sorry citizens sir I didn't get back to you regarding the labs.

I am thinking of junking them so not a big waste to cut a hole in them.

First problem is I have no idea how big the hole should be. How does it relate to fs? I need to put two together and measure the sq mt idge.

Next is to do a plot outside rite now would be impossible. I could do one in the workshop but what would it mean? Two labs standard and two labs bc. But as I said inside measurement. I would get wholey flamed for that on this forum.

As always good work sir.

Remember four labs stacked like that would be 2.4 mtrs high, about. not possible I think two at a time.

Do you take into account how the hole/ mouth area is radiating into 2,0 pi for some frequencies but 4,0 pi for other frequencies

Partially due to cabinet stack height, partially due to the irregular hole shape with it's variable distance to the edge of the boundary loading

mobiele eenheid wrote:

The hole looks like an eye in a pyramid, Illuminati confirmed

aahaha, I cant unsee that now.

mobiele eenheid wrote:

Would increasing the hole not simply decrease average horn path length if it's indeed the limiting factor with regards to efficiency

If you look at that hole, then divide it by 4, the cross sectional area per cab is actually really small, <25% of the mouth area of a BG horn. If I make it ~40% of the cross sectional area, it might shorten the horn by 50mm but I doubt that will have a tangible impact compared to not properly coupling with the exterior of the cabs.

mobiele eenheid wrote:

What simulation software did these simulations use? I am not saying AKABAK 3 is perfect or even that I have mastered it but it is special when compared to other software used by the DIY community. It doesn't just simulate a straight horn path, that approximates a folded horn. It simulates the actual 3d model of the horn, including the exterior panels, the shape of the driver diaphragm, all the folds, any mistakes made when folding the horn in the design phase. Maybe these sims wont perfectly overlay a real life measurement but I have some confidence that they will tell you what the impact of a change is. Eg. the BC arrangement in my above post does not improve efficiency buy does improve LF extension.

KaphaSound wrote:

Nice work, how did the final result here stack up against hornresp? From what I’ve been seeing Akabak seems more defined but the larger fluctuations in spl are very similar.

If you look in the BG horn thread, the sims there reflect this sim to a much greater degree than I was expecting. They seem to tell a similar story, increased extension with a peak in sensitivity at ~35hz

KaphaSound wrote:

I definitely still think 2 pi isn’t the most accurate with a face that large. Even if only 40% of the wavelength is diffracted the directivity gains there will still be noticeable, but especially in the higher sub frequencies you’ll see some extra sensitivity. My main concern now with this type of horn is group delay. Also from reading some of Danley’s comments he said the trick is in something along the lines of trying to best couple the flare rate to the flat face at the mouth (which I think just means having a slow enough increase in cross sectional area to best suit low frequencies while still expanding fast enough to almost be completely flat at the end of the horn). It’s a tricky beast, but done right I still think this design is the future of bass horns.

As far as AKABAK 3 goes, by 2pi, I just mean its sitting on an infinate plane. AKABAK 3 takes the exact dimensions of the exterior of the cab in to account. I dont think the halfspace/quater space discussion is relevant here as I can just model it as if its the real world cab sitting on the ground.

Are you referring to the Danley quote Jo bg posted below?

Jo bg wrote:

in order for that to couple to the interior horn, the interior horn has to have an end area large enough to provide something close to the needed expansion rate to mate up to the exterior expansion.

This makes me think you cant just do this to any FLH, horns with high T values may work better as the expansion rate at the end of the end of the horn will be higher.

Jo bg wrote:

Interesting threads on diyaudio about Bc and its predecessor Bdeap from some years ago.

I think there has to be a limit on how small the hole can be before it impacts efficiency and coupling. You cant put 145dB of bass through a hole the size of an orange with out it liming the amount of energy that can pass through, and I have a hard time picturing that wave coupling to a 2m x 2m baffle.

Tomorrow I am going to do some more sims, might try 3 different sizes and overlay the graphs, when I get the right size, I might try some different shapes. I will also do a visualisation of the sound pressure at the mouth, maybe we will be able to see if its coupling.

Jo bg wrote:

Nice threads lately, this place seems more lively thanks!

Thanks for the support, it is nice to know other people are interested in what I am working on. I'm in one of the worlds toughest lock-downs, away from my home city so I don't have a lot of better things to do with my time.

doller wrote:

Sorry citizens sir I didn't get back to you regarding the labs.

I am thinking of junking them so not a big waste to cut a hole in them.

First problem is I have no idea how big the hole should be. How does it relate to fs? I need to put two together and measure the sq mt idge.

As always good work sir.

Remember four labs stacked like that would be 2.4 mtrs high, about. not possible I think two at a time.

I am going to do some experimentation with hole size, if you are interested I may build a labhorn model and try it. I am also interested in what would happen if you did this with two horns, with the horn mouths on the ground. Would it still work? I might sim this too.

Sorry for the wall of text guys!

Edited by citizensc - 03 September 2020 at 11:28am

if you compare the bc218 measurements you`ll see that the bc config adds little to the low corner compared to the gain in directivity due to the `boundary`involved?!

This is 4 standard BGs in a standard config compared to the largest cutout. About 1dB improvement in sensitivity and 3hz lf extension.

Here are some top-down view maps of pressure at 63hz

BC1765 cutout - Key got a little messed up on this one, the values are correct, colours are just shifted.

I am thinking the BG horn is not a good candidate for boundary coupling, the pressure maps above show the standard BG horn to radiate from the whole mouth. The BC horns have a hot spot around the cutout but this reduces by ~9dB at the edge of the baffle. The 15BG100 is a relatively high Qts, this means to achieve reactance annulling via the method layed out in the letters to the editor section of the Leache paper, you need a high T-value. High T-values do not have fast flare rates at the mouth of the horn. If I find the motivation I might try to design a dedicated BC horn using the 15SW115 or even the 15DS115 then repeat this analysis on it.

Let me know what you think, does my hypothesis make sense to you?

citizensc wrote:

Standard BG

BC 1110cm^2 cutout

BC 1400 cutout

Wow, top contribution citizensc!!!! Clap

Very encouraging results. I see hypercardioid dispersion patterns for the BC version Shocked

Remember four labs stacked like that would be 2.4 mtrs high, about. not possible I think two at a time.

Instead of trying to achieve a 2x2 array, it would be easier to just have an horizontal array, place the horns with the mouth downwards and have the aperture on ground level.

taken from:https://www.grc.com/acoustics/an-introduction-to-horn-theory.pdf