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BioBrian

Gestating large sensitive passive 15" augmenting subs

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Posted (edited)

My nearly-4-year 4-way speaker project has reached a plateau. A little Tassie GTG today made me reflect that it's all been worth (some of) the struggle.

 

The design and build thread is:

 

 

The most recent photo:

 

182193155_Brians4-waysand3-waycentre.jpg.2b80ef4e70594cde8d9e7b5697f6b545.jpg

 

But this is a hobby, and this is SNA. Things must proceed. The words "so, it's basically the same as last time" really touched a nerve today (just kidding, you-know-who 😉). This must never happen again!

 

I've played a few low tones and walked around the space - 10 x 6 m, with 3.2 m concrete ceiling, and noticed nulls, halfway along room, at the sides. Not huge, but a bit of a sag around 50-55 Hz.

 

It's a passive system. No DSP. The design aimed to maximize bass extension by doing what few seem to do: use a large vented box. These are 240 litres, with twin 102mm diameter vents.

 

So the new intention is to build 2 subs (stereo of course) to augment the bass, and place them in the "null" areas. This can hopefully coincide with the same distance from the listening position, so there are no time-delay issues.

 

The best option would seem to be in the joint of wall/ceiling, behind the listening position, a bit more than half-way back into the room. How to get them there was an unresolved discussion today. They will be around 320 litres internally, and likely approach 100 kg each; securing them is the easy part, getting them there is another thing. A "sub-crawl" would necessitate a reversal of gravity.

 

I initially thought the best driver would be the same as my 15" SBA woofers, but I was recently tempted by some gorgeous Acoustic Elegance TD15H drivers, so decided to model them in WinISD.

 

It was all settled for a couple of days, then I simmed the SBA's more recent "long-throw" version of my woofer. It's a bit confusing, as the TS parameters are very similar, including X-max.

 

The 3 drivers in this graph are:

 

SB42FHC75-6      black (mine)

SB42FHCL75-6     red (new long-throw)

Acoustic Elegance TD15H   blue

 

These all have LP (hi-cut) filters applied. The black one is the same as my 4-way woofer, cutting off at 100 Hz, as with its present passive 4th-order filter.

The blue trace, the AE driver, was very appealing, but both are completely blitzed by the (red) FHCL, so I've ordered 2 of these. Also, the 3" voice coil and greater power handling* (*Edit: wrong - see later posts) appealed.

The red and blue traces have a 60 Hz 2nd order filter applied. Of course, this can be adjusted in real life to match the 4-ways and the room.

 

203294586_3x15inchwooferWinISDsims.JPG.fed52d31e722d08de578b82c63d2dae9.JPG

 

The box sizes are 300 to 320 litres, all vented.

 

So I can't be saved from the spend, but maybe your comments can help me with the next stage - designing the boxes for the wall-ceiling corners? As a cube, internally they could be around 700mm each way, but I anticipate angled baffles, from 45 degrees to whatever into the room. Looks like they'll need to be about 1.6m "wide".

 

Edited by BioBrian
wrong link, error re power handling

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I have seen sub built long and thin, almost like a pillar mid wall in a dedicated home theatre that were pretty cool.

Just a thought

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Thanks Brian for having us to visit at such short notice.

 

Thoroughly enjoyed the company and the music, definitely better sounding with the door open.

 

This is the latest photo I have of your system, apples and herbs.

BioBrian_270419_SNA.jpg.eda7c6aa63b4815bb832b89b4bd2e2c0.jpg

 

Will eagerly await the new sub configuration whatever it is.

 

Sphere calculator?

https://www.calculatorsoup.com/calculators/geometry-solids/sphere.php

 

These links may help with positioning? 

https://amcoustics.com/tools/amroc

https://www.bobgolds.com/Mode/RoomModes.htm

http://www.sengpielaudio.com/calculator-roommodes.htm

 

Best of luck with them, to quote @Upfront "don't be shy", yell if I can assist.

 

 

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6 hours ago, Wozza_Lee said:

sub built long and thin, almost like a pillar mid wall

This would certainly help with finding the best spot, and could be an interesting feature, as you say. I've sort of been led to believe that box shape (hence internal reflections) is almost irrelevant at these frequencies (bottom 2 octaves: 20-80 Hz). This would allow freedom not to be found in the upper octaves, where baffle step is also an issue.

 

I should say that there's fairly aggressive competition for wall-space though, and forget the floor!

 

The wall-ceiling junction looks most likely at the moment, as there are grooves in the Bondek ceiling for "Bon-nuts", which take heavy weight, and can be slid along the grooves if needed, to find the right placement.

 

I'm kind of hoping that the triangular box shape could match future room treatments, such as a continuation of the box-shape along the corners, but made of fluffy stuff with adjustable reflectivity surfaces. This could be material, holy plywood, metal, slats, etc, on their front surfaces. It may be too shallow for effective bass traps, but it's a thought at the moment.

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Russell, it was very refreshing to have you and Linc down yesterday. Thank you (both) for all your advice and discussion. Nice pic from your phone, too.

 

Your post above has given me quite a lot of food for study! It's the first time I've come across these mode calculators, etc. And I thought I was just building a couple of subs 😓.

 

Still wondering about the spherical enclosures - that would surely test the notion of box-shape-being-irrelevant. They could be stunning, especially if made of concrete and dangling from the ceiling (and you walk into one in the dark). New meaning to the phrase "balls in the air"? To get the 330 litres, with bracing, vent, and reasonable wall thickness, they'd need to be about 1m diameter. That's a statement.

 

It was kind of disturbing that the speakers sounded better with the Rolladoor open. That's not going to work well in winter. Or spring/summer, with all the bird noise. I'll try to get around to organizing all the mattresses and insulation into more correct places at some stage. The last 2  queen mattresses, that are currently leaning in one side of the room, seem to have swallowed some life out of the mid-range, but that's really hard to be sure about. I think they need to move to hanging vertically, somewhere out from the back corners.

 

If the door is acting as a limp mass membrane, maybe it needs tuning. It should be doing something towards diffusion, with all those horizontal folds. I've tried hanging batts in the middle there, but need to do some more serious tests.

 

One day we should have a REW party, with somebody who knows how to do it (not me).

 

I think the new subs need to happen first. They might have a future in somebody's home one day. Form will follow function, but I'll try to keep the shape a bit useable, hopefully.

 

 

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Does anybody have experience using Green Glue to laminate speaker box panels?

 

I used to think its benefits would be negated by having to have joins at the edges, but even in the middle of panels it may help.

 

And I used to think it stayed sloppy forever, but someone recently said it sets hard.

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Does anybody have experience using Green Glue to laminate speaker box panels?
 
I used to think its benefits would be negated by having to have joins at the edges, but even in the middle of panels it may help.
 
And I used to think it stayed sloppy forever, but someone recently said it sets hard.
I use rubberised adhesive (most contact adhesive is rubber based and will stay flexible).

Although wall thickness really isn't that important with a sub (as you said frequencys down that low are 15-18 meters long).

Its all about braces, for a sub that size I would have a skeleton with 2 vertical and 2 horizontal braces full size of the inside of the cabinet interlacing

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17 hours ago, BioBrian said:

but both are completely blitzed by the (red) FHCL

Your chart doesn't show that.

 

17 hours ago, BioBrian said:

designing the boxes for the wall-ceiling corners?

What frequency range do you intend the boxes to cover up to.  If it's ~100Hz or lower with a relatively steep cutoff (eg. 24db/octave) - then the shape and angle of the bafle is of much less (if not zero) relevance.

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2 hours ago, BioBrian said:

Does anybody have experience using Green Glue to laminate speaker box panels?

Yes - don't bother trying to laminate two boards together with an "elastic" material if the purpose is to achieve some type of damping or vibration resistance.... and the frequency of operation is <400Hz.

 

Use extensive bracing to "lock the walls in place".     Like "B&W matrix" bracing .... but you don't need to quite go that extreme IMHO.

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17 hours ago, BioBrian said:

Also, the 3" voice coil and greater power handling appealed.

"greater power handling"?

 

The AE TD15H-8 - 500W (cont).

AE_TD15H-8_spec.jpg.9f6f6e83fc81589683200256fc0034c1.jpg

 

The SB42FHCL75-6 - 300W.

SB42FHCL75-6_spec.jpg.c923a18c24fa0d905b32fc9a249cd3fc.jpg

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3 hours ago, BioBrian said:

Does anybody have experience using Green Glue to laminate speaker box panels?

I got better results using significant bracing. Think B&W Matrix.

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18 hours ago, BioBrian said:

The blue trace, the AE driver, was very appealing, but both are completely blitzed by the (red) FHCL

You and I have a very different idea of 'blitzed'. By the time it's in room, you won't be able to tell the difference based on those sims. Also remember, that the sims assume the drivers are completely linear throughout their excursion based on their T/S or small signal characteristics so cannot and do not take into consideration the surround, spider and motor linearity. A driver can have a specified Xmax, but only a fraction of that usable because of said non-linearity.

 

Do you actually have a measurement system?

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Posted (edited)
1 hour ago, soundbyte said:

"greater power handling"?

OK, this looks like I made a glaring error - thanks Russell. Brain overload.

 

I think either driver would suit the intended purpose in this case. I came away from the search thinking that because of the frequency performance of the AE drivers, they were less of a pure sub. This was supported by a post from Paul Spencer back in 2015 about the TD18H still needing a "sub" below.

Edited by BioBrian
word missing

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@davewantsmoore and @A9X, thanks for your posts re my comment "completely blitzed". I know I have a tendency for overstatement and cheap theatrics, but to me the SB42FHCL version looked better in WinISD.

 

As a passive driver sub, I want something that will produce 20 Hz with the least effort, and the least overshadowing by, say, 30 Hz.

 

The 3 traces were for drivers in the same sized vented enclosures, so include output from vents. Not sure about response using more than 2.83 V, but maybe you guys can see this.

 

In the same graph, the "L" says it puts out 3dB more at 20 Hz, and nearly 5 dB at 10 Hz (not that I'm bothered about 10 Hz).

 

Could you please explain what I'm not getting? Does it mean WinISD is not fit for purpose?

 

It may be not too late to cancel the SB order, as Wagner apparently didn't post them on Friday.

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Another issue that's come up since ordering is from reading a comment by @almikel, also I think from 2015, that has pointed to the SB "L" having an Le of 2.1 mH, and the AE 5 times lower, this having a significant effect on lower frequency response. My error here is that I had thought 2.1 was pretty low.  Not sure of the implications here.

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1 hour ago, A9X said:

I got better results using significant bracing.

2 hours ago, davewantsmoore said:

Use extensive bracing

 

2 hours ago, davewantsmoore said:

Use extensive bracing to "lock the walls in place".

 

I think this part is in hand - my previous builds didn't raise any doubts, from memory.

 

Awesome footage of the B+W building! Putting the hard-ish foam in the back is interesting.

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