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Gestating large sensitive passive 15" augmenting subs


BioBrian

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6 hours ago, BioBrian said:

Some may disagree, but it sounds good

It's sound the same as any other filter with the same transfer function ?

 

 

EDIT:   I've clearly spent too much time on forums where the auto-moderator would merge your 5 posts in a row, into one.  Sorry.

Edited by davewantsmoore
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2 hours ago, davewantsmoore said:

Denman Marine.

I got my 2nd lot (a good while ago) from there, but when I tried again later, they weren't doing it any more.

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On 07/05/2019 at 11:56 AM, BioBrian said:

but it means accepting bi-amping, which is another mental shift. (Not against it, but it changes long-term amplifier choices).

Why?

If you were to biamp in the method you're talking about, ie LF vs everything else, I fail to see how it makes a difference. The LF amp can be anything with sufficient power, and the other amp can be whatever audiophile fashion appeals to you at the moment.

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A couple of very large boxes arrived on Friday. Nice looking drivers, but not as exciting as the AE ones.

 

This 'L' version (the ones in my 4-ways are SB42FHCL without the L) is very similar in specs and appearance, but there are some subtle differences. The dust cap dome measures 90mm diameter in the old ones, and 110mm in these. Maybe this has something to do with the wriggle in FR in this version, where 40 Hz is louder than 50 Hz, but I'm not at all concerned with this now. It's measured at 315 mm mic distance. Maybe the larger dust cap gives a different rigidity to the joint between voice coil and cone. Surely it's not to make people think there's a 4" voice coil?

 

Also, the roll surround is a bit wider - 25mm instead of 22mm. Just a little more wobble room, or anticipating a heavier life, being labelled a 'sub'?

 

It's tempting to give them a burl in the present boxes, but that's blatant procrastination.

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I also got 3 sheets of 30mm BB plywood. Thought there might be some left over for at least one other project, but it looks like I should have got more. These are big boxes!

 

So with the present box sketch, it's looking like I'll be laminating the baffles from 24mm BB and something else behind. Still working on dimensions, and a method to get them together nice and square - worse than boat-building!

 

I lost a post the other night - not sure what happened - but it said I also saw some beautiful Meranti plywood at "The Bunker". It has a lovely red colour, nice smell, and is flat and silky smooth with no knots, front or back. This could be even better for speakers than BB. And $188 per sheet. Might try that next time.

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7 minutes ago, BioBrian said:

but not as exciting as the AE ones.

Oh dear, but then again you don't see the backside when they are mounted in a box, not a problem for my system, OB, you see it all.

8 minutes ago, BioBrian said:

It's tempting to give them a burl in the present boxes, but that's blatant procrastination. 

Nothing to lose is there?

Only a few minutes work!

Unless they are different mounting holes? ?

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1 hour ago, soundbyte said:

you don't see the backside when they are mounted in a box, not a problem for my system, OB, you see it all.

Ah ha, I'm starting to get the attraction. Trading bottom octave for viewing the backside. Tough one ?.

 

No, I don't think they are perverse enough to change the mounting holes. But I think intent listening to a pair of subs could wear a bit thin after a few minutes, then it'd be getting on with swinging the hammer anyway.

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On 12/05/2019 at 11:52 AM, soundbyte said:

It's tempting to give them a burl in the present boxes

Did you change your mind and try them in your present boxes?

 

They would not be "subs" in the present boxes with the crossover that is in place now?

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2 hours ago, soundbyte said:

"subs" in the present boxes

I probably wouldn't call them subs, no. My '4th Order' LP filter has a -3 dB point around 100 Hz..

 

These new drivers have a larger VAS, and ask for a larger box. Not a big deal when the oldies are in 240 litres, but I'm wrestling the figures to give the newies 320 litres net.

 

These are some old near-field measurements with coils & caps:

 

Black & green are severe, using 22 and 12 mH coils (sealed and vented, not showing vent output)

Grey is just a 12 mH coil plus cap

Red is more like what I settled on - the driver basically gave the finger to my 4th Order electrical! Not a sub...

 

596114899_SB42olderLPexperilments.JPG.5aec6fc0ff4e2126afd2fb39aec44560.JPG

Edited by BioBrian
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1 minute ago, davewantsmoore said:

do you mean

Not at all, no. We have the same problem as before - these pics should be in the other (4-way) thread.

 

It's going to look much more like the grey, black or green traces, I would think. The black is still only 12 dB/oct from 40 to 80 Hz though, even with big LCLC filter. I know it would be easier with DSP, but my way (not many others to include, alas) might give more even room response, not dropping away too fast. Will have to wait'n' see.

 

And of course the vent output will bump up the output below 40 by a large margin.

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20 hours ago, BioBrian said:

the vent output will bump up the output below 40 by a large margin.

I see, OK.

 

I was going to say I was more talking about 20 to 50Hz.

 

... the upper end rolloff should be whatever is needed to integrate with the next driver up... whether that's 12dB/octave or 24, and what shape the knee is.

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The drawings are threatening Leonardo:

 

1389832224_boxelevation.jpg.b560e0a16a2881874cb30428c9cd0a0d.jpg

 

That's where they are planned for the room.

 

From above, the "B" distances are planned to be equal, for timing:

 

1979320315_roomplan.thumb.jpg.0367c7b010d494c3814c709d671faa02.jpg

 

And the boxes themselves - I have the volume worked out at around 350-360 litres, which will allow 30 litres for driver and vent volumes, plus bracing.

 

At the moment, they look like this, but I wonder about internal reflections, and whether the drivers should be offset from the baffle centre:

 

1942313306_boxplansmall.jpg.d014509438c36b7534aa385041fdc1de.jpg

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Would anybody like to comment on any of the artworks above? Are they even legible?

 

I'm almost ready to start sawing, so any feedback would be welcome now, rather than when it's too late. Expensive wood, and it's looking almost like boatbuilding - not such a normal box.

 

I'll probably have to start baffle-down, and build back from there.

 

I've shown the triangular nature of the box, as it hugs the wall and ceiling, but I forgot to say that it's 1500 mm long/wide. If stood on end, that's the same height as my DTQWT-12s, as in my pic on the left. Big.

 

A rough calculation puts the driver cone about 300 mm from the large rear/side walls. That's half a wavelength of 554 Hz, which should be well out of range of this sub. But my question about placement of the driver toward one edge remains - would it help to not have the same reflection from both walls??

 

Bracing is still in the head, but I'm anticipating 4 cross-braces (about 350 mm apart), and 1 fore-and-aft, right up the middle. It'll probably need some others, to halve the longer open surfaces.

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Have a look at these Brian, the first one is a spherical enclosure with woofer (sound familiar?),

https://www.cepro.com/photo/10_loudspeakers_munich_high_end_show/2/

 

The second one is very similar to your speaker layout,

https://www.cepro.com/photo/10_loudspeakers_munich_high_end_show/7/

 

Not looked at all the offerings at the Munich High End show but there are quite a lot of sites with pictures of the shows offerings about.

 

https://hifipig.com

A number of reports here.

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Love it, thanks. I haven't kept up enough with the Munich posts, so will get onto it. I find a lot of these new arty builds would be using skills a bit out of my DIY experience. A bit further from 'au naturel', too.

 

Not a lot of bracing in the Cabasse - could save a bit of work, haha.

 

It brings me to the extreme of my internal reflections question - the sphere would be the worst offender for making a peak at this frequency - I wonder how this is dealt with?

 

The Odeon Audio one is fun. It's kind of saying 'this is what you should have done to make it work'. Looks a bit unstable, whereas mine looks truly rooted ?. Not sure that the bottom horn could be doing anything useful, but it's attractive. After 15 years of working in the Odeon Theatre in Hobart though, I'm not sure I could live comfortably with the name. (We were rehearsing the 1812 in there once, and some dudes from the army let off shotguns - it was a fairly resonant acoustic without an audience. The performance proceeded with different methodologies. Brings a new meaning to the need for 'bracing').

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On 17/05/2019 at 5:27 PM, BioBrian said:

It brings me to the extreme of my internal reflections question - the sphere would be the worst offender for making a peak at this frequency - I wonder how this is dealt with?

The enclosure is too small to affect the sound in the frequency band where the woofers operates.

 

Just like your subwoofer enclosures.   The boxes are small compared to the sound waves.   The boxes have no effect.

On 17/05/2019 at 5:27 PM, BioBrian said:

Not sure that the bottom horn could be doing anything useful

It's too small to be doing anything below about ~400Hz.

 

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3 hours ago, davewantsmoore said:

The enclosure is too small to affect the sound in the frequency band where the woofers operates.

 

Just like your subwoofer enclosures.   The boxes are small compared to the sound waves.   The boxes have no effect.

I'm still uneasy about this. All waves reflect off a hard surface. Waves of any frequency will have about 2 ms before they crash into the cone again, off the back wall(s), however long the wave is. How can this not have an effect?

 

I dug these up from early measurements of (here we go again) my OTHER sub boxes, which are about 750 mm deep, with a flat, unpadded wall behind the driver. Both nearfield measurement, 250 ms gating, one of driver, the other of one of the twin vents:

 

888476816_2016-08-21SB42ventnf250ms.JPG.4d9cd9c7f62df31352289ba6d650d183.JPG

 

Can you say what the nasties are at 222 Hz from both? (Not floor reflections, as mic heights were different).

 

Is it just a coincidence that it's the same distance as a 222 Hz wavelength, driver-wall-driver? Sorry if I've asked this a million times before, but I'm not getting it yet. In such a big box, this is not out of the pass-band.

 

There's also a slight glitch in the impedance curve about this frequency.

Edited by BioBrian
trying to make sense
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1 hour ago, BioBrian said:

How can this not have an effect?

The wave is so so so long that the extra distance it travels to bounce around inside the box, is a tiny fraction of a wavelength.

 

If you take a wave.... and then a copy of it delayed by a small fraction of a WL .... and combine them, they add almost perfectly (ie. there is no effect).

 

Quote

Can you say what the nasties are at 222 Hz from both? (Not floor reflections, as mic heights were different).

 

Is is just a coincidence that it's the same as a 222 Hz wavelength, driver-wall-driver? Sorry if I've asked this a million times before, but I'm not getting it yet

No.  Not a coincidence at all.   It corresponds to about an 80cm delay .... so very roughly ~40cm deep box.

 

 

This is why I said previously "assuming that your have an appropriate low pass filter on your subwoofer".     If you don't have a lowpass that is strong enough .... then yes, what happens at higher frequencies is relevant - which makes the internal (and maybe even external) shape of the box, important.

 

A typical subwoofer would be cut out by 80Hz.     So a -6dB point = 80Hz, with a 24dB/octave slope.   This puts the driver at least  36dB down by 220Hz.     ... but the slope of your rolloff is only 6dB/octave, which is insufficient.

 

The solution here is to set an appropriate frequency response for the driver.   (Recalling that the frequency response of the driver is the only thing that matters).

 

The 6dB octave loss pass of the subwoofer .... also won't match the high pass response of the midwoofer, which will be 12dB.octave if sealed, and 24dB/octave if vented.....   ergo, your subwoofer needs a steeper filter.

 

 

If, on the other hand - if you really want to have this very low slope.   Then you'll have to redesign the box like a midwoofer.    Reduce the depth to move the error, and/or stuff the box to absorb the error.    That's really not a good choice though ..... as even forgetting about this path-length error, I would not want a driver which is under high power and high excursion (eg. while it's playing 20Hz) .... so also be contributing to playing 200Hz.

 

 

 

It seems like you are using a method which kinda goes  "I've decided I'm going to do X Y and Z .... and I'll try to deal with whatever result I get as best I can".     A much much better approach is to define the result you want.... and then figure out you need to do to get it.

 

Success depends on "getting the desired result" ..... not on whether you used (or not) method X Y and Z to achieve it.

Edited by davewantsmoore
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How about providing all the info on the design incl WinISD input page, port length and diameter, location re driver and if there is any internal stuffing used. Then it might be obvious and a solution offered. Playing 20 questions to try to help you is dull.

Z curve too.

Edited by A9X
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Hi Brian,

It's been a while since I've been on SNA, and this is what I've missed.

Already 10 pages long, so I've read only the front and rear covers because it's in the wee hours of the morning and I have to get up at 5am.

I can only give you some brief info at this stage (PM me for more). 

Spheres don't require internal bracing as the shape itself does that already. Bracing reduces flat panels from naturally vibrating. Flat (and/or parallel) panels will produce internal reflections that may present themselves to the back of the cone. Spheres don't exhibit this characteristic, but do suffer from power compression if the enclosure volume is not adequate.

Luckily, I did manage to solve that problem back in 2006.  The prototype is still collecting dust in my shipping container. If I recall correctly,  Upfront had a privileged viewing of it's insides whilst I was living there. Not sure if he can remember or explain what he saw.

 

Those "nasties" look like 'break-up modes' that large drivers normally exhibit. Since they'll be out of the operating frequency range there's no cause for concern. Placing them in the corners utilising the walls and ceiling as a pseudo horn is a nice touch.  Wouldn't worry about offsetting the driver in this application. If you want to be really creative, you could extend the enclosure, turning them into a transmission line.

 

Hope they'll be ready to for a listen when I next pop down your way.

 

Keep up the good work.

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