SKRAM Baltic Birch Twins (build) - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 53 Old 01-26-2020, 08:20 AM - Thread Starter
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SKRAM Baltic Birch Twins (build)

Hello All,

I have been happy with 8 SI HT18s in sealed boxes for the past 6+ years. Being a fan of horns, a recovering member of Subs Anonymous, and hearing a few of Ricci's designs, I was thinking of adding an skhorn to the mix.

However, life/work got in the way and the skhorn did not happen. During that time, Ricci had since released the skram design. Two skrams should be just as good or slightly better than 1 skhorn, right? And the Eminence 21" NSWs were on sale at that same time? The stars seemed to be aligning, so that seemed like a good time to make some wood chips.

Others seem to have had success running sealed drivers up to 30hz and then letting the 6th order bandpass skram/skhorn drivers takeover and run up to ~100hz. I will look to follow what has worked for others. This thread will hopefully cover the success of that journey.

To start that journey, I modeled the skrams in CAD (a huge thanks to Ricci and DSL1 for sharing the dxf files as a basis):






After finishing the skram parametric model to match Ricci's specs, some modifications were made to help with CNC'ing and assembly.

Then each component/piece was nested on a 4x8 sheet to begin the CAM work for cutting just 2 skrams with the grain of the sheet going in the direction I preferred on each piece.
The top row 4x8 sheets are cut twice. Bottom left is cut once. Each of these sheets are 18mm Baltic Birch.
The bottom right 4x8 sheet would be cut once and is 12mm Baltic Birch. Several pieces will require double sided work (the pieces with handle recesses and the back for the hatch recess).


Let's start making some chips. Here is the top right 18mm BB sheet for the first skram that I ended up slightly modifying the layout for:

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Last edited by klipsch; 01-26-2020 at 08:52 AM.
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post #2 of 53 Old 01-26-2020, 08:20 AM - Thread Starter
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Time / distance alignment

Used REW and a Umik-1 to get the perceived distance of each sub in relation to the Left channel speaker from a position just behind the MLP. The goal was to get the 2 skrams, 2 DO sealed subs on top of the skrams, 1 DO sealed sub between the skrams, and 1 DO sealed sub behind the MLP aligned.



The receiver crossover was set to 80hz and volume set to -15.0.

The skrams were high passed at 31hz with an LR 24db/oct and despite the picture, I removed the port blocks. The skrams were run will all ports open.

The sealed subs had no high pass.

Below is the progression of the sweeps that were run. Each was labeled, but is probably hard to read. I think I have things aligned fairly well at this point, though. I measured each sub by itself. Then measured it with each other sub individually. Then added more subs to the mix. Each time I added a sub to the mix, I was hoping to add around 3db. If not, I adjusted the delay until the sweeps got close to that 3db gain. I have a null around 78hz just behind the MLP which I believe is a room node at that spot since switching the receiver crossover to 120hz didn't alter it. However, maybe I should check the L&R alignment with the subs a bit more to figure that out...



The above is a result of adding 4ms to each skhorn, 1.5 ms to the DO sealed between the skrams, and inverting the nearfield sub, while leaving the 2 DO subs on top of the skrams as the reference at 0ms. The single sub pre-out on the receiver is set to 2 feet greater than the L channel to phase align those subs with the L & R channels. Hopefully I did all that correctly and now all subs should be in-sync with the L & R speakers.
lr sealed = 2 DO sealed subs on top of the skrams
c sealed = 1 DO sealed sub between the skrams
skrams = skrams
NF sealed = 1 DO sealed sub behind the MLP (nearfield)

I added a low pass to each of the sealed subs at 35hz LR 24db/oct and ended up with the following:



I ended up enjoying too much music with this config, so no EQ filters have been applied yet to smooth things out. These skrams and NSW drivers are just stupid ridiculous. I've been seeking a more pronounced chest compression for music and that search is over ... @Ricci for the win again
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Last edited by klipsch; 02-09-2020 at 05:14 AM.
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post #3 of 53 Old 01-26-2020, 08:27 AM
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Awesome, that will be a very potent set up. Just a note, I don’t see any pics in your first post if you had any. Sounds like you were supposed to.
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post #4 of 53 Old 01-26-2020, 08:43 AM - Thread Starter
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Quote:
Originally Posted by gpmbc View Post
Awesome, that will be a very potent set up. Just a note, I don’t see any pics in your first post if you had any. Sounds like you were supposed to.
Thank you for the note regarding the pictures. I uploaded them as an attachment to the post. I am able to see them in the post when I am logged in.

Hmmm...I'll try editing and re-uploading. Maybe something weird happened
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post #5 of 53 Old 01-26-2020, 08:49 AM
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How are they sounding with those SH-50's? Would love to hear your setup, any chance you'd CNC two more flat packs and I'll drive from Ohio and pick them up

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post #6 of 53 Old 01-26-2020, 09:08 AM
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Quote:
Originally Posted by klipsch View Post
Thank you for the note regarding the pictures. I uploaded them as an attachment to the post. I am able to see them in the post when I am logged in.

Hmmm...I'll try editing and re-uploading. Maybe something weird happened
I can see everything now. 😎
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post #7 of 53 Old 01-26-2020, 11:18 AM
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Where in Pennsylvania are you? I would love to see/hear these when they are finished!
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post #8 of 53 Old 01-26-2020, 04:43 PM - Thread Starter
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Quote:
Originally Posted by mcallister View Post
How are they sounding with those SH-50's? Would love to hear your setup, any chance you'd CNC two more flat packs and I'll drive from Ohio and pick them up
Still working on that setup! Ohio is 6+ hours west and would be quite a drive for a few hundred pounds of wood and metal

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Where in Pennsylvania are you? I would love to see/hear these when they are finished!
Hello from the great Commonwealth of Pennsylvania. I'm over in the southeast part of the state

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I can see everything now. 😎
Thank you again for the note!
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post #9 of 53 Old 01-26-2020, 05:02 PM - Thread Starter
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With each piece cut for skram 1 and a quick sanding of a few of the cut edges, it was time for a dry fit. The top side in the picture (right side when skram is upright) needed a few love taps in the front to fully fit flush. One loose clamp across the top prevented the top and bottom (left and right sides in the picture) from moving in the dry fit. Things fit very well, so gluing should be ready to go. Will do some pre-staining in the hard to reach areas to make finishing a bit easier too. In addition, the front piece needs a cut at 45 degrees on bottom for that extra super special sound output it will make.




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post #10 of 53 Old 01-27-2020, 02:22 PM
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Hello from the great Commonwealth of Pennsylvania. I'm over in the southeast part of the state



Bummer. I'm in the northwest part of Pennsylvania!
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post #11 of 53 Old 01-30-2020, 06:44 PM - Thread Starter
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Here is some more progress on skram numero uno...

Who likes to look at wood, adhesive and clamps?
Get your PL3 and clamps fix below ...

Glued the internal pieces and then put glue on the right side of the skram (bottom of picture). The left side of the skram (top of picture) is not being glued just yet, but is being used to keep things where they should be for the next stage of gluing. I thought this would work well for keeping things aligned.


Another shot while in the process of doing some quick and dirty stain work for places that will probably never be seen again once fully assembled.


From the back/top of the skram for this first round of assembly. Nothing to be done at this step other than waiting for the pl3 to cure and do its thing.


"Bet you can't get 10 clamps on the hatch assembly!" ... challenge accepted ...



A day later, I went for the homerun and attempted to glue all the remaining pieces. "You can never have too many clamps" ...



The pl3 gave a lot of work time which was needed for doing this many pieces at once ... I may or may not have wondered if doing the rest at once was a good idea ... but it seems to have all worked out.

For the adhesive, I bought 2 28oz PL3 tubes, but ended up using about a half tube of pl3 or a little less all in for this first skram. I think I should be able to do both skrams with just this one tube. For those that may want to store open tubes, I have found this process to work well: I wrapped some aluminum foil around the nozzle to make a cap, then removed the foil cap and set aside. Next I used petroleum jelly around the base of the nozzle and re-added/slid on the aluminium cap. The jelly and aluminum seal pretty well and apparently the moisture in the jelly does not interact with the PL3. To seal the tube a bit more, I put some cling wrap around the whole nozzle/aluminum cap/jelly. PL3 cures with moisture IIRC, but this seems to keep the moisture out for several weeks in a climate controlled storage area.

On another note ... while assembling, i realized I made a mistake. I forgot to stain the inside bottom piece while I was clamping everything together. Woooopps. I'll remember that for the twin skram coming after this one. It shouldn't be too bad to add stain in there ... I hope.

Next up is sanding, staining, and applying some lacquer. These are ultimately going behind a screen, so I am not planning to make these glossy or buffed. However, in case they end up on the other side of a screen some day, I hope to make them look respectable.

All in all, I think the build is going well ...
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Last edited by klipsch; 01-30-2020 at 06:47 PM.
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post #12 of 53 Old 01-31-2020, 09:57 AM
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Good looking work.

FYI when you go to install the drivers it is tight in there. The side handle holes help out.
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post #13 of 53 Old 02-07-2020, 01:53 PM
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Is this done yet? I'd really like to see some measurements. This is on my short list of possible next projects.

Looks nice so far.

Why do the braces protrude through the hatch lip?
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post #14 of 53 Old 02-07-2020, 04:29 PM - Thread Starter
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Used an orbital sander first with 80 grit and then with 220 grit to get the baltic birch decently smooth for staining.









Used minwax's ebony stain to match the other boxes in the room where these skrams will be going.



After the stain dried, some minwax brush on lacquer was applied. Here is the first coat. Since these will be going behind a screen, the intent is to not make these glossy or too reflective. This is my first time using a brush on lacquer. The stuff smells terrible and definitely needs to be applied outside. Even with sliding glass doors open, windows open, fans going, etc. this stuff didn't leave the room for 24 hours. The next box will definitely be done outside of the house!

Using the pl3 and some water, I attached the metal handles on the hatch and the top. I used the water to wet the underside of the handles before applying the adhesive. Based on the dry fit, I figured the ability to remove the side handles in the future for driver replacement or repair would be a good move. Using some gasket tape that I cut in half lengthwise, I outlined the inside and outside edges of the side handles to hopefully get a nice tight seal.



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post #15 of 53 Old 02-07-2020, 04:52 PM - Thread Starter
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Quote:
Originally Posted by Samps View Post
Is this done yet? I'd really like to see some measurements. This is on my short list of possible next projects.

Looks nice so far.

Why do the braces protrude through the hatch lip?
Make that list shorter

I liked having the braces go in to the hatch for rigidity during the gluing stage and for extra assistance with the fit/assembly. It may not be structurally needed, but that is the route I stuck with for what I thought we be facilitate ease and simplicity.
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post #16 of 53 Old 02-07-2020, 05:28 PM - Thread Starter
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Using some dish-soap in water, along with some 600 grit wet/dry sandpaper, I tried to smooth and knock down the layers of lacquer. I really liked using the lacquer (outside of the smell) as no sanding was needed between coats. I don't think I had enough soap at first as I was getting a little bit of a white "waxy" build up. Once I added some more soap, the white waxy stuff went away and wet sanding went pretty smoothly. Once I thought things looked and felt pretty good, I wiped the cabinet dry. Using some 0000 steel wool, I went over the cabinets again for a light dry sanding. This really gave a nice smooth and matte finish which hopefully protects the wood and doesn't reflect behind the screen. Using a damp cloth, I wiped the cabinets down a few more times and carried it over to the new resting place.



There were some spare pieces of 1" thick foam from the diysg 88s I built years back. Those seemed like a good idea to help reduce any badness that might bounce around inside the box. Maybe I could or should add more ... I tried to keep everything away from the ports as much as possible. The same gasket tape around the handles was used around the hatch (cut in half and run around the inside and outside again).



Inside shot of the 1" padding. The open cell foam is from a chain fabric store and is meant to be used for crib bumpers, outdoor seat cushions, and such.



The obligatory driver shot:


Putting the driver in was not too bad. I grabbed the driver with the cone facing down. My grip was over / under; one hand supinated and the other pronated. Both hands were across from each other grabbing the frame. This driver is quite "top heavy" when the magnet is on the top, but this grip worked very well for me to control and lower the driver down in to the skram. It really was not difficult at all. However, if the side handles were both in, I would imagine it would be an extreme PITA.



I have used SDS heavy duty simpson strongties in the past and they have worked well. I used them again here. They are 1/4" x 1 1/2" with a 3/8" hex head. I tried to make sure the screws would not be underneath the handles and be as far away from any near edges of wood as possible. The impact driver fit nicely in the cab and the driver seems very secure. I did run gasket tape (a thin strip that was cut in half) on the inside edge of the driver cutout. I don't think I need to do this for the NSW, but I figured it couldn't hurt ...



NSW through the handle hole, now you see it



Now you don't

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post #17 of 53 Old 02-07-2020, 05:37 PM
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Beautiful work!
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post #18 of 53 Old 02-07-2020, 05:40 PM - Thread Starter
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First skram is built

Using some left over 12 ga wire, two same length cables were made with some solder and crimping. This skram got one of the cables.



The clearance between the handles and driver can be seen here, along with the soldered wire ends to make inserting the wires in to the terminals a lot easier/cleaner.



All wired up


And ready to go
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post #19 of 53 Old 02-07-2020, 06:12 PM - Thread Starter
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skram 2

After some more CNC work



Some more dry fitting







And some more pl3-ing and clamping









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post #20 of 53 Old 02-07-2020, 06:28 PM - Thread Starter
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and more staining and lacquer-ing (outside this time). It was cold here in PA for the 2nd skram. I ended up applying the lacquer in about 43 degree weather outside under cover. The temperature seemed to have no ill affects on dry time or curing process. However, there was no smell in the house which was a big improvement over the last one.





The skram twins are now in the same room



Since the alignment of the holes with the first skram driver worked so well, I tried to duplicate that alignment as best as possible with the second skram and driver.



All secured



with these



Foam cut and applied with some 2P-10 activator adhesive (without the glue)



And then moved and all hooked up



The skram twins are joining 6 HT18s in 3 DO sealed boxes behind the screen with 2 more HT18s behind the first row of seats. The plan is to utilize the skrams and pro driver for the "upper" bass frequencies and utilize the HT18s down low (under 30ish hz). Integration and measurements to come later ...
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Last edited by klipsch; 02-11-2020 at 04:29 PM.
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post #21 of 53 Old 02-08-2020, 06:21 PM
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And then moved and all hooked up



Tell me about your LCR speakers? Are they DIY synergy horns?
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post #22 of 53 Old 02-09-2020, 05:07 AM - Thread Starter
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Tell me about your LCR speakers? Are they DIY synergy horns?
Bill Waslo and a few others have made and shared diy synergy / unity horns. I am not part of that prestigious group. I bought these synergy horns pictured.
They are SH50s from DSL.
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post #23 of 53 Old 02-09-2020, 12:18 PM
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Bill Waslo and a few others have made and shared diy synergy / unity horns. I am not part of that prestigious group. I bought these synergy horns pictured.
They are SH50s from DSL.
Looks impressive! I have only seen videos of them in action, and love the concept. Hopefully i get to hear a demo one day.
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post #24 of 53 Old 02-09-2020, 02:16 PM
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Looks great. How's the integration going?
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post #25 of 53 Old 02-10-2020, 05:44 AM
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Looking good!

________________________________________
DIY Mid Hi Speakers Build - SKHorn Build - Gjallarhorn Build SOLD - Othorn Build SOLD - SEOS 12 Build - Other Gear: JVC NX5 - Seymour Matinee Black - QSC CX4.5Q amps x 5 - QSC Q-sys 110f - Speakerpower 6000 x2
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post #26 of 53 Old 02-10-2020, 01:17 PM - Thread Starter
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Looks impressive! I have only seen videos of them in action, and love the concept. Hopefully i get to hear a demo one day.
There is something about the full point-source of the synergy horn and the crazy crossovers in them that my ears appreciate more than any other speaker I have heard. It does some "spacialization" stuff that I have not heard from many speakers (the JBL M2s come to mind as another). For example, when I listened to Chocolate Chip Drip for the first few times on the SH50s, I had to double check I didn't have 7 channel stereo or some other dsp program on ... multiple times in the track I believed I had sounds to the side and behind me which made me second guess the receiver config ... I also hear things in various tracks that I just do not hear in other speakers. When we did a GTG back in 2014, I realized I was not the only one who thought so ... everyone always says that every speaker design has trade-offs and everyone's ears are different ... I agree, but for my ears, the SH50s are the current reigning champs ... if you happen to be in PA, stop by for a demo

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Looks great. How's the integration going?
I did a lot of playing around last night for fun. That dip I have at 78hz is from my failed integration, and not the room like I mentioned before. I was wrong on that front. I did a lot of sweeps with different xover points, different roll-offs for the xovers, running the front 3 as large with "extra bass" (sending bass below 120 to the sub preout), etc.

I have been using the minidsp 10x10 plus the receiver in different permutations for which will handle the frequencies being sent to each channel. I think I've made things more complicated by doing so, but I did enjoy the different results and learning, both on the REW graphs and what my ears heard with the different configs.

I think my plan at this point is to xover the front 3 with the receiver around 80hz. I enjoy music just as much as movies in my space and would like to have both content optimized. Running a lower crossover might be slightly better for the music. I am trying to do both music and movies with one configuration, but maybe I should just go with 2 configs (1 for each). One crossed lower for music with extra bass (4 way designs with L and R separation for the subs) and one crossed at 80 in the receiver for movies with no extra bass.

I ran a few demo scenes with a few of the configurations I liked during the "playing around" with The Book of Eli scene with the cannibals, the opening car/deer/hunting scene with I Am Legend, and the Edge of Tomorrow opening. These NSWs in the skram never even blinked with the sinbosen 20000q powering them. That makes me want to try the skrams with 3 ports blocked to get around a 14hz tune. I want to get the current config working first though with a 31hz tune as the HT18s are pretty flat in my room down to 10hz.

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Looking good!
Thanks sir! How are your skhorn and skrams doing these days?
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post #27 of 53 Old 02-10-2020, 01:42 PM
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I ran a few demo scenes with a few of the configurations I liked during the "playing around" with The Book of Eli scene with the cannibals, the opening car/deer/hunting scene with I Am Legend, and the Edge of Tomorrow opening. These NSWs in the skram never even blinked with the sinbosen 20000q powering them. That makes me want to try the skrams with 3 ports blocked to get around a 14hz tune. I want to get the current config working first though with a 31hz tune as the HT18s are pretty flat in my room down to 10hz.


Thanks sir! How are your skhorn and skrams doing these days?
Look forward to your impression.

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post #28 of 53 Old 02-10-2020, 06:18 PM - Thread Starter
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Front LCR set to Large in receiver

Minidsp routing LCR signal to each sub

LCR high passed at 50hz bw 24db/oct

skrams high passed at 31 24db/oct and low passed at 90 with bw 24db/oct

sealed subs low passed at 38db with bw 24db/oct and high shelf PEQ of -5 per HT18 pair at 20hz.

1/48th smoothing with receiver at -15.0 and at 0.0

Not too bad of a place to start ...

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post #29 of 53 Old 02-10-2020, 07:31 PM - Thread Starter
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For more fun, I crossed everything at 100hz

This is running 6 sealed HT18s and 2 skram NSWs with the L&R SH50s

HT18s are from 0hz to 100hz bw 24db/oct

skram NSWs from 31hz bw 24db/oct to 140hz bw 24db/oct

No nearfield HT18 on at this point and zero boost applied. All cutting PEQ filters. Sounds very good. Feels very good. Will integrate the nearfield HT18s and PEQ the SH50s later this week.

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post #30 of 53 Old 02-11-2020, 01:04 PM
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The more I look at these the more I like the finish you put on these.
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