SevenSixTwo - InDIYana 2018 Coax Design

[rpb]

Well... Tonight the engineer in me is annoyed. I did some quick studies on both the sealed coaxial section and the vented woofer section with DATS. I measured different levels of poly fill stuffing from gross over-stuff (16oz in 0.4 cuft) to totally empty while monitoring Qts (Qtc for sealed enclosure) in DATS. BassBox model said I should get around 0.78 for Qtc, but I couldn't measure lower than 0.89 when grossly over-stuffed. Of course that sounded awful even with the impedance sweep. I settled on a natural sounding impedance sweep with around 6oz of fill for a Qtc of about 0.91. I'm sure this will be fine, but I get perplexed when the analysis doesn't match reality well enough. Similarly, I took some foam lining out of the vented cabinets to see if my Fb would rise back to 31hz. I got excited when removing the foam from the top of the port and the ceiling in that chamber measured an Fb of 32hz while I held the driver in place by hand. Unfortunately the moment I put the screws back in place for the real seal the Fb dropped right back down to 27.8 Hz. I'm having trouble modeling a vented cabinet with my port geometry in BassBox that ALSO has an Fb of 28Hz. BB insists my Fb should be 31Hz. I am probably making a mountain from a mole hill, but again, it bugs me when I can't get the simulation tools to match reality. Guess I'm putting too much faith in the underlying assumptions that are used in these programs. Since the only way to bump the Fb up at this point is to shorten the port, I'll be living with them as they are and seeing what I can get from XO designs. I was t setting out to make an EBS alignment for this cabinet, and the idea of decreased power handling doesn't thrill me since the esoteric driver has fairly high power handling specs. It feels kinda wrong not to let that sucker slam when it can, but I'd have to rebuild the cabinets from square one to change the port now. Guess that's what I get for trying a slot port! -- Sent from my SM-N900V using Tapatalk

You could cut the port from the bottom of the box with a router.

[1100xxben]

What length port did you model, and what is the physical length of your port? When you put a slot port in that utilizes the bottom and side walls like you have, it will effectively add length to your port. Also, since your port is on the bottom, placing it on a large flat surface (like a table top) could also extend the effective port some more. This could be part of the reason for attaining lower actual tuning than you have modeled.

[1100xxben]

Thanks Marvin! Yeah... gravity clamps are the best Back in my solar car days in college we used old lead acid batteries for the same thing. Three cheers for high density materials!

I know I'm a little late on this one, but I had a woodworking project where I needed some serious "gravity clamps", so I bought a dozen bags of water softener salt. Fairly easy to move around and I knew I wasn't wasting any money because I would eventually use them. I also had a few old brake rotors and a 1 HP motor sitting around that I threw in top.

Another alternative is to assemble everything on a workbench that is open underneath and use ratchet straps wrapped around.

[KEtheredge87]

What length port did you model, and what is the physical length of your port? When you put a slot port in that utilizes the bottom and side walls like you have, it will effectively add length to your port. Also, since your port is on the bottom, placing it on a large flat surface (like a table top) could also extend the effective port some more. This could be part of the reason for attaining lower actual tuning than you have modeled.

Hi Ben, I made several iterations of bass-box models and port length adjustments before zeroing in (check out post number 70 in this thread). I had measured with DATS until the Fb matched BassBox. The port length is modeled as 12.75" long x 4" wide x 0.75" high. The internal width of the whole cabinet is 8 inches, so I made a slot port tunnel from MDF and glued it to the bottom of the cabinet before opening up the baffle's slot entrance with my router and flush trim bits. The baffle thickness itself is 1.25", so the actual port length inside the cabinet is ~11.375". On the front baffle, the slot port has a flush face without roundover. This is likely to transfer to the final design even though I did my CAD work assuming I would add a 3/8" roundover to that edge for a bit of flaring. Inside the cabinet, the slot port terminates with a 1/4" roundover applied to the internal edges of the slot to be a little bit of a flared end. I doubt this little of a flare really translates to the modeling programs well, so I guess you could consider this a single flush end with free other end?

You make an interesting point that I hadn't considered regarding the port effective length. The recent test sweeps I had been doing were on the carpeted floor with the cabinet laid on it's side. I'll take another sweep here in a bit with the cabinet back up on the testing stand. That should remove any possibility of external influence.

[KEtheredge87]

OK. Thankfully I was doing something silly in BassBox. While I was accounting for known volumes of stuff inside the cabinet, I included the volume occupied by the slot port itself. I was double counting that, since the program already takes into account the volume occupied by the port (including the wall thickness). Geeze... I wish my mistakes were easier to catch!

I also triple checked all settings on the BB study, and ended up replacing the driver values with my most recent DATS T/S parameter measurements, which surely helped the realism factor a bit. While all of that was spinning around my head, I decided to take out a bunch of the lining foam, assuming that I was either choking the port, using way too much lining foam, or just accidentally augmenting the effective length of the port from the inside (thanks 1100xxben for sparking the idea). I left a bit of foam on the back wall, but nothing else, and this gained about 1.5 Hz of Fb.

Now, my Fb is measuring at 28.9 Hz, and BB is modeling at 30.4 Hz. I'm comfortable calling measured Fb within 1.5 Hz of the prediction as good enough. This gets my F3 back into the mid-low 30's without as much of an EBS-style alignment, where I was flirting with F3 in the low 40's during my mistake earlier today.

Maybe... just maybe... I can get back to taking some honest to goodness XO design measurements tomorrow! What a detour today has been.

[augerpro]

Looks like you're working through the variables well (use measured TS, account for your volumes, etc.). One major factor that doesn't get near enough attention though, is end correction, specifically the fact that it is just assumed by software and the user never considers/questions it. Most software just assumes one, some assign one based on user settings and let you see/change it (unibox), and the best (Soundeasy) don't bother with an end correction at all. They leave it to the user to determine, because if varies so much with different construction methods there is no point in guessing. So you do have to have some experience in working out what you need for EC, but in the process you'll learn a lot more about how different construction methods impact it.

[KEtheredge87]

Looks like you're working through the variables well (use measured TS, account for your volumes, etc.). One major factor that doesn't get near enough attention though, is end correction...

Hey Brandon, you've got that right! This particular slot port doesn't quite match any of the pictures that bass-box includes in it's port suggestions. Part of me wanted to call it "Two flush ends" since I had one legitimate flush end opening on the front baffle, and a barely rounded over internal entrance. The only thing keeping me from that is that the port has 3/4" of material and then the floor of the cabinet on the bottom side of the opening, and 3/4" of material thickness on the top side of the opening, rather than a full length baffle, which is what the picture seems to depict. I've got to be getting some internal reinforcements since the port opens near the floor, rather than floating in the middle of the cabinet like BassBox's picture.

Unfortunately, I don't get a real chance to fine-tune the end correction value in Bassbox, I'm left to pick from their options. The two flush end choice gave a predicted Fb that was almost identical to my measurement, but I hesitate to use that since it feels inappropriate to the actual cabinet.

[augerpro]

What EC did Bassbox spec? Can you change it? If so, put in 100%, then backwards sim the Fb until it matches the measured Fb in DATS. Calculate the actual end correction. Now you know that for a smallish box, with a certain lining/filling, and slot port with approx. that design, that is the end correction you should use in the future.

[KEtheredge87]

Hi Everyone, Quick update this morning. I finally got to take some FRD measurements for all the drivers last night. I fumbled through the blending process for nearfield and farfield measurements and came out with what I think is correct. PCD is doing something interesting though. All of my FRD response files appear to be smooth, yet there's a funny little discontinuity in two places (biggest at 1500Hz, little one at ~5500Hz).

Wonder what that's about?

[PWR RYD]

Boy that tweeter response is going to be a bugger to work with.

[wogg]

Looks like you're done.... 40Hz to 20kHz +- 20dB

[KEtheredge87]

Boy that tweeter response is going to be a bugger to work with.

Hey Craig... yeah, probably going to be a challenge. I'm hoping I can do enough with it to have it sound ok. I'm also hoping i made the blended files correctly. I measured my farfield at 32 inches as opposed to the 20 inches that I figured on after reading Jeff's white paper. Maybe this tweeter response is why no one has left any reviews on the PE page for the FTX0617 coax . Time will tell what I can do!

[KEtheredge87]

Looks like you're done.... 40Hz to 20kHz +- 20dB

Haha, who knew a three way XO would be so easy! I think I'm going to re-create the blended files tonight and see if that little anomaly in the blended PCD response doesn't go away. Unless I figure out some place that I royally screwed up post-processing my raw FRD files, It's time to jump headlong into PCD and get to designing.

[jhollander]

The little blips might not be anything depending on if your measurements were smoothed. Anything less than 1/48 smoothing might be hiding a spike you will want to see.

IMO, typical tweeter in a deep guide. It should fill in off axis, but that will be the x-o challenge.

[KEtheredge87]

The little blips might not be anything depending on if your measurements were smoothed. Anything less than 1/48 smoothing might be hiding a spike you will want to see.

IMO, typical tweeter in a deep guide. It should fill in off axis, but that will be the x-o challenge.

Thanks John, I used 1/96 smoothing when recording with the OmniMic just to make sure that everything I could possibly see would be brought into PCD. I know that everything gets 1/48 smoothing by default in PCD. I was intrigued when all the individual files came through relatively smooth at 1500 Hz, but the system combined line has that odd blip.

I did take ~20 degree off axis measurements of the coax as you recommended, but I haven't imported those into PCD yet to see how they look. I assume those 20 deg files do not get combined with nearfield measurements?