Faux Pas

[DrewsBrews]

I went outside and did some quick and dirty measurements. I'm not equipped for this really. Different amp, so wouldn't be a 1-1 comparison. Brown: 7.5" cardboard tube. Green: original 4" port.

I did a super dirty measurement about 2ft away aiming at the woofer (holding the mic) just to see what that gave me. (violet)

So the ~100hz peak in the measurements up to this point appears to be a room mode. Makes sense as my room is just a few inches over 11ft wide. Most of my measurements are within 1ft of the center between walls. Except the measurement I did against one of those walls, which did not have the 100hz peak at all.

But the mid 70s peak must be something internal.

[Steve Lee]

It would be nice to apply some EQ and see what it needs in XO from ear experimentation . . .

[DrewsBrews]

It would be nice to apply some EQ and see what it needs in XO from ear experimentation . . .

I am listening to it as I go, but just one speaker. I disassembled the crossover for the moment. I've got some days off the week after next off so I'll probably do my final tweaks and assemble the crossovers then.

I tried an open box GRS 12PT-8. It was a little flatter on the response peaks and didn't seem to have the 70-80hz peak, but lost too much sensitivity to be acceptable to me.

I've got some 3" precision port flares that have the exact same flange OD as these 4" ports.. Since the MCM woofer picked up so much low end with the extended port, I'm thinking of just linking together the 3" port flares for a roughly 3"x5" port and using that.

[DrewsBrews]

Ever since I saw the port response, something kept telling me I had seen a similar response before...

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(Example of the bottom end response of a transmission line)





















So I've pretty much made a transmission line. Which, is not too surprising as that probably happens in most tower configurations to a certain extent. In the one tower I did previously I deliberately put the port about 1/3 up the tower in an effort to stay away from these kind of issues. This time I really didn't have a choice unless I did rear port. But at least now I know what it is I'm up against. The foam must be reducing the second plateau and completely killing the 3rd. It could be better though. Someone mentioned rock wool previously. I've got some fiberglass insulation I could replace the foam at the bottom of the enclosure with, and cover it with some scrap grill cloth to keep it somewhat contained. hmm.

[DrewsBrews]

3" double flare port seemed to do fine without chuffing during stress testing. Unless things rattling in the house just covered up any chuffing noise lol.



I've used 2" precision ports before and they have a nice satin texture on the mounting ring portion. The 3" version uses a much larger glossy texture that reminds me of cheapy blow mold tool cases. Neither that texture, nor the gloss of the rest of the port jived with the rest of the speaker. I sanded off the texture, then rubbed the whole thing down with steel wool to give the rest some tooth and sprayed a matte clear.

Per the instructions: "Note: On Plastic Maximum paint adhesion and durability is achieved in 5-7 days.".... Ugh, better follow it or I bet the screw heads will crack the clearcoat.

[Wolf]

Where did this information come from? It may give the lowest possible anechoic F3 for a given woofer (in sims) but there are other important things to consider.

Even peaks only seem to make tuning very close to box resonance, therefore tuning to the Fs of the box/driver system. This is known as the best way to keep Xmax to a minimum. It is not however the way to get the lowest F3, which usually involves tuning quite a bit below Fs.
Agreed there are more things to consider.

Wolf

[DrewsBrews]

I think my point was missed.

My resulting enclosure happened to be close to the calculated default enclosure size. Not saying it is actually ideal in any real sense.. I was just using the impedance peaks from that enclosure as a telltale leading to a theory that the actual woofer parameters are not too far off from the published specs.

[DrewsBrews]

Messing around in WinISD with the new port... At say 35watts they breach the meager 3.75mm xmax at ~28hz, 96.6db. If xlim might be about double xmax, then danger zone limit for that power level is ~22hz @90db. Port air velocity at that power level peaks ~20.5m/s. Hopefully the double flares help handle that, if not then.. oah well I guess. The rest of the woofer output peaks @~109db by that point. Probably much louder than I would normally be listening to unless I was just testing the limits. Right now my listening distance is right around 2meters. Might add a couple feet once we rearrange the living room.

[DrewsBrews]

Not much to report yet. Focusing my days off to do some drywall work at my home. Decided to cut out early today to keep from burning out my arms/shoulders... So I sanded out the silicone and paint from the port holes. Found my clamps were way too big to fit in there. Made a run for some tiny clamps so I can glue in little blocks to bolster the location for the precision port screws.

[DrewsBrews]

I got the ports installed. I went to take a new set of measurements and had a facepalm moment. Hard to explain without making a whole paragraph, but it seems my receiver will auto split bass to both speakers and if I forget to unplug the other speaker it will skew the results. I've never noticed this before and don't know if I somehow enabled the setting while trying to get hdmi audio working for a dvd player I plugged in the previous week... or if it has always been doing that when I forget to unplug the other speaker.

Once the other speaker was unplugged it became apparent the new port really wasn't making the difference I thought it was. The whole situation was frustrating enough that I put it all down for a few days to reflect.

Today I decided to pull a Dayton PA310 and give it a try. I really hate to pilfer from a completed build, but they are currently not in use anyway. It seems a bit flatter and with better bass response. And without losing sensitivity. Not sure if the original port would have done better/worse. hmm....

[Steve Lee]

Glad you had this little epiphany and decided to go for a sanity check with the PA310 driver, Drew.

Interested in your further findings as I am working a PA310 into a cabinet vol of 1.3 cu ft net right now . . .

[DrewsBrews]

Finding some time in the holiday whirlwind...

I still wasn't convinced this project was worthy of the PA-310. So I tried some of the dirt cheap MCM 12s in 4ohm flavor and took over the living room this afternoon. Think I may be near ready to finish them up.

I was able to go back to 2nd order on woofer-mid XO. Total component count of 10+4 resistors overall.



Green=On axis, Blue=~30deg off axis. Big woofer dips I suspect are room interactions/floor bounce... Or because they are pieces of crap. I'll stick with room interactions for now to make me feel better

Next up, distortion



Distortion from the mid shoots up quite a bit down low, showing it really aught to be crossed above 1k to live its best life. But I suppose it is still within reasonable levels? That tweeter is certainly holding its end of the bargain!

And finally the resulting impedance...



Not exactly stellar but I've seen some pretty funky curves that were deemed acceptable so I really don't know what I'm talking about.

Anything I should look into fixing before getting out the soldering iron?

[Wolf]

I'm seeing 57dB for the 850Hz HD peak, what output was the fundamental at? 95dB?
if you are -25dB, that is the minimum attenuation for distortion modes while -40dB is considered standard allowance. -50dB is where it is then completely inaudible.

Wolf

[DrewsBrews]

Looks like 95.7db @850hz. So pretty much -38db at that point.

I know I'm stretching the capabilities of $20 drivers. I'm not surprised they aren't perfect. I'll stew on it a bit. Maybe I'll muster up enough effort to try bumping up the XO point a bit.

[PWR RYD]

You made those ports look SO much better! I'm going to copy what you did with them. One thing I have done with my Precision Ports in the past is add 4 more holes for twice the mounting screws. Four just didn't look or feel like enough.