My First DIY Speaker - Mini Bass Array Module (Teaser)

[weltersys]

Originally Posted by mattlong8

My attempts to make the fiberglass black kind of suck but oh well.

It might save what little sanity I have left if no one else suggests I build a horn. Thanks for understanding.

Matt,

At the rate this project is going, probably best to forget about horns or bass reflex cabinets too .

You need a boat load of pigment to get glass black.
Most fiberglass products use a colored gel coat, then hand lay the uncolored glass or use a chopper gun to blow the glass.
Even when the glass is pigmented all the way through, it turns white when scratched. And of course, that white stuff is very itchy, when I used to use fiberglass we painted the cabinets as soon as road abrasion sanded off some of the high spots.

I can remember our truck sleeper berth was always itchy from glass fiber brought in on our clothes, I'll never glass a speaker cabinet again unless it is for use in a permanent outdoor installation.

Anyway, (hopefully) someday you'll look back on this project and laugh, I know I have had a chuckle re-using parts from projects that took hundreds of hours but never worked very well.

Have fun, and good luck finding work you enjoy!

Art

[mattlong8]

Thanks Art.
I feel better knowing that I'm not the first person to have a lapse of reason that resulted in hundreds of hours of work. I have a bunch of questions for you but I'm going to get back to it for now. I awoke this morning to my girlfriend telling me that her new to her car will be getting here tomorrow(a day early). The arrival of the car has been my drop deadline for finishing as I will lose the bulk of the garage space that I was using for most of the construction. I hope all the electronics work when I start testing this evening.

-Matt

[mattlong8]

Well, close but no cigar as they say. I got it put together but the paint is making one of the handles fit poorly. I'll have to fix that after cleaning up the garage. Also when I plugged it in, the regulator on the axillary power supply for the fans blew. I'll have to make sure everything is ok tomorrow.

Oh I missed your post earlier Joseph. Thanks! I appreciate the encouragement, I needed it right now. I just hope some useful data comes out of all this and is not just a horror story sound guys tell around the camp fire. At least my sanity is holding for now, my mental outlook is better than last night.

-Dumb ***

[mattlong8]

Well Folks,
Prototype B is done. In the last stretch when assembling and installing the electronics I changed the design to enclose the amp and dsp separate from the rest of the cabinet. I had some 5/8 sheet acrylic left over from another project and made the electronics enclosure out of that. Not only is it quite heavy but it takes up a lot of space. After building it and dry fitting I saw that it came between 2-3 inches from the internal port flare. I knew this was not good at all but at that point I just wanted to be done. I didn't permanently mount the enclosure to the cabinet but I'm not sure I can get it out with out removing a cross brace. When I did a first listening test the output was low and loud but I definitely heard port resonance and harmonic distortion. The measurements I took a few days ago confirmed this. Although I missed my target alignment I do feel the results will be beneficial to the next (and final) prototype and I hope to the diyAudio community at large.

I deleted the impedance measurement I took on accident but I was pretty close on the tuning freq of 33Hz. I it appeared to be about 32.5. I only kept the near-field measurements as even the 1m was dominated by room modes. All measurements were taken indoors with a DBX reference mic, Presonus 1818 preamp, and Macbook pro running Smaart 7.3.

Transfer Function 4"(pink) and 6"(blue) from midpoint between woofer and ports no smoothing


Transfer function unwrapped phase


Transfer Function Magnitude. Green: 1" inside port, Yellow:1" from top of woofer just inside the surround, Pink 4" from midpoint between woofer and ports.


Phase same as above


Group Delay of Woofer and Port


40Hz and 51.6Hz sinewave measured separately.


150Hz and 300Hz sine wave measured separately.


49hz sine(pink), 180Hz sine (blue), 49Hz+180Hz dual sine(white)


Girl friend is home, Analysis soon to come.

-Matt

[weltersys]

Matt,

Glad for you to be done, some projects seem to take forever.

Looking at the port output, the tuning looks to be about 37 Hz.

What voltage were the sine wave tests done at?

Art

[mattlong8]

Doh,
I was about half way through a write up and lost all the text when I clicked go advanced =(.
Anyway, Hi Art! Thanks for checking in.
The impedance graph that I lost had a minimum between the peaks between 32 and 33, the transfer function measurement has 33Hz as the -3dB point and yet the port measurement indicates 38Hz. Do I just have my lingo wrong? I get confused between Small, Beranek, and contemporary guides using different terms. I'm under the impression that Fb=F-3=tuning freq.
I don't have a measurement of the drive voltage, I was pushing the pink noise fairly hard but for less than a minute for each measurement so as to not go in to thermal compression. I doubt that the vent was compressing even though it was whistling at 300hz . I suppose since the alignment is off I could have been around xmax. With the amp inside the cabinet I didn't have it setup to measure. I'll see if I can run some more tests today as I don't feel like doing all the calculations along the signal path to figure out what voltage the woofer was seeing.
I'll also take some measurements with the crossover points in the miniDSP not bipassed. I suspect that the large amount of harmonic distortion, especially odd order, that the port will still resonate even with a low pass 24dB BW at 120Hz.

I'll also retype my analysis of the measurements later today, this time in a text editor first.

Any suggestions on specific measurements welcome.

-Matt
PS here is an image from the analysis work I've started:

[weltersys]

Originally Posted by mattlong8

The impedance graph that I lost had a minimum between the peaks between 32 and 33, the transfer function measurement has 33Hz as the -3dB point and yet the port measurement indicates 38Hz. Do I just have my lingo wrong? I get confused between Small, Beranek, and contemporary guides using different terms. I'm under the impression that Fb=F-3=tuning freq.

I'll also take some measurements with the crossover points in the miniDSP not bipassed. I suspect that the large amount of harmonic distortion, especially odd order, that the port will still resonate even with a low pass 24dB BW at 120Hz.

I'll also retype my analysis of the measurements later today, this time in a text editor first.

Any suggestions on specific measurements welcome.

-Matt
PS here is an image from the analysis work I've started:

Matt,

I simply run a sine wave sweep and measure the excursion minima to determine Fb, it is easy to see the difference of a 1 Hz on a cabinet like you (and I ) built.
The port resonance (and frequency response) of both our cabinets look the same, even though cabinet dimensions are quite different, so I can't agree with your "Proposed Sources of Resonant Peaks", they are simply the result of the port tube.
Hornresp also shows the same peaks and dips (though exaggerated) as our cabinets exhibit, regardless of the dimensions chosen for the box, as long as the volume remains the same.

A word of caution, the Lab 12 can be burnt in a matter of minutes with sine wave testing when approaching Xmax in your cabinet, it can only handle 200 watts of sine wave continuously.

Art

[mattlong8]

Thanks for the help Art.
You input has been very helpful.
I know who I'm going to for future sanity checks

-Matt

[weltersys]

Originally Posted by mattlong8

Thanks for the help Art.
You input has been very helpful.
I know who I'm going to for future sanity checks

-Matt

The first couple months of 2008 were trying to my sanity, testing Lab 2x12" and other speakers in various designs.

No actual alignment seemed remotely close to the low end response on line simulations suggested, and I was not using any simulation program at the time.
I went through dozens of different cabinet tests, all sine wave at 5 Hz increments, I did not have Smaart at the time.

Later, when I got some B&K 4004 microphones and Smaart, I found out the mic that came with my Terrasonde Audio Toolbox I had used for all the testing was -6 dB at 30 Hz .
What I thought was a slightly falling response at 40 Hz was actually a bit of an increase in level...

I've included those tests below so you can see you are "not the only one".

Art

[mattlong8]

Art,
I missed this last message you posted and it's ironic that I saw it now. Sure enough my reference mic is probably about 6dB down at 30Hz. And some how I keep forgetting to compare my measured response to your double 12. Clearly 300hz isn't a result of port resonance.
I've started a photo gallery on my facebook page(yes I know it's annoying to have it on FB) with some pretty good details on parts of the process. I think it's worth a look.
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