The first thing I did with the freshly built prototype was to measure the impedance of the speaker. This impedance corresponded very well with the impedance predicted by Mathcad. This is good to know, because this means that Mathcad has modelled this speaker correctly, and thus my measured T/S parameters were correct, and I could safely assume that other characteristics were properly modelled.
This image compares simulated impedance (top) with measured impedance (bottom). As you can see, the peaks are in the same place and also have the same amplitude (exept the larger peak, which is about 2 ohms lower, which is not really significant)
The next thing I did with the freshly built prototype was determining the effects of different amounts of stuffing on the bass-response.
This is the measurement setup:
Although this was done in a small room, this doesn't have much effect on the measurements, which were all nearfield.
This image gives an impression of how I stuffed the TL. I lined the walls of the front "chamber" with 28mm printex and filled the open space with baf wadding. I left the other half of the speaker completely unstuffed. This image shows the speaker with about 75 grams of baf wadding installed.
The next 2 images show the effects of stuffing on the output of the woofer and the terminus. As you can see, small changes have a large effect. One piece of baf wadding weighs approximately 150grams, and is roughly 50cm by 75cm. Half a panel is pretty small, but has a measurable effect on the bass-output.
I decided that using between 150 and 225 grams of baf wadding would give the best results, and that I would decide on the final stuffing by ear (and keeping within 150 and 225 grams)
With these results I was convinced that the basic design of the speaker was good, and started with the construction of the final speakers. While building those, I continued using the prototype for measurements.
Next, I mounted the tweeter in the baffle and did some frequency response measurements. After a false start (my first measurements turned out to be wrong, which also rendered my first crossover a fluke), I got some good measurements. I then started on the final crossover design. This is where the tweeter placement turned out to be a less fortunate choice. One of the tests that shows if you have proper phase tracking in the crossover region, is that the response shows a deep null if you reverse the polarity of one of the drivers (Good phase tracking is necessary for proper summation of the drivers over a large listening area). When I reversed the polarity of one of the drivers of my speakers in LspCad, it didn't give a deep null. Why didn't I chage my design at that point? 2 reasons: I sit at a very fixed point, namely behind my pc. Which lessens the need for a good response over a wide area. The other reason is that I was already nearly finished with the final pair of speakers, and I didn't want to go through the trouble of spray-painting a second set of speakers.
After some simulating in LspCad, I had some preliminary crossover designs. And to determine which one sounded best, I setup the speakers in the living-room and connected them to my pc through an Onkyo HT amplifier.
The crossover that sounded best was quickly found, and only needed a little tweaking (I changed two components one E12 step). However, the bass was a little weak. That was weird, because the bass was noticably weaker then before, when I was listening in mono to the protoype speaker. It tured out that they way I stuffed the speakers was the cause of this. In the open speaker in the image above, you can see one large piece of baf wadding that goes al the way from top to bottom. What's not very visible is that the bottom piece of baf wadding bends around the fold in the TL and lies flat on the bottom. In these speakers I folded that last piece of baf wadding 180 degrees, and added a small piece of baf wadding in the bottom of the speaker. After I unfolded this piece of baf wadding and removed the small piece from the bottom, the bass was back in full glory. Folding the baf wadding probably made the density at that point so high, that it caused the bass to suffer. There is still more damping in this speaker than in the prototype, but the bass is at least as good, probably better. The highest density is in the top of the speaker, behind the tweeter (more than 1lb/ft³). Behind the woofer it is a little less, and the bottom half of the front of the speaker has about 0,5lb/ft³. And all the walls in the front half of the speaker are lined with 28mm pritex. All in all, this makes for a very nice bass response, with that sweet TL flavour over it.
This is the filter I decided on:
It results in this frequency response (the output of the terminus is not added to this plot, thus the dip at the tuning frequency):
There is a slight lift in the lower frequencies (50-200 Hz) which is also noticable when listening to the speakers. It gives a somewhat "poppy" character to the music, which actually is kind of nice. If I get tired of this (which I probably will), this small lift is easily corrected by adding some more damping material. The impedance of the speaker is very friendly and never goes lower than 4ohm (at 5000Hz), the phase angle never becomes larger than 30 degrees.
Then: how do they sound? Well, first of all, they're a huge improvement over my previous pc-speakers (a pair of jamo compact 700, standing on the floor). I just love the bass from these speakers, very nice. At this writing, I have only listened a few hours to the speakers. And I like them, nothing really stands out, which I consider to be a good thing. And considering how they're set-up, not a small feat. In july, I'm going to take these speakers to a small gathering with more diy speakers. I'll add some of their comments here, then.
Did I mention that they're difficult to photograph? It's because of their dark color (which isn't even that dark if you see them with your own eyes), the camera has difficulty to get enough contrast. Especially when it isn't dark outside