The Choti HT
This was my first DIY speaker design as well as my first time trying to do a high gloss finish. You can see some of the issues (surface scratches) in the finish in the image. I wanted to make get my feet wet in DIY speaker building without spending too much money, so I figured I would make a small speaker for new surrounds to add to my HT system. My goals for this project were:
I decided on the Dayton RS100-4 as the woofer. Modeling showed good low end performance in a small box and it had reasonable sensitivity. The Aura NS3 goes a little lower overall and can also be put in a pretty small box, but the RS100 has much lower distortion levels and better sensitivity. at only a roughly $25 difference pair, I chose th RS100. To keep costs down and fit the small form factor I went with the Dayton ND series tweeter. I looked at the ND16FA and ND20FA. The ND16FA was a little cheeper and actually had slightly better distortion performance in its useable range, according to the graphs on Zaph's website.
After playing around with some different box tunings, I decided on going with a ported 1.6L box after displacement that is 6.5"H x 5"Wx 5.5"D made out of 1/2" MDF. It uses a 5" long by 1" diameter port that tunes the box to 75hz. The 1" port is a little smaller than optimal and will just peak above port velocity limits at around 70-75hz. I have these crossed at 80hz and since they are being used solely for surrounds, it is not really an issue for me. A 1-1/8" port will prevent exceeding limits under pretty much all the power the RS100 can take, but needs to be 6.5" long and doesn't fit in the box. The tweeter is offset slightly to minimize diffraction and placed very close to the woofer. Be careful with the cutouts, as there is only about an 1/8 inch between the tweeter and woofer.
For this design, I used manufacturer's published data as well as data from Zaph Audio for my frequency, impedance, and T/S data. The data was run through Jeff Bagby's Response Modeler Excel Spreadsheet to simulate the baffle step and diffraction, as well as extract minimum phase for both the woofer and tweeter. The acoustic offsets were estimated at around 0.020 meters. I didn't use full baffle step on this design because they were going to be placed close to a wall, and you can see this as a shelf in the modeled response starting around 1.5 kHz.
The crossover is a second order electrical, approximately second order accoustic on the woofer and third order electrical and acoustic on the tweeter crossed around 3.8khz. Based on some advice from Wolf below, I have modified my original crossover a bit. The new schematic is provided below. This actually makes the phase line up even better between the woofer and tweeter, so overall I'd call that a win. The cap in parallel with the inductor on the woofer needs to be 0.15-0.2 uf. If you remove this, the woofer breakup will have a significant impact on the sound. When I built these, I used all NPE caps and they cost right at $100 excluding lumber and PVC pipe for the port. The price on the ND16FA went up a little after my purchase though, so it is just a bit over now. If you want to build a set of 5, all 5 speakers can be built out of one Home Depot handy panel and one 2 ft long 1" PVC pipe.
I picked up a basic measurement mic after building these and went back and measured the response to see how they matched my model. Measured response matches the modeled response fairly closely. I't's nice to know that the modeling works pretty well. The distortion numbers are not a valid representation of what you should expect. I was having some issues with HolmImpulse on my setup giving me weird distortion readings.
Overall, I am very happy with the way these turned out for my first build. The soundstage is much bigger and way more spacious than I expected from such a small speaker. The bass is actually pretty surprising given the high f3, but obviously they don't dig supper deep on their own. I would imagine paired with a small sub, like one of the Tang Band 6.5 inchers, it would make for a great desktop speaker. The top octaves have a bit of a rise in them as seen in the measurements. That was measured on stands out in the middle of the room. Again, they are designed to be placed close to the wall or on wall mount stands, which should fill out the lower octaves a bit. Below you can see a picture when they were being tested, sitting next to a pair of Paul Carmody's Overnight Sensations to give you a sense of size.
Builds and Comments
PE forum member lowpolyjoe from New York has built several sets of Chotis for himself and family, including some with a built in amp. He modified the cabinet slightly, making them a bit taller and using a slot port. His comments were:
'They sound pretty good to me. Strong low end for something this small. I was just listening to the radio while setting up to do measurements, so it wasn't exactly critical listening, but i'm happy so far. There is one problem, which is to be expected - at loud volume with something with a heavy low end (pop dance music was on the radio) the woofer gets messy hitting its excursion limits. Running these from my Denon as "small" surrounds i'm pretty sure they will end up crossed over high enough to avoid this issue."