First design+build - 2 way bookshelves w/ TCP115

[JonathanPenner]

Hey all,

I was hesitant to post this since there are some pretty big names on this forum. But I was inspired from a recent post about posting designs so people can discuss and learn from them. I know I've made some mistakes with this build, but I'm here to learn!

I started this hobby at the start of this year and I was immediately fascinated by it. It’s an amazing combination of woodworking, acoustics, music, physics, electrical engineering, etc., and each aspect of it is incredibly deep. Much more than the “stick drivers in a box” that I saw when I was introduced to it (sealed RS100s for computer speakers)

I wanted to try designing my own speakers, but first I wanted to learn as much as I could and try to do things “right” instead of jumping the gun to building. That’s part of why I didn’t just build a kit - I really wanted to go through all the theory myself, even if I made some mistakes. My day job is as a computer programmer, and I would equate it to copying and pasting some code instead of learning the language and really understanding what it’s doing. Or to put it differently, I'm reinventing the wheel so I can learn more about wheels.

After months of reading countless articles and scouring through forums, here is my first build! A 4L vented enclosure, (accidentally) tuned to ~42Hz with the Dayton Audio TCP115-4 and ND25FA-4. Dimensions are 6"x8.5"x8" WHD.



These were meant for our small TV / games room. We watch TV and listen to music at relatively low levels, 65-75dB. I didn’t intend for them to be for super high SPL analytical listening in a large room, so these are perfect for their intended use of background music, TV, and video games. They happen to go low enough that I’m not intending on building a subwoofer. The TCP115s are pretty remarkable.

I like high quality audio, though admittedly haven't listened to a lot of great systems. The best I've heard is some KRK Rokit 5s with the 8" sub. Those are super tight and image really well, but are incredibly revealing of every flaw in a track, which can be pretty miserable. The next best I've heard is my Sonos One SL pair + Sonos Sub. Listening impressions of my build compared to the 2.1 Sonos setup is they're remarkably close. The Sonos setup has better midrange detail, but can sound cold and harsh. These bookshelf speakers play almost as low for music as the Sonos Sub (~38Hz) and sound warmer and mellower. Sometimes I forgot which system was playing and had to check! I haven't heard the standard starter DIY kits like the C-Notes or Overnight Sensations, but I'm really curious to hear how these compare. There is definitely room for improvement, but all in all I'm really pleased with how these turned out. Feedback is more than welcome!

Note these are simulated responses. Unfortunately I don't have measurement equipment, so I can't post the actual results. And yes, I know it was unwise of me to design a speaker without measuring the drivers in the cabinets. A microphone and a DATS box are some of the next things to acquire so I can redo the crossover around the actual responses. I have some other projects in mind, and for those I'll definitely do the approach of build an enclosure, measure the drivers, then design the crossover based on those measurements.

[JonathanPenner]

I wanted low-end extension and was willing to compromise on a drop in volume from 50-80Hz. SpeakerBoxLite said that a 4L cabinet tuned to 48Hz would need a 1/2" tall slot port that was about 17.5" long. I thought I would be clever (whoops) and make a channel running along the back of the cabinet. What I learned later was about air friction and introducing bends in ports reducing efficiency. I believe that made them tuned to about 42Hz since they play reasonably loudly down to that point and then fall off a cliff at about 36Hz.



1/2" MDF construction. I was working with a very old table saw from the 1960s where the fence adjusted in two places, meaning it was really tricky to get straight cuts. I also used a bendable blade for the rabbet joints, which meant that everything was just slightly off. A millimetre here and there meant I had to do a lot of cleanup later on.




I made the slot vent out of 3/8" plywood I had from another project. I made a bunch of 1/2" strips to make the assemblies.




Dry fitting all the cabinets. I had to do quite a bit of trimming on the joints to get things to fit together. It was at this point I wished I was more careful about getting straight, accurate cuts. I've done larger-scale construction before (a cabin), but nothing this small with tolerances this tight.




Cutting circles took a bunch of trial and error to get (almost) right. I made a really bad circle cutting jig and went through a few test pieces to get the drivers to fit. I say "almost" right since the cutouts needed some widening for the flush-mounting of the drivers using a rabbeting bit and a sanding spindle.




I made the vent assemblies by gluing little "sleds", rounding over the inner corners, then gluing the sleds with a spacer in between them.

I made the mistake of assuming my cuts were the right length. I ended up needing to redo one of the assemblies because by the time I glued it all together it was slanted about 1/4" horizontally and wouldn't fit in the enclosure. More lessons learned....

[JonathanPenner]

I realized later there would be smarter ways of laying out the clamps and weights for the glue up. This worked though!




The fronts of the vents ended up being a hair too short to reach the front baffle. This cabinet was the worst, but both were slightly off.




I ended up needing to add some strips of wood that were flush with the front baffle to seal up those gaps. I also added some mattress topper to the walls. I'm honestly not sure if it really is doing much, but I feel better for putting something there!

[JonathanPenner]

My first attempt at assembling the crossover was on a scrap piece from an IKEA wardrobe we cut apart. This was my first time soldering too!




I did a test fit of everything and trying to tape up gaps. Unfortunately I didn't account for how large the magnet on the TCP115 is and the crossover had to be really wedged inside to fit!




A couple hours of desoldering later and I had all the components again...




For my second attempt I made sure the layout was compact enough to fit in the enclosure. I used two pieces of craft board, taped them together, and drilled all the holes through them both. It worked out much better.

[JonathanPenner]

I flush trimmed all the sides and rounded over the baffle edges. It was pretty cool at this point to have something that looked like speakers! You can see the burn marks on the tweeter cutouts from where the sanding spindle widened them.




I used bondo spot putty to fill all the seams and cracks. I know the trench method for seams works better in the long term, but I didn't have the right router bits to actually pull it off. We'll see how these end up looking in a few years.




I don't have any pictures of the primer, but I sealed the MDF using the classic BIN shellac primer sealer. Four coats, sanding between coats two and three. I did a similar thing with the black rattle spray can here. This picture is after two coats.

I learned the hard way to make sure all brush strokes are really, truly sanded out before painting. They're subtle, but they show. I wasn't going to attempt a piano finish or even a gloss finish, so the enclosures are a bit more forgiving, but next time I'll definitely put way more time and effort into the surface prep stage.




For the stain I used a Varathane mahogany stain + poly all-in-one can since I already had some lying around. I had better luck with it when I stained our dining chairs. I'll be the first to say this is pretty rough.

As convenient as the all-in-one stain+poly was, I've really come to appreciate having many small passes of specific products to build up a finish. Despite my test pieces turning out alright, I wish I had used a separate stain or top coat. I think I sanded too high as well, so the stain couldn't absorb as well into the wood. I am pretty sad the character on the wood grain is lost with this. Eventually I'd like to try to refinish the baffles.




I wired everything together, added some fibreglass stuffing, and mounted the drivers! Here they are in their current home.

[Chris Roemer]

Fun! Looks like a great 1st project.

It LOOKs like you simulated baffle-step loss (on the woofer). Without any "Z" data dialed in (which is the depth - or "time of flight" distance back to each voice coil) your phase relationships are probably screwed up (in your model), and THAT (mainly) determines how your 2 drivers "sum" (around the crossover freq.).

There's almost never any (good) reason not to have the tweeter and midbass as close as reasonably possible. I'm running something like this on MY desktop, but I traded cab depth for height - since you usually like to have the tweeters near "ear level". It's also pretty common to use the tweeter center as the "design axis" ("0 0 0" for XYZ), then the woofer's X would be 0 (if centered below the tweeter), and you'd have offsets for Y (the distance the woofer's center is below the tweeter's) and Z (the depth that the woofer's v.c. is behind the tweeter's). Still, these probably sound better than my 1st project.

You can verify your cab's tuning freq. (Fb) by laying the speakers on their backs and putting a few grains of rice on the woofer cone (and running some sweeps). When you hit the tuning freq., the rice grains will hop around the least (as most of the SPL will be coming from the port - at that freq.). Also, while (open cell) "mattress topper" (a.k.a. "egg-carton foam") lining a vented box is pretty common, stuffing it is not so much (typically sealed boxes get stuffed, while vented boxes get lined). SOME will add stuffing to a vented box to help tame any boominess (caused by mis-tuning or also by T/S parms that varied too much from "spec").

[JonathanPenner]

Thanks Chris!

Yup, I simmed battle-step loss in VituixCAD. And yes, the phase is pretty wonky. About 50 degrees out of phase at the crossover point. While I had punched in all the relative offsets of the drivers (and using half the driver depth to estimate the Z), it was only until after I had ordered the components I remembered about phase alignment. That and some of the midrange voicing issues are two of the things I'd like to address in the next version of the crossover. I wanted something a bit more "V" shaped but didn't realize just how audible the 4dB drop would be in the midrange.



Sitting on the couch now my ears are only a few degrees above the tweeters. If I slouch more it's perfect

About spacing, I can definitely see no reason to have the drivers not as close as possible, though I figured the rule about no greater c-t-c distance the crossover wavelength more as a limit. There's 3 7/8" between them, which puts the wavelength at 3500Hz and I cross at 2850Hz. Though I guess I'd still get lobing issues past that point, since at 3500Hz the woofer is only down 10dB?

I heard the trick about rice and tried it, but the rice didn't move at all from about 55Hz - 42Hz, so I wasn't really sure what to make of that. Splitting the difference, maybe I did actually hit the 48Hz tuning?

I didn't at a ton of insulation. I listened to it first and it had a lot of boominess. I added too much and the mid bass was cut down too much. I took a fair bit out so there's only about a fist-sized wad in each and it opened up the mid bass again.

[djg]

Lotta work for a couple of little speakers, huh?

Thanks for all the great pictures.

[JonathanPenner]

djg - a lot of work, but so much fun! I’ll be moving into a new house (with a garage!) next month so don’t want to start the next project just yet, but I’m really looking forward to it.

[scottvalentin]

Awesome job! Sell that old table saw and get a nicer used one off of Craigslist!! That will make your next build far more enjoyable.

[JonathanPenner]

Thanks Scott! lol, I was working at a friend's garage and he bought a modest set of tools a long time ago and has stuck with them. A measurement mic, soldering iron, and table saw are on my list!

[wogg]

Really well written, thanks! These are the sorts of threads that make this place valuable, working projects with lots of lessons learned in the process.