Here are a few pictures of my latest speaker building project.
I used the 24.5 x 72 x 1.125" workbench table top particle board they sell at Menards for $14.99 a sheet. The finished speakers are 19 lbs each. You might think this thickness is overkill for such a small speaker. But what I was trying to do was to increase the mass of the speaker as much as possible to reduce self-cancellation caused by the entire speaker moving back & forth at low frequencies.
Gross internal volume is 3 liters (5.30x3.55x9.75"). The port is 26" long with a 1.6" inside diameter. This tunes the system to somewhere in the 40-42Hz range. A 10" long by 1.25" inside diameter bass trap eliminates the 1st and 2nd port resonances at 260 and 520Hz. This trap is stuffed with a small amount of Acousta-stuf and dacron material. Tuning was done experimentally by changing the trap length and stuffing density, on the fly, as I measured the near field response with OmniMic.
Drivers are the Tang Band W5-2053 subwoofer, the Techtonic Elements TEBM46C20N-4B midrange, and the Morel MDT-39 tweeter. Crossover frequencies are 320 and 2400Hz. Sensitivity measures somewhat low at 82-83dB/2.83V/1 meter.
Because I am using a 4 ohm subwoofer and a 4 ohm midrange, the overall system impedance dips to about 3 ohms in the 90 to 150Hz range (see graph). I have played the speakers at moderate to somewhat loud levels with a number of amplifiers and have not, as yet, had any problems with amplifier overheating or the tripping of amplifier protection circuitry.
I measured total harmonic distortion at various SPL levels with OmniMic and then transferred this data to a spreadsheet (see attached). Subwoofer distortion at 80Hz appears to be the weakest point, with midrange distortion at 300Hz a close 2nd. When I listen to the speakers at the 86-89dB level, there is absolutely no sense of strain, but when I push them to either the 93-95 or 95-97dB levels, I can hear a very serious dynamic compression effect starting to set in.
Any comments or questions are welcome. Thanks for looking.
Bill
Comments
That was A LOT of work! I like your port resonant trap. Do you have a Z measurement with and without? Also, NF/port summed with and without?
If you have ever seen the weird shape of Vivid speakers, even the Nataulus thats what L. Dickie (the desiger) uses the cabinet shape for. That, and diffraction. It is very cool stuff as a whole and I think your exexution of dampening a port's resonance is awesome.
Great document package, too. 3 ohms Z for a 4 ohm speaker is no big deal. Many commercial 8 ohm speakers drop to 3.8. It is a little reactive and 80HZ looks to be a hell of a load which could account for the distortion spike as you are pulling a ton of current and back EMF will be at a maximum. Would not be a problem with amps, just make them complain a little and by the looks of it, it is likely more of something that is measurable....Not meaningful.
Specially in such a tiny speaker!!!!!
I bet they actually are stunning at regular to moderate volumes. I am very impressed with this design.
Thanks, Kevin. I went with particle board because it is available in the 1-1/8" thickness verses 1/2" or 3/4" for MDF. I cannot find 1" MDF in my area. I would have had to glue up two 1/2" boards, but when I do this I run into problems with the boards warping when the glue dries. Even when I clamp boards in rails for a week, when I remove the clamps, I get warping.
I do get some flaking with particle board, especially in the driver cutout holes. But it is not too bad. I just fill it in with bondo.
Thanks for the kind words. They do pack a real punch for the size, which is what I was going for. I did not measure Z with or without the traps; only NF SPL at the port exit with and without the traps. I now have the ports glued together, so I would be unable to re-measure at this point without cutting them apart.
You might be able to find MDF stair tread at one on the big box building supply stores. Home Depot has them. 1-1/8 in. x 11-1/4 in. x 4 ft. MDF with a bull nose one edge which is big enough for small boxes and could be joined at the edge if you wanted bigger panels. http://diy.midwestaudio.club/discussion/272/1-1/8%20in.%20x%2011-1/4%20in.%20x%204%20ft.
Ron
Javad
Bill S.
Thanks for the tip, Ron. I was actually thinking about using the stair treads that you mention to build these speakers. But I had to go with the larger benchtop material because I wanted to be able to cut the faceted baffle angles off the end of a long piece of stock. So I first ripped the benchtop down into two 12x72 inch pieces on my table saw. Then I sliced the baffles off the end of this long piece of stock using my dual-bevel sliding miter saw. The stair tread material would only have allowed me to build the box about 11 inches high.
Bill
You could cut a liter off like I did.
Creative paint job, Bill
Bill
PS. I didn't close my eyes.
Bill S
When Ben picked up my speaker to put it on the stand, he first waved it to the crowd and there was a slight uproar of laughter in the crowd. As a photographer, whenever I hear laughter in the crowd, I quickly raise my camera and fire as fast as I can. This pic was the result.
Bill