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The Plumber's Delight
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
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
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
Ron
Bill
Sehlin Sound Solutions
One of the aspects that made Bill's intersting was the trap he used to greatly reduce the port resonance. And that with his design vs the other entry using the same woofer with dual prs, Bills was noticeably more sensitive though tuning was very similar.
However there is no denying that Bill's implementation of this design w/in the rules resulted in a very optimal speaker. While it was quite a bit larger than many entries, Bill did some very neat things with the port tuning and the resonant H-chamber to really justify his effort and the use of the extra space.
Javad
When Wolf set the 3 liter criteria, one of his stated objectives was to be smaller than popular "small" speakers such as the Overnight Sensation (which is something like 4 to 4.5 liters). The overall aesthetic impact of the "Plumbers Delight" is of something larger than the Overnight Sensations. The upside of the approach is that it brings attention to the port resonance issue and demonstrates a solution. I don't want to speak for John H., but I strongly suspect that this design had an influence on what he did for his silver Peerless based nano entry, which was a more elegant solution based on the same principle.
If I were running such a competition, I would set the size criteria based on external size, which would have created a more level playing field and maybe spurred some different innovative solutions. But then, this speaker probably would not have been built - and I am glad it was built...Food for thought.
Sehlin Sound Solutions
I agree Scott that Johns solution is more appealing and elegant. However, as John can attest he was not able to reduce both peaks as Bill did. I'm sure with further design that he would though.
Ben and I will be discussing next year's theme soon. And he will again use a poll to determine what it will be.
One question for those that attended. What did you think of the tournament format?
My only suggestion is make more "open play time" for the theme speakers, my speakers only played for the demo music and never came out again, would have been fun to have another 5 mins to show off what they did well with my own reference music.
Javad
The original port was 26" long with no trap. And it sounded and measured bad, with large 1st and 2nd port resonances. I was going to give up and re-tune the box from 40Hz up to about 50Hz by cutting the port down to about 12" or so. This would have created a peaky 50Hz boom box. But then I came up with the trap idea and, because the port was on the outside of the box, it was very easy to construct it and tune it. I had OmniMic running continuously taking near field port measurements as I swapped out different trap lengths and stuffing densities on the fly. When I got done, I had a big pile of rejected trap stubs laying on the floor.
Update:
Mike, you asked me a while back if I had run a Z curve with and without the trap. But the traps were already glued in place.
Well, I accidently bumped and knocked one of the port traps off one of my speakers. So, before gluing it trap back on, I ran a system impedance curve comparing trap verses no trap. The purple curve is with the trap removed & plugged with a rubber stopper; the green curve is with the trap in place.
As you can see, there is no difference at the 1st & 2nd port resonances of 260 & 520Hz. There is a slight difference between the two curves in the 40 to 80Hz range. My guess is that the 1st & 2nd port resonances have little to no effect on the impedance curve because these resonances are created by the smaller volume of air inside the port and therefore tend to be isolated somewhat from the larger amount of air inside the box. As to why the slight change from 40 to 80Hz, I have no idea.