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Old 3rd July 2013, 07:11 PM   #1
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Default 30 HZ from a 10 X 7 box

I am building a few Bookshelf speakers for the Grand-kids. Using a Low Vas speaker so I can get a +- 3db 30-20K from a 10 X 7 X 7 box. I need some advice. when can you use just a hi freq blocking cap, a first order, and when do you need a 2nd order crossover.
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Old 3rd July 2013, 09:52 PM   #2
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For low distortion I want to use the lowest order crossover I can use.

Besides very low distortion, I wanted very low bass – 30 Hz, a flat frequency response +/- 3db. To get these I needed a relatively “big” enclosure. The “biggest” enclosure is an infinite baffle.

Infinite baffle = 5 X Vas

For Tang Band W3-1876S 3”
Vas = 0.03 ft^3
5 X 0.03 = .15 ft^3

Anything over 0.15 ft^3 is an infinite baffle and will give me “extremely clean and uncolored sound with very low bass output capabilities”. So my tiny ~.3 ft^3 box is an infinite baffle.
Some sub-woofer formula for infinite baffle use:
Vb = 10 X Vas which is 0.30 ft^3.

To insure I made the internal:
Vb = 10 X 7 X 7.5 = 0.304 ft^3
after subtracting the volumes of the speaker and port tube.

It had to have a high Qts-above 0.40 to work properly in an infinite baffle. This Woofer has a Qts of .43. Higher Qts’s are more appropriate for the Chebyshev alignment to eliminate boomeness, with the side effect of lower bass.
I tuned the box to 33Hz, a true Chebyshev would be tuned in the 20’s and require an equalization boost in the low frequencies an fs of 33 Hz does not.

I do have a couple of other problems. Vas is the ratio of reactive to resistive elements. So a low Vas speaker has more energy losses to the resistive elements so is lower efficiency. Matching a tweeter to a 76 db sensitivity speaker takes some looking, or lots of resistance in the crossover.
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Old 3rd July 2013, 10:49 PM   #3
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How about a graph?
All drivers simulated on power that reaches xmax at ~35Hz

Grey: w3-1876S, 0.3 ft³ tuned to 33Hz, 10W
Green: w3-1876S, 1 liter tuned to 55Hz, 6W
Blue: w4-1720, 0.3 ft³ tuned to 55Hz, 3.5W

In other words, consider these: https://sites.google.com/site/undefinition/speedster
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File Type: gif temp.gif (4.5 KB, 751 views)
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Old 3rd July 2013, 10:49 PM   #4
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[ATTACH]TB w3 -1873 in enclosure 1.jpg

TB w3 -1873 in enclosure 2.jpg[/ATTACH]Thanks Dump-truck.
Here are some other graphs

The tweeter I will be using is the Dayton Audio DC28F-8
Frequency range: 1,300-20,000 Hz, 8 ohm at 2K
SPL: 89 dB 1W/1m
The tweeter drops off below 1500.
The woofer drops off above 2K, 8 ohm at 15-18K
If only a cap, 10 uf
Crossover if used 1800 Hz

Last edited by fwbutler; 3rd July 2013 at 11:03 PM.
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Old 3rd July 2013, 11:08 PM   #5
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I guess I just don't see the very low max. output and -4dB in the 60Hz region as being a good tradeoff for getting to 30Hz. The reduced output in the most important bass region is going to be noticeable.
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Old 4th July 2013, 12:24 AM   #6
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my thinking, not yet put to the test, is that it is easier to equalize the 40 to 80 Hz that is 3-4 db low, than to try to resurrect sub 60 hz music that is -40db. This was discussed in Thiele & Small 's papers. Yes dumptruck I agree, without equalization it would weaken the music. The kids boost the bass to distortion normally trying to get the very low non-existant bass. If you or anyone has experience in this I'm open to ideas
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Old 4th July 2013, 12:28 AM   #7
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And for the low output. Their parents would kill me if i made them 100 db boom boxes that would disturb the whole house. They have enough power to get 85+ db sound out of these.
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Old 4th July 2013, 01:35 AM   #8
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And you'll lose even more output from the EQ. Getting bass at 30Hz that actually sounds like anything requires more like 100dB+. I may be wrong, but I'd put some money on the suggestion I gave you in post #3 (or another vented design with that woofer, there are several out there) sounding like more bass, EQ or not. Speakers that struggle to get above 80dB do not tend to impress anyone. Just use higher output designs and drop the upper mids as needed to make them sound beefy, that's my advice for small speakers meant to run full-range.

Last edited by dumptruck; 4th July 2013 at 01:37 AM.
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Old 4th July 2013, 02:22 AM   #9
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Still cant get deep bass, in a small box from high Vas speakers.

From this 5 watts should give output in the 80db's.

I built a set of 40" exp folded rear firing horns, 40 Hz at 86 db on less than 2 watts but rarely use it that loud, normally in low 70's at less than 1% distortion.

When I added 6 db of equalization it only added .05% at 76 db.

Here is 30Hz from the .188 ft^3 Vas woofer W4 1720 in small .4 ft^3 box

Should have the small speakers built and tested in a week, will share the test results. It will be interesting as formula and software, even test eq are not ears.

Do you have a preference as to type of enclosure?
Attached Images
File Type: jpg TB w3 -1873 in enclosure 2.jpg (196.8 KB, 714 views)
File Type: jpg w4-1720.jpg (60.3 KB, 698 views)

Last edited by fwbutler; 4th July 2013 at 02:26 AM.
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Old 5th July 2013, 06:09 AM   #10
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Quote:
Originally Posted by fwbutler View Post
Still cant get deep bass, in a small box from high Vas speakers.
No, you sure can't. I was talking about the 1720 tuned higher being more impressive sounding than the 3" "subwoofer". My opinion/experience is that deep bass is worth nothing at limited SPL because you can't even hear it. Better output 50-80Hz would steal the show, and you're limiting yourself there a great deal by trying to reach 30Hz. Nothing wrong with trying it, just don't proclaim it works great without good basis for comparison.

Quote:
Originally Posted by fwbutler View Post
Do you have a preference as to type of enclosure?
Nope. Whatever works best for the driver and the application.
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