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Sealed boxes are straight-forward.
The bigger the box the closer the box Qts approaches the free air Qt. Fs / F3 / F10 all decrease. In the chart the top curve is the smallest box and the bottom curve the largest box (IEC standard volume). The optimal flat (butterworth) box is in the middle. All these are applicable to the WR125S & WR125ST. |
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<<< WR125 + ApexJr tweeter (1.8 uF cap XO) sealed prototypes. Tangent TM3 donor box, cut down to just under 15 litres. This combo of drivers & box work amazingly well. |
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Just completed -- a set of 25 litre (0.87 ft3) sealed bipoles. Our favorite so far. Drawn with WR125ST, implemented with WR125 + ApexJr SuperT (XO = 1.8 uF Solen) and FR125 in a completely reversable box (so called the Bipolar-Bipoles). Picture before finish. Note: a rev of this design with slightly changed dimensions is in the works. Some discussion on this design.
<clik images for larger pictures> |
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Simulated Box Parameters
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VB (litres) |
2.6 |
4.25 |
9.43 |
20 |
200 |
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Qts |
1.0 |
0.853 |
0.707 |
0.634 |
0.568 |
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Fs |
120 |
103 |
85 |
76 |
68 |
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F3 |
95 |
88 |
85 |
85 |
90 |
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F10 |
65 |
56 |
49 |
46 |
43 |
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Modeled using True Audio MacSpeakerz |
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<clik chart for larger chart>
The sealed box is most often executed using sheet material which yields a rectangular box with the least amount of effort. It is best to keep internal dimensions from being integral muliples of any other dimensions. One of the best ways is to use ratios based on irrational numbers.
The table below uses one of the world's favorite irrational numbers. This is the Golden Mean -- 0.618... & 1.618... Feel free to use other irrational numbers (ie: epsilon, square root of a prime number (don't use root 2 twice), pi). Of course you can use any face to mount the driver -- as long as it is big enough.
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Rectangular Box Internal Dimensions (using Golden Ratio 0.618:1:1.618)
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VB (litres) |
2.6 |
4.25 |
9.43 |
20 |
200 |
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Height |
8.8" 222 mm |
10.3" 262 mm |
13.5" 342 mm |
17.3" 439 mm |
37.3" 946 mm |
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Width |
5.4" 138 mm |
6.4" 162 mm |
8.3" 211 mm |
10.7" 271 mm |
23.0" 585 mm |
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Depth |
3.3" 85 mm |
3.9" 100 mm |
5.1" 131 mm |
6.6" 168 mm |
14.2 361 mm |
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Boxes with walls that are non-parallel tend to be better. Slanting the 2 largest panels is not that much harder and yields what is termed a "pyramidal" box (even though not a true pyramid but a trapezoidal cylinder). It is easy to change a rectangular set of dimensions to a "pyramidal" box. If you subtract n units from the width of the top of the box, add n units to the width of the bottom of the box. The shapes below give examples based on the ratio used in the table.
You'll need to be a bit more creative
for even wilder shapes -- spheres, ellipoids, use of found objects like pots...
