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Old 17th October 2004, 01:33 AM   #1
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Default calculations

Can some one check my calculations to make sure they are right.
here are the specs.

Driver Properties
Name: 55-1875
Type: Standard one-way driver
Company: MCM Electronics
No. of Drivers = 1
Fs = 31 Hz
Qms = 2.193
Vas = 404.9 liters
Cms = 0.171 mm/N
Mms = 154.2 g
Rms = 13.7 kg/s
Xmax = 4.4 mm
Xmech = 6.6 mm
P-Dia = 405.5 mm
Sd = 1292 sq.cm
P-Vd = 0.568 liters
Qes = 0.46
Re = 6.8 ohms
Z = 8 ohms
BL = 21.07 Tm
Pe = 400 watts
Qts = 0.38
no = 2.528 %
1-W SPL = 96.18 dB
2.83-V SPL = 98 dB

vb:13.34 cubed ft
f3: 31.23hz
Fb:31.1 hz
vbin: 23064.8140131 in cubed

P-vd is the volume of the driver i think, so i have to add this to the vbin: 0.568 liters(is also 34.6614866 inches cubed) + 23064.8140131 inches cubed = 23099.4754997 inches cubed

my total volume of my box has to be 23099.4754997 inches cubed

for length of port it is 1.5 inches
radius of port is 2.995 inches

and if i want to use the golden ratio to get the dimension of the box what would i have to do?
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Old 17th October 2004, 02:31 PM   #2
Ron E is offline Ron E  United States
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Firstly, your port is way too small. A 3" port with an 18" woofer is not enough - even though this woofer doesn't have a long throw. While your alignment will work, it is a fairly large box.

I wouldn't worry about making a golden ratio box. That is more important for midrange, it is more important IMO that your box is well braced and has some absorptive lining on the walls. To figure out golden ratio dimensions you first take the cube root of the volume - the cube root of 23064.8 is 28.465 inches, so that means your box if it were a cube would have internal dimensions of 28.5 inches on a side.

Math:
23064.8^(1/3)=28.465 inches
28.465*28.465*28.465=23064 cubic inches

To get a golden ratio box, you multiply and divide the cube root by 1.618034.
Your box would then be 1.618034*28.465 x 28.465 x 28.465/1.618034 or:
46.0 x 28.5 x 17.5 inches (rounding to the nearest half inch). Multiply that out and get the volume 22942.5.

If that size doesn't daunt you, I would go for two 6" ports, each 5.5 inches long. Using a small port will cause lots of air rushing noise.

There are other dimension ratios that you can use - if you get dimensions by multiplying and dividing the cube root by the same number, you will always get the correct volume. Try factors of 1.2 or 1.25 (two other recommended ratios) to see how the dimensions come out.
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Old 17th October 2004, 08:11 PM   #3
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what do you think about a 4th Order Bandpass Systems. What would i have to do.
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Old 17th October 2004, 11:25 PM   #4
Ron E is offline Ron E  United States
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Quote:
Originally posted by lXxAndyxXl
what do you think about a 4th Order Bandpass Systems. What would i have to do.
What are your goals?

You can make a smaller vented box if you want to, you could make a 200liter box tuned to 35Hz with 2 - 6"dia x 11.5" long ports. This woudn't go quite as deep, F3 would be 40Hz, but it would be exceedingly loud with very little power.

You could make a bandpass - one that might work is ported volume 84 liters tuned to 68Hz and sealed volume 106 liters. This would have a lower F3 of 40Hz and upper f3 of 117Hz. The rolloff on the low end is only 12dB/octave, where the ported is 24dB/octave, so this would have a bit more bass than the equivalent sized ported box.
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Old 18th October 2004, 12:38 PM   #5
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All things considered I hope you haven't already purchased this driver. You don't say but it appears you want to use it in the home as a woofer, and there are far better ways to go. A smaller driver with a longer throw will allow you to accomplish the same task from a far smaller box with much better results. This driver is intended for professional use and it really isn't well suited to anything else. For subwoofer use it can be easily outdone in both bandwidth and SPL by a small (3.6 cu ft) folded horn loaded with an eight (MCM 55-2421).

www.bilfitzmaurice.com
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Old 19th October 2004, 12:58 AM   #6
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i know the box is going to be pretty big. I am going to throw some wheels on it so i can roll it around if i need to. I can really care less about the size of the box that dont matter to me i just wanted to get the max out of these woofers. I wanted to go with the 4th order bandpass system and was wondering what i would have to do to calculate the size and demensions of the box.
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Old 19th October 2004, 04:27 PM   #7
Ron E is offline Ron E  United States
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Quote:
Originally posted by lXxAndyxXl
i know the box is going to be pretty big. I am going to throw some wheels on it so i can roll it around if i need to. I can really care less about the size of the box that dont matter to me i just wanted to get the max out of these woofers. I wanted to go with the 4th order bandpass system and was wondering what i would have to do to calculate the size and demensions of the box.
I think your best bet for performance would be a big (200-377L) vented box, properly accompanied with a subsonic filter. Be careful with trying to put in the full rated power, though - especially without a subsonic filter. Wht kind of amp are you planning to use, some of the plate amps have built in subsonic filters that might work with this setup.......
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Old 19th October 2004, 04:36 PM   #8
Ron E is offline Ron E  United States
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Quote:
Originally posted by BillFitzmaurice
www.bilfitzmaurice.com
Maybe you mean:
www.billfitzmaurice.com
My keyboard has been droping lettrs lately, too! And I have this real problem with typing teh word teh (or the)....

Any new SB articles lately?
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Old 19th October 2004, 04:53 PM   #9
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i was going to go with a Mackie 1400i amp since it has a built in crossover. Right now i have 2 jrx125's powered by a cerwin vega amp a-200. i cant figure out how to use the formulas for calculating a 4th order bandpass,
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