Newbi: Box Volume and Port Length

[69stingray]

Hello everyone. Great forum here.

I have finally decided to build my our subwoofer box. My home theatre / stereo system does not have one. I will be using a 10" sub. Can someone calculate the desired volume, port length and diameter? I have tried some of the "online" calculators and I get screwy numbers. Here are the specs:

10" diameter
Fs = 35
Vas = 0.07363
Qes = 0.39
Qms = 11.3
Xmax = 0.0051
Qts = 0.377

[simon5]

Can you give the units for the Vas and Xmax measurements please? It's cu.ft and inches or something else?

[Ron E]

Originally posted by 69stingray
10" diameter
Vas = 0.07363
Xmax = 0.0051

I am assuming that Vas is in meters^3 and Xmax is in meters
Try the online calculators again with Vas=73.63 Liters and Xmax=5.1mm

[69stingray]

Thanks Ron E., that is what I figured out around midnight last night. They are in m^3 and meter.

I retried the online calculators this morning and came up with the following:

Inside volume: 2,936 in^3
Port Diameter: 3 3/4"
Port Length: 11 1/3"

Does this at least sound right?

[simon5]

The data the online calculator calculated is ok.

It won't be a strong subwoofer, not much output under 35 Hz.

If I was you, I would use a 30W amplifier on it, and put the driver in a 100 liter internal box with one 3 inch diameter port 11 1/4" inches long for good output down to 18 Hz.

Good luck!

[69stingray]

Thank you simon5-

For my own education, how did you come up with those numbers? The volume is considerably higher then the online calculators.

In regards to amps, I was thinking about using Keiga KG-3100, which is 50W. What other options are there?

[simon5]

I found these numbers with slight trial and errors method in WinISD Pro, not the online form of it. I try to match a theorical room gain curve to have a nice and flat response down very low in frequency.

To compare the two enclosures easily, you can look at :

http://www.linearteam.dk/default.aspx?pageid=isdonline

Enter Vas (in liters), Qts and Fs.
Enter Vb = 48.12 liters for your idea, then enter Fb = 35.3 Hz (11 3/4" x 3 3/4" diameter pipe).
Click on Draw it, you'll see your curve.

Then to see my curve, enter Vb = 100 liters and Fb = 20 Hz (11 1/4" x 3" diameter pipe).

You'll see that you have better output down low.

You can use your Keiga with no problems especially if it's rated at 50W RMS continuous, just don't use too much power when playing movies with deep bass like LoTR. If it's a linear volume knob, stay under 1/2 if possible hehe! If not possible, you can go higher, then lower the volume when you hear too much distortion for your taste. If the rating on the Keiga is a bit too much optimistic, you can probably use the full power output.

But then, as you can see, you lose some output in the 30-40 Hz region, so it depends on how you like your sound.

[69stingray]

Thanks simon5, I will play with those numbers.

I do have an idea - I am actually using two 4-ohm woofer wired in series to get 8 ohms. I was just going to make one box with a divider for two equal volumes. Can I have two different volumes?

One volume tuned for a lower freq. and the second volume tuned for a slightly higher freq? We I run into out-of-phase problems or cancellations?

[simon5]

You can do that, but it's hard to make it sound good I think. You may find that the higher tuned subwoofer have a bad transient response. You better tune them both at the same frequency, like a single 200 liters box with 2 subwoofers. You could go with a single vented subwoofer and a single sealed subwoofer, probably better... but I prefer my 200 liters idea!

[69stingray]

OK, I have completed my box design, I think.

I went forward with the 200 liter internal volume. I am wiring two (2) 4-ohm subs in series and using 2 ports.

The internal dimensions are 18-5/16 x 18-5/16 x 18-6/16. The brace has five (5) 8"-diameter holes. I plan on using 3/4" MDF for everything.

The tuning freq. works out to be approx. 29.5 Hz

QUESTIONS:
1. What does everyone think?

2. I will be using a 45W amp. from Parts Express
http://www.partsexpress.com/pe/showd...number=300-784

3. What about stuffing? I have read some are in favor and some are not.

4. A guy at work suggested that I mount one sub facing forward and the other sub facing up. What do you think? It will be positioned in the corner of the room BEHIND the TV and the rest of the speakers.