Detrmining box size and port for sub

[regal]

How do you determine the box size and tuning frequency? I have entered T/S parameters for an 8" sub into the ported.xls spreadsheet from diysubwoofers.org. The "optimum" gives an F3 of 42 hz which is the lowest of the alignments shown. But it really isn't low enough. What would be stopping me from going lower with the tuning frequency? It just seems there is an infinite # of possibilities which give better graphs. Unfortunately Madisound doesn't know the xmas of the woofer. But I am guessing 10 mm all the graphs show going high around 20hz.

Is there anywhere on the web that tells you how to make a selection?

[simon5]

You can use Unibox or WinISD Pro if you don't want to use a standard alignment and want to tune further.

BTW, 40 Hz isn't so bad for a 8 inches subwoofer... to go lower, you usually need to use a bigger subwoofer.

[regal]

So its OK to use a non-standard alignment? How do you know when you tune too low?

[simon5]

Yes it's ok to do it.

You'll know because the SPL graph will peak around the tuning frequency. When you tune lower, it's harder to have a flat frequency response curve. You'll also significantly exceed Xmax lower than the tuning point and higher than the tuning point.

If you still achieve flat FR and don't exceed Xmax then you found out a good alignment for your subwoofer.

[rabbitz]

What you want to do is post the driver brand and type as someone might be able to help you more. Download a copy of WinISD as well, which will make modelling easer.

You're normally not going to get a real low F3 with a 8" driver and what you want to look at on a sub is the F6 and even F10 as room loading will lift the bass. Try an alignment with a less steep more gentle roll off as with room loading, you will get a lower response. The F3 might appear to be a bit higher in the response, but will give a much better performance. I've found most of the time, the optimum result from the software doesn't give the best in room result.

I try not to tune the port below the driver's Fs, but that is very driver dependant. You can always try it, and if it doesn't work, you can always cut off some of the port.

[rcw]

In the article about compliance scaling on Rod Elliots website at
www.sound.com.au
You can find out how to derive a set of parameters to put into modelling programs that let you pre-select a particular f3 or box size.

[J.k.Wilson]

you can to www.carsterio.com/help/articles.cfm they offer a ton of free calculators for deffrent box costructions.