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leadbelly 26th January 2004 03:27 AM

Please comment on my W8-740C design
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I'm about to begin building a small sub using a 100W plate amp and a TB W8-740C for someone other than myself. I'm trying to keep costs down as much as possible, hence I was hoping to get away with just the single driver in a ported alignment, instead of adding the 10" PR as Creative Sound does.

I get the following response in a modeling spreadsheet (Brian Steele's). The red line is Vbox=Vas=23 litres, Fbox=26 Hz, port is 3" x 5.75". The blue line is 1 ft3 sealed for comparison.

I love the F3 @ 30 Hz.

Comments? I know RAW recommended a 1 ft3 ported alignment for the W8.

leadbelly 26th January 2004 04:36 AM

Some other construction details:

Box, 1/2" baltic birch, heavily braced (max unbraced span 5").

Front-firing, with single port on the bottom, 4" legs.

For grill, I think I will use these:

Unfortunately, they are 10" since PE does not have 8". I'm hoping they will still look OK. Anybody used them?

GM 26th January 2004 05:47 AM

Due to the high inductance (Le), F3 referenced to the XO point will be considerably higher than 30Hz. Depending on the XO point, the 1/2 space F3 could be as high as 90Hz. Look at the huge broadband peaking around 100Hz on the factory FR plot to see what I'm referring to. Simple programs don't show this, so many sims bear little resemblance to reality.

Don't know what's caused you to get only a 5.75" long vent for a 3" diameter, but it's off by at least a factor of 5, and probably closer to 6 for a typical golden ratio cab. There's a reason CSS uses a PR.

It can be shortened somewhat by using a ML-TL layout, but even then I still get 24" long for a 3" diameter.

Also, due to the long vent length, the XO point will be limited to ~200Hz and there will be some peaking at Fb, so some damping of the vent will be required for best transient response.


leadbelly 26th January 2004 02:00 PM

Thanks, GM! It's great to get that kind of critique before starting up the saw!

You seem to know the driver already. Is there another good alignment for it other than PR?

Levc 3.1811mH
Zo 113.9893ohm
X-max 12 mm
Vas 23 Litr.
Qts 0.30
Qms 10.53
Qes 0.30

GM 26th January 2004 03:08 PM

You're welcome! I don't 'know' the driver beyond its published data, so my comments are only as accurate as they are. That said, I don't know what app. this thing was designed for except possibly car audio where there's lots of cabin gain to compensate for the high inductive roll off. For home use, the vented gain isn't all that much whether built as max flat or EBS.

I'm not saying don't build an EBS, I just want you to understand what sort of performance to expect and that you're going to have to build a Daline type cab if you don't use a PR.

Really, Vb = Vas, with a 9" long vent (~40Hz Fb) and some vent damping is probably about the best alignment for home use. You get ~89dB eff./40Hz, rising to ~92.5dB/100-180Hz before rolling off, making it a relatively compact/low distortion, economical wide dynamic range midbass/LF driver to fill in below a FR driver.


leadbelly 26th January 2004 03:37 PM


Originally posted by GM
Really, Vb = Vas, with a 9" long vent (~40Hz Fb) and some vent damping is probably about the best alignment for home use.
Pardon my simpleness, but do you mean a heavily damped 3" port 9" long, or a 2" port just damped enough to cut port noise?

GM 26th January 2004 04:52 PM

No, 3" dia x 9" long with just enough damping to flatten the peak at Fb.


leadbelly 6th February 2004 08:12 PM

I am now re-thinking my design and think I would rather use a downfiring design. I just really like how my sealed Tempest sub looks and sounds downfiring.

So, my idea would be to have both the port and driver on the same, bottom, surface. As for driver sag, it doesn't concern me much for a $40 driver.

Any opinions?

GM 6th February 2004 10:02 PM

Works for me, its mass/compliance ratio is extremely low, so sag shouldn't be an issue.


RAW 7th February 2004 01:41 AM

try ported
2- 2" X13.5 in my 14" cube ext. using .75" MDF.
Takes a full 150 watt plate amp with out a hick up;)))
The PR yes cost is more I am finishing the test results for the PR weighted at 250g I think that was it.

have fun


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