Please comment on my W8-740C design

[leadbelly]

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]

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:

http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=260-440

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

 

[GM]

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.

GM

 

[leadbelly]

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?

SENSITIVITY 1W/1m 84 dB
FREE AIR RESONANCE 28 Hz
Levc 3.1811mH
Zo 113.9893ohm
X-max 12 mm
Vas 23 Litr.
Qts 0.30
Qms 10.53
Qes 0.30

 

[GM]

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.

GM

 

[leadbelly]

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]

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

GM

 

[leadbelly]

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]

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

GM

 

[RAW]

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

Al

 

[leadbelly]

try ported
2- 2" X13.5 in my 14" cube ext. using .75" MDF.

Al, is that 13.5" port length each? If the box is 14" external, 0.75" walls, the internal dimension is only 12.5". How does the port fit?

 

[seangoesbonk]

try ported 2- 2" X13.5 in my 14" cube ext. using .75" MDF.

Is this the one I heard at NWDIY 2003? If it is, I would definately recommend it.

Al had 3 downfiring subs in his HT room. He asked me to guess which one was playing. I was wrong! It was the smallest one! He flipped it over to reveal the TB 8" vibrating like crazy!

It sounded pretty damn good, all things considered. I had heard a couple of different configurations using that driver, including dual PRs, and the dual ported definately sounded the best.

 

[RAW]

Thats for the kind words.
John chapman and Bob thought I was trying to give those sitting in the front row a cheap hair doo!

Anyway as it has been pointed by Sean that yes the TB ported will just do fine.I think he stayed in the HT room all day talking to Rick till the time of the draw
So he did listen to a few subs that were played that day.As well as the 5.1 Spires.


As for the port yes, they are each 13.5"
well take a 7" piece , glue them in the bottom of the cabinet the a 90 elbow then a 6" piece of PVC.Hand clean out the final 6" piece end to make it not sharp on the inside.I do this just to make a small radius by hand.Not needed but I do it anyway.
the inside of the 90 on the short side is 1.5" for a total on the port of 13.5"
Glue them in place with a poly glue then round them over with a router and a 1/2"round over bit.FLARED PORT.
I can shoot a picture in the AM and post it.

And yes you are correct on the new cabinet size I have for them.
Wait a tube kit coming for the Shiva and CSX 10".
More later.

Al

 

[RAW]

I will take this down to Seattle I do think so anyway.

So it will get a samll show against the Tempest designs that will be there.
You going down Sean?

Al

 

[leadbelly]

I just wanted to thank you for all your help and post a picture of progess. The baffle is 3/4" MDF, all other sides 3/4" Baltic birch ply; cutout is for 100W plate amp. This is with 2 coats of stain; more stain and clear coat to follow.

 

[RAW]

That is looking very good.

An externally hosted image should be here but it was not working when we last tested it.

Just like this as well.

Al
Let us know what you think of the finished product.

 

[leadbelly]

Thanks Al.

What is the height of those feet? I was thinking of making up some spikes, but wasn't sure if that was necessary to keep it from moving.

 

[RAW]

3" I think
When I do the lock Miter 14" I have a 3.5" foot.


Looking great good work

 

[DurdyD]

Hi Leadbelly,

I've been following your progress on this as I'm considering something similar. I like the looks of your cabinet and am curious to know what method you used to put it together - can't tell from the picture. I'm guessing miters and splines for the plywood with the MDF baffle set into a rebate. Close?

 

[leadbelly]

Hi Leadbelly,

I've been following your progress on this as I'm considering something similar. I like the looks of your cabinet and am curious to know what method you used to put it together - can't tell from the picture. I'm guessing miters and splines for the plywood with the MDF baffle set into a rebate. Close?

All 6 sides are 45 deg mitres. Of course, not all 12 joints are tight, so I added 10 small braces in the middle of each panel-to-panel joint.

This actually is a twin of another since I and a friend are building 2 simultaneously. The other one actually came out better, all joints tight enough to not need bracing, and I passed that on to him since he is new to woodworking and has hardly any tools.