Let's build a custom Subwoofer for Carlman's basement theater

[richidoo]

Thanks Shane, I know about that real MDF. I prefer the lightweight soft powdery shit from Brazil (homedepot) because it is so much lighter. But it burns up blades and router bits. Carl has already bought the MDF from BuilderSupply. We'll see how that looks. I need a new blade too. That Trek speaker killed my blade.

You are correct about the box resonance. The pass band is 20-50Hz. Double thickness MDF's resonance will be way higher than 100Hz, so it will not resonate. So we are only concerned with flexing under internal pressure, which will be significant. Bracing and wall thickness will minimize that. Maybe some angle steel or just vertical 2x4 braces screwed in. The side walls will only be 8inches tall, so they are no problem.

Using factory driver specs:
Qtc .5 = 90 liters
Qtc .55 = 65 liters
Qtc .71 = 34 liters

SPL at 25Hz
Q  .5 = -9.64 dB.
Q .55 = -10.18 dB
Q .71= -12.48 dB

So of the boost switch gives 10dB we are all set. Just pick the Q to taste.

An interesting FAQ for subs. http://www.diysubwoofers.org/faq.htm

Carl is planning to build a pair of dual driver OB servo subs to place next to the main speakers for music. So deep bass for music will be very accurate. In that sense, would you recommend higher Q?

As for living with another person, I used to remember why, but it's been so long I forgot. Now I live with 4.  

I have WT2 software, but it is a year old. I am registered so I'll try to download it. Thanks Mike!

[richidoo]

Here is the plot of the driver with factory specs at Qtc = .55  (65liters)



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[richidoo]

Can the flooring surface of the platform lie directly on top of the box, or should there be a gap for vibration isolation? I guess it's whether you want to feel it in your feet. I think that would be annoying?

6 square feet (2x3') footprint will give about 100L volume inside a 1.5" wall thickness box.  So framer spacing of 2 feet should be enough, with 2 square foot extra unobstructed space inside that framing bay if we need it, which we will need for the extra volume we wasted by tilting the driver to fit.

To be on the safe side, make a center framing bay with studs spaced 26" on center, the bay is centered on the length of the platform. I'll send you a dwg Carl.

[bpape]

Realistically, you're not going to prevent feeling some of the vibrations if you're sitting on the platform. 

Personally, if it were me, and the driver will tolerate it, I'd build a 1/4 wave pipe from 2 of the cavities.  Talk about fast and tight... 

Bryan

[Carlman]

It's the last row in the theater, I won't be sitting on it anyway.. So, no big whoop to me.  That's essentially the 'make out couch' so a little vibration from underneath may be nice.

[richidoo]

Personally, if it were me, and the driver will tolerate it, I'd build a 1/4 wave pipe from 2 of the cavities.  Talk about fast and tight... 

Great idea Bryan!

Like this, but folded into thirds to fit:


The exit opens at the 1/4 wavelength you desire to boost. There will also be a minor null at a higher frequency.
http://www.t-linespeakers.org/projects/exolinear/rhinos.html
I have quarter wave simulator, but many brain cells would die in the effort.

[bpape]

Actually, I think you generally would prefer to have the pipe length be at the free air resonance of the driver so the backwave helps to damp it.  The one problem is that it can tend to want to unload below the tuning so I'm not sure it's optimal for HT usage - but it'd be outstanding for music and concert vids.  According to their specs, that's around 22hz.

The other option to extend response and avoid chuffing is to use a passive radiator.  Their doc shows tuning the PR from 10-25Hz.  If you tuned it low and used the boost in the amp just above 20, you could likely have some serious extension relatively smooth into the teens.

Bryan

[richidoo]

Yes, that's cool. The T/L is too complex for the schedule and the patience of the client anyway. I like the tall pillars though.

I'll check the out doc about PR. Thanks

[Scott F.]

I found the software CD for the woofer tester 2 unit I have.  I'm pretty sure I know what group of boxes the actual measurement unit is hiding in.  I'll try to dig it out tonight or tomorrow. 

Slightly off topic... You might want to contact Smith & Larson who are the folks who make the Woofer Tester 2. They have new software which is a free upgrade and it's much improved over the original versions. They ask that you contact them to get a login to download the software but it's worth the effort.

Their site is www.woofertester.com

Cool, I'll have to update my software.

Thanks Mike 

[bpape]

Just a quick link for an easy calculator for volume adjustments for bracing.

http://www.hometheatershack.com/forums/enclosure-volume-calculator/

Bryan

[richidoo]

Cool, thanks Bry

[Carlman]

Just wanted to say thanks to both Rich and Shane.. Shane dropped off the woofer tester and a sack o' nickels and I took mdf and the sub to Rich this afternoon.  We got started on testing and had a nice visit.

Thanks again, I'd be very lost without your help.

-Carl

[richidoo]

You're welcome! Least I can do for all the help you've given me.

WT2 is working fine, thanks Shane. I'll post the results when I have run it a few times for confidence.

This driver is humongous. I'll post a picture later. Welded steel box would be appropriate for this beast.

Yesterday afternoon, we had a pair of Chinooks strafe our house in formation. They flew directly over my back yard at about 500 feet. There is no sound like that, 12 blades beating a 4 rotor chorus of supersonic shock waves at 11Hz. I wonder if this sub can do that? I would pay to see that again.

[richidoo]

Shane suggested that I break in the new driver before measuring it. I was thinking about what signal to use. A disco song would have some bass, but nothing to put the sweat on this driver. So I used SoundForge to create a custom torture track.

At first I created a track of pink noise low passed at 80Hz. It did not seem to be very loud. I boosted it to clipping, but RMS was not very strong. I needed more energy down low. My daughter liked the noise sound though, she came up to me while I was editing it and said "I hear something weird in my bedroom."

I decided to use a 20Hz sine wav mixed 100% with 33Hz sine. I put the 20 in left channel and 33 in the right to get some overtones from interference. Combined the channels to mono, then convert mono back to stereo, I saved both forms if anybody wants it we can upload it to this server. It is one minute long, so you can put it on repeat.

I will try to setup my 80W AudioSource amp into bridged mode and run the mono track on the driver for a while. The surround seems very soft and pliable to me. Spider is pretty tough though.

The waveform looks like this:


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[Carlman]

That curve kind of reminds me 'Kilroy was here'.. ha.  Glad we've got time for break-in.  The platform is built.  I have the ability to remove the solid plywood top to route wires and such.  The plate amp has a home in the back corner.

Total space is 24" W x 50" L x 11.3".  I'm not sure why it's not 11.5... but it isn't.  I feel short-changed.

I did some basic right-triangle math to figure out if we'd still have enough room and we 'should'.  But I don't know how to figure out how the magnet comes into play in the dimensions.  Using a combination of a2 + b2 = c2 and sinA = a/c.. I can figure the hypotenuse by the angle of the triangle.. but.. those are straight lines.  By my calculation we have 15.56" of room from the middle front of the plane of the driver to the middle rear at a 45* angle.

But, when you put that big circle around the bass of the driver, I'm not exactly sure how to factor that... but it could get very tight.. It might have to be a 40* angle... and it could be less than that... I have plenty of room to notch out the plywood on the top as needed.  But at a 45, I don't think it'll fit in an 11" high box if my fuzzy Math is right.

-C