Subwoofer driver choices for DIY sub.

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Hi Bill C,

Look for the paper by Salvatti, Devantier and Button with the title: "Maximizing Performance from Loudspeaker Ports". Harman - Scientific Publications Publications

In general, the port should be radiused at both ends, with a small flange being added to the port on the inside. If the port diameter appears to be too small use a larger port tube or use multiple port tubes, it's the cumulative cross-sectional area that matters, e.g.: use two 3" ducts instead of one 4". Find the length using a program like Hornresp or WinISD.

Regards,
 

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Flare flow separation

Be careful of how you interpret the word flare.
A simple radius is great, at lower SPL's.
At higher SPL's flow separation can occur.
A better "flare" has a simple angle rather than an exponential radius that the flow can not stay attached to.
Big 45 degree angles make good "flares" as do 22 degrees on a side symmetrical "flares".
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Look at the screen snip at the bottom.
If the angle approximated by the first 1/3 of the flare shown was continued to the same OD as the spherical flare, then the flow separation and consequential turbulence would never form.
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I have read that at high SPL's and excursions, the traditional flare will decrease the effective flow area causing compression.

Dave
 

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Hi AndrewT,

Look for the paper: "Vented-box geometry and low frequency reproduction: the aerodynamical approach" by Morkerken, Parzy, Pellerin and Polack. AES 112th Convention, Munich, Germany 2002 May 10-13.

Or in short, a smooth laminar flow is desirable, and the vortices (turbulence) create the "blowing noise".

Regards,
 
Glad Oliver tipped us off about using wood rings and a router to finished the inside of a PVC port. Can remember many times sanding/polishing the end of a pvc pipe thinking this can't be doing much.
[a hole would need to cut in box anyway, so router/hole cutter would be set and ready to use]
 
Driver Properties
Name: Goldwood GW-12PC-4
Type: Standard one-way driver
No. of Drivers = 1
Fs = 32.9 Hz
Qms = 3.9
Vas = 2.59 cu.ft
Cms = 0.184 mm/N
Mms = 127.3 g
Rms = 6.747 kg/s
Xmax = 5 mm
Xmech = 7.5 mm
P-Dia = 259.8 mm
Sd = 530 sq.cm
P-Vd = 0.265 liters
Qes = 0.44
Re = 3.7 ohms
Le = 1.7 mH
Z = 4 ohms
BL = 14.88 Tm
Pe = 225 watts
Qts = 0.395
no = 0.572 %
1-W SPL = 89.72 dB
2.83-V SPL = 93.07 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square (optimum)
Vb = 3.304 cu.ft
Fb = 27.56 Hz
QL = 6.501
F3 = 31.06 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 4.001 in
Lv = 10.04 in

A 20" cube with 3/4" material will be about the right size, should handle 200W on program material, and go to 27.5hz with just a bit of room gain (4dB), should be able to hit 110dB in room with your 100W plate amp.
 
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Ugh...that stuff, I tried it for a pair of speakers a few weeks ago, I didn't think it worked well at all. I think I'd go for the MDF over the lightweight plywood.

Look for Baltic Birch, it's not that hard to find. $40 for 1/2", $59 for 3/4".

** I may put the light plywood pair up on swap meet - "free boxes".

What's wrong with the poplar? I've built subs with birch faced poplar several times, it's plenty stiff and light, but I perfer to coat it internally with damping material. I certainly wouldn't use it for mains without a thick coat of damping material.

Until I get an answer to this question, I'm going to assume Doug doesn't know how to design a speaker.
 
Driver Properties
Name: Goldwood GW-12PC-4
Type: Standard one-way driver
No. of Drivers = 1
Fs = 32.9 Hz
Qms = 3.9
Vas = 2.59 cu.ft
Cms = 0.184 mm/N
Mms = 127.3 g
Rms = 6.747 kg/s
Xmax = 5 mm
Xmech = 7.5 mm
P-Dia = 259.8 mm
Sd = 530 sq.cm
P-Vd = 0.265 liters
Qes = 0.44
Re = 3.7 ohms
Le = 1.7 mH
Z = 4 ohms
BL = 14.88 Tm
Pe = 225 watts
Qts = 0.395
no = 0.572 %
1-W SPL = 89.72 dB
2.83-V SPL = 93.07 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square (optimum)
Vb = 3.304 cu.ft
Fb = 27.56 Hz
QL = 6.501
F3 = 31.06 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 4.001 in
Lv = 10.04 in

A 20" cube with 3/4" material will be about the right size, should handle 200W on program material, and go to 27.5hz with just a bit of room gain (4dB), should be able to hit 110dB in room with your 100W plate amp.



Thanks djk I understand this and it's perfect and this is what I'll use as I acctually know what I'm doing. Thanks Everyone for all the help it sure paid off asking for help.

BillC
 
Bill, post some pics if you can.

Turbo, If you like HD plywood, it's fine with me.

I take issue with baseless statements. You're certainly not getting 4x8 sheets of real baltic birch for $59 (more like $100+). Poplar and birch have the same density to stiffness ratio, poplar is 20% lighter and less stiff. I personally think 5-ply birch faced poplar is stiffer than multi-ply birch, due to the smaller number of thicker plys.

Fact is, for $37 for a 4x8 sheet, poplar ply is a good material for a speaker cabinet on a budget, and certainly better than MDF (lighter AND stiffer) for a large subwoofer cabinet which the topic of this thread.
 
Some reading/learning and testing the best types of wood (constructing methods) for loudspeakers.
BBC RD 1977/3 Factors in the design of loudspeaker cabinets.

Think I've read that before. Any time you use any plywood in a pair of main speakers you really should damp it internally. I damp my plywood subs, but have tested them without the damping and it's very tough to notice a difference. Just put together a small PA system where the subs are coated with accust-x paint and the tops have a ton of dynamat (peavey plastic cabs).
 
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