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[Howard Ordemann]

Hello everyone,

I am looking to make a box for the peavey black widow low rider 18" woofers (1808-8HPS). The ones that are in the QW series subs. I was wondering if someone would be able to draw me up a quick box plan for a single and dual cab that would be right for these drivers. I have done a little research on the WinISD program and feel very lost when using it. I have attached the driver specs. I want to make 2 duals or 4 singles and just don't have the cash to buy those cabs right now. Any help would be greatly appreciated. Thanks

[Art Welter]

Howard,

Peavey has a PDF that suggests three box alignments:

Small:  5 cubic foot net volume, 2) 6” diameter 15.25” long ports, FB 37 HZ, F3 40 HZ
Medium:  6.75 cubic foot net vol., 2) 6” diameter 12.75” long ports, FB 34 HZ, F3 35 HZ
Large: 9 cubic foot net vol., 2) 6” diameter 12” long ports, FB 30 HZ, F3 31 HZ
The large box reduces power handling by at least 15%.

Use a steep filter (like a LR 24  dB per octave) set for about 5 HZ below whatever tuning you decide on.

I would suggest making the cabinet dimensions truck integers, division of 90 inches.
22.5, 30, 45 inches will all fit into trucks nicely.

Art Welter

[Reginald Dennery]

I would suggest checking out www.speakerplans.com. Use the search button and you will find a few for the lowrider. I believe the b&c18 is suitable as a single reflex box and the low rider might work well in a g-sub. There are quite a few people on there who might be willing to help you

[Dave Rickard]

here you go--

[Howard Ordemann]

Thanks for the answers so far everyone. Still a few more questions before I fully understand. That net volume is calculated by the just the inside dims of the box right? I don't have to take the account for the driver? Also, If i go with the medium box it says I need (2) 6" diameter vents 12.75 inches long. Am I just reading this wrong cause that seems huge to me?

[Dave Rickard]

Howard Ordemann wrote on Sun, 10 May 2009 19:57

Thanks for the answers so far everyone. Still a few more questions before I fully understand. That net volume is calculated by the just the inside dims of the box right? I don't have to take the account for the driver? Also, If i go with the medium box it says I need (2) 6" diameter vents 12.75 inches long. Am I just reading this wrong cause that seems huge to me?

Ideally, you account for the driver volume, the volume of the ports themselves, and account for internal bracing.  The port dimensions seem reasonable.  

Do it once, do it right.

[Art Welter]

Dave’s exactly right.

If anything, the ports recommended are a bit small.

Port displacement should be much larger than the Xmax displacement of a speaker, or when you hit the speaker hard and low, the air gets blown out of the ports, whistling instead of working.

(Edit to correct technical error pointed out by Phillip Graham on Mon, 11 May 2009 22:39)

[Phillip_Graham]

Art Welter wrote on Mon, 11 May 2009 22:34

Dave’s exactly right. If anything, the ports recommended are a bit small. Port displacement should be much larger than the Xmax displacement of a speaker, or when you hit the speaker hard and low, the air gets blown out of the ports, and the speaker unloads.

To the OP,

Art is right about the ports being small for a driver of this Vd.

However, the latter statement about port "unloading" is technically incorrect.  As the velocity of the air in the port increases (due to smaller port or larger driver excursion) the Reynolds number of the air in the port increases, resulting in potential turbulent flow with the port, which in turn changes the acoustic impedance of the port.

As the Reynolds number increases, the port impedance becomes increasingly resistive, and the resulting response more like a sealed box.  So, while the driver is not "unloaded" it is misloaded by a sealed box with low Qb.

This will change the driver acoustic response, and driver excursion--how will depend on the box alignment.

Additionally, since the voice coil temperature has typically risen greatly at high power, the driver's effective Qes has also changed substantially.

A rule of thumb is to try to keep the port air velocity under 20m/s, but even this value has caused a substantial change in the acoustic impedance of the port (dramatic increase in resistance, moderate change in inertance).

A couple of AES preprints on this topic:

N.B ROOZEN, J.E.M. VEAL, J.A.M. NIEUWENDIJK, Reduction of Bass-Reflex Port Nonlinearities by Optimizing the Port Geometry, AES preprint 4661.

J. VANDERKOOY, Nonlinearities in Loudspeaker Ports. AES preprint 4748.

[Tim Padrick]

The QW218 is a split box.  The dimensions that the factory uses for each driver are:

Inside width 19.5"

Inside height 23"

Inside depth 23"

So the volume (before subtracting the driver and port) is 10315.5 cubic inches = 5.97 cubic feet for each driver.


Port width 19.5"

Port height 3.5"

Port depth 10.5" (this is to the outside of the baffle).

So the port area is 68.25 square inches, which is equivalent to a tube of 9.32" diameter, two tubes of 6.59" diameter, or four tubes of 4.66" diameter.  (Remember that this is per driver.)  That's a lot of port, with a 10.5" depth.

(Check my math before cutting!)