Hi m8t the usb is reverse loaded, you need to wire the cabs up with the polarity inverted too.
Below is work doc i got a few years back hope it helps with some of your questions,
USB Bass Bin
USB project
What is it? What does it sound like? Where can I get one?
What drivers should I use? Design background and tekky info
Terms and Conditions of use
What is it?
The USB bin is a high efficiency, high power bass bin designed for general PA use and raves. It uses two 18 drivers in a band-pass horn configuration; that is, it has a short horn which gives it a high efficiency over 80hz, and a band-pass action due to its large rear and front chambers which allow for a shallow roll off extending sub bass response.
What does it sound like?
The USB is an extremely efficient bass bin, with a very clean solid sound without any honky peaks or distortions. It works great as a kick bin above subs (like Labs) but also on its own. This is because the horn mouths couple together as you add more bins and the huge power handing allows you to also push the USB low to give out serious sub bass.
Where can I get one?
The USB is a bass bin that you can build yourself. The design is fairly simple as far as horns go, and no trouble for an experienced speaker builder. To save hours of time and to ensure a totally professional finished product, we highly recommend buying the USB as a FLAT PLACK. They are priced around 200 each and are 5-axis CNC routed from two 8x4 sheets of 13 laminate birch ply. The panels just pop out and slot together in minutes.
(Speak to Insomnia on this site regarding flat backs i think he may be able to help you!)
Alternatively, you can download the plans.
What drivers should I use?
The USB uses two reverse firing 18 drivers. Some drivers are more suited to the USB than others. I advise you to do some background reading so you understand what loudspeaker T&S parameters are and what they mean.
Basic guidelines:
Fs: 25-32Hz
BL product: 21-27
Qts: 0.25-0.35
Xmax: >6mm
Voice coil: 4in or more
Power handling: >500w rms
From my experience with this type of bass bin, a lower Fs, Bl and higher Qts value would give you a slow, loose sound with more sub. Likewise, a higher Fs, Bl and lower Qts value gives a tighter, punchy sound with excellent efficiency over 60Hz but a much steeper roll off below this.
The USB was designed to use the Celestion Frontline FTR18-4080FD 1000w.
Other drivers which will work:
PD184
Celestion FTR18-4080F 600w
PD186 (although this may not fit)
Fane Colossus 18B600
Fane 18XB
Beyma 18P1000
P-audio challenger 18LF
Eminence Magnum 18LF
Eminence Magnum 18HO
B&C PS100
B&C PZB100
B&C TBX100
(speak to tony a.s.s. on this site regarding B&C drivers)
Eminence Omega pro 18
REMEMBER:
1) You must use the same driver throughout your stack. Mixing and matching different drivers in the USBs will seriously degrade performance.
2) You must invert the polarity on the drivers to keep the USB in phase with the rest of your system.
Design background and tekky info
Following the success on the Lab Sub, it was decided that the USB should be designed as an accompanying bass bin with the same front dimensions to allow for attractive stacking and ease of truck loading. A traditional horn design was considered, but the band pass configuration was settled upon as it allows for a wider response allowing you to use the USB on its own. By making the USB to match the lab sub however, it was no longer possible to mount the drivers in the traditional forward firing configuration. It was therefore decided that the drivers would be reverse loaded and fire backwards through the magnet. In doing so, the front chamber has been made smaller, decreasing efficiency slightly around 60Hz, but the rear chamber is now larger, increasing sub bass response. Apart from this, there is no loss in output (as a driver acts like a piston) and there are actually some advantages:
1. The magnet now has a direct heat path to the outside and a cooling effect due to airflow over the magnet. This eliminates heat build up in the driver chamber, decreases power compression (loss in output due to heat) and allows for greater power handling.
2. The magnet acts like a phase-bung; to an extent, reducing path length differences and giving a much cleaner sound and flatter response.
3. Mounting the driver is a lot easier
4. There is protection of the paper cone due to the basket.
5. It is actually possible to reach round the horn and check on the magnet temperature!
The optimum horn mouth size was found mathematically to be 800cm2. From this a fairly shallow hyperbolic-exponential horn flare parameter of 0.3 was chosen to maximise horn loading, and approximated using 3 conical sections. The design then went through many revisions to make everything fit within the space we had and underwent extensive modelling in David McBeans HornResp program to optimise the front and rear chamber volumes. Eventually a design was decided upon. To keep costs down, every effort was made to allow the USB to be built from 2 sheets of 8x4 with minimal wastage. For this reason, it was decided that the cross-brace should be cut out of the central brace, giving the USB its attractive swept bracing. Finally a prototype was built. So far were really happy with the results. A crude measurement of the response has been done using a sin-wave generator, multimeter and Db meter, and our ears confirm what you can see a very flat response above 100Hz, and a slow smooth roll off below this.
Terms and Conditions of use
1. The USB design and plans are protected by UK copyright laws and international treaty provisions.
2. Use of the plans is strictly limited to production of max 6 units per person and/or company. If you wish to build more a licensing agreement can be arranged.
3. Distribution or reference to the plans is strictly forbidden without prior permission and reference to
www.dissident-faction.co.uk
.
4. This design is provided "AS IS", without warranty of any kind. We take no responsibility for use and/or performance of these designs.
5. Sale of fully produced product for profit without prior permission from copyright holder is strictly prohibited.
USB bass bin, Copyright David Trotter 2006. All Rights Reserved.
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