Twin 18" PA-Sub 6th-Order Blowthrough in Trunk... Sanity Check.


recon440

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Sep 22, 2011
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Hi everyone,

I'm in the process of designing a new enclosure for next season and would like to get some advice.

To give some context, the vehicle in question is a 2001 LS430 which has a sealed trunk which only has a ski-pass, which is suboptimal for conventional bass-reflex designs. At the current power level (~1500W Fosgate T1500-BDCP) I was using two 12" Alpine Type R's in a 6CF ported alignment tuned to 37Hz. This setup had immense lows (30-35Hz) but fell flat on it's face above 45Hz due to the trunk and ski-pass acting like a poor 6th order, not to mention annoying trunk rattle. Here's a photo of that:
vented.jpg

Next, I built a 4th order bandpass (1.8cf rear chamber / 3.8cf front chamber) and the output was somewhat better and more flat but the lows were not as powerful. To remedy this I modified the 4th order to a 6th order by porting out the rear chamber using a pair of 3"x12" PVCs. This gives a net tune of ~33Hz on the rear chamber and ~75Hz on the front chamber. Output from 30Hz to 80Hz is quite impressive and the box response pretty much the same as the Hornresp model to the ear. Now I'm looking to get even louder and the bottleneck is essentially the power handling on the Type Rs.

This is what the modelling looks like in Hornresp. Alpines @ 1000W in a 6CF vented (grey) vs Alpines in 6CF series 6th-order bandpass (black curve):
typer_v_vs_6thorder.PNG


Below is what the box looks like now:
6th_order_1.jpg 6th_order_2.jpg 6th_order_3.jpg 6th_order_4.jpg


For this season, I am planning to go louder with more power (~4000W). I am targeting to be very loud between 35-100Hz for which I have two options:
- 2 x 12" DC Audio XL m2 (each 2200W RMS)
- 2 x 18" B&C 18SW115-8s Neos (each 1700W RMS, 3400W program power)

I actually have both sets of drivers as I am too impulsive and had to pick up a pair of B&Cs cause the price was too good (~350USD a piece brand new). The B&Cs are monstrous in terms of cone area with a motor size that is unassuming compared to ferrite offerings even though they house a 4.5" inside-outside would coil... LOL. Engineering and build quality is really impressive on these drivers.
driver_comparison.jpg



The DC XLs are attractive in that they are compact and can essentially be retrofitted into the existing enclosure but the B&Cs are even more compelling to me for a few reasons: the wow factor of two 18s, the ability to run them outside of the bandpass as a bass-reflex as PA-sound rig, and the weight (~28lb a piece which is half of the XLs). The motor force on the B&Cs is impressive and on paper the only place where they fall short in comparisons to the XLs is xmax (14mm vs 26mm) but I don't think this is an issue due to cone area and the fact that they have a huge xmech of 60mm p-p which is atypical for a PA driver.

Obviously to fit the 18's I will be redesigning a box into two piece sections: the rear low-ported section which houses the drivers which will then sleeve into the front section which will be build to conform to the front aperture of the trunk. Optimizing these in Hornresp I am working with the following sections:
- Front chamber: 6.6cf tuned to 74Hz (front port 5cm long, ~75sq in)... preferably would like more port area but it's all I can do with the ski-pass. Port velocities even at 3000Ws are still <40m/s which is not bad.
- Rear chamber: 3.8cf effectively tuned to 40Hz (twin 4"*8" PVC) as a bass reflex, but drops to 36Hz when loaded into the 6th order alignment with the front chamber.

The sleeve looks something like this with the 3.8cf loading into it:
schematic.PNG



If I were to go with the DC XLs then I would just use the existing 6CF box that the Type Rs were using but with more power.

When I compare the B&C to the DC XLs, the B&C dominates everywhere on paper.
Here is a comparison of the B&C 18s (dark) and DC XL 12s (light grey) normalized to @ 3400W.
Even at 3400W I am within the rated xmax of the B&Cs of 14mm (black line) from 30Hz and upwards. In fact the xvar on the B&Cs is 16mm and with xdamage 60mm peak-to-peak there is no risk of unloading these things that hard even at 20Hz.
bc_vs_dc_3400w_output.PNG
bc_vs_dc_3400w_displacement.PNG


The main question is whether there is something I am missing because this all seems too good to be true. Has anybody else use pro-drivers for such an application? With current sims the DC XL's will generally be better <35Hz but with that being said I don't think that they can even remotely hang with the pro-drivers in terms of a wide broad-band response from 40Hz-100Hz, which for me is the sweet spot of bass and where real loudness is perceived. I'm not really interested in doing hairtricks. In fact I'm thinking the B&Cs should model closer to reality than the DCs due to the fact that their inductance is much lower thanks for the shorting ring (1.9 Le for the B&C vs 5.2 Le for the DC).

Thanks for any advice...
 

THATpurpleKUSH

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Output on PA drivers drops fast below fs, those PA 18s drop like a rock below 40hz.

They will make a decent SPL number but dont expect them to play low.
 

hispls

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I find it very hard to believe that you will make enough airspace to really do those 18s justice. If the DCs will model well for you they should hold up to at least double the power than your R's can manage.
 

recon440

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Sep 22, 2011
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Output on PA drivers drops fast below fs, those PA 18s drop like a rock below 40hz.

They will make a decent SPL number but dont expect them to play low.
On the Hornresp sim the cross over point where the XLs start overtaking is at 35Hz on the above designs.

Out of curiosity I tweaked port lengths while keeping volumes the same and see how the B&Cs would do in each alignment:
- B&Cs using 30cm rear ports vs 20cm (black)
- B&Cs using 15cm front port vs 5cm (grey)

As you can see the output using the longer ports (black trace) give huge low-end gain at 30Hz at the expense of 3-5dB on the top end (40-100Hz). I don't usually play any modified music and prefer to get loud in the spectrum where typical music is mastered so I'm thinking I could be happy.

Capture.PNG



Fs on the B&Cs is 32Hz. When loaded into that alignment it brings the fundamental frequency up to 36Hz based on the length of the front port. If I lengthened the ports I can get effectively get the fundamental frequency all the way down to 20Hz if I wanted. At the end of the day in the case of these B&C drivers, on paper they have the best of both worlds with an xmax much higher than typical pro drivers with the higher efficiency which is surely beneficial >50Hz.


In terms of a simple bass reflex, B&C recommends a ported alignment 5.3CF tuned to 35Hz. Overall in the case of a 6th order the total net system volume will be ~10cf+ (depending on how tightly to the trunk I can build the front chamber "sleeve"). Generally in series 6th order, the low-tuned rear chamber is much smaller than the front chamber so I think in terms of the design objective this is well in the real of a useable volume (~40% of what would be recommended for a standard bass-reflex requirement). This will also the design constraint so that the rear chamber can actually be fit into the trunk in one piece... LOL
 
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Buck

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On the Hornresp sim the cross over point where the XLs start overtaking is at 35Hz on the above designs.

Out of curiosity I tweaked port lengths while keeping volumes the same and see how the B&Cs would do in each alignment:
- B&Cs using 30cm rear ports vs 20cm (black)
- B&Cs using 15cm front port vs 5cm (grey)

As you can see the output using the longer ports (black trace) give huge low-end gain at 30Hz at the expense of 3-5dB on the top end (40-100Hz). I don't usually play any modified music and prefer to get loud in the spectrum where typical music is mastered so I'm thinking I could be happy.

View attachment 24618


Fs on the B&Cs is 32Hz. When loaded into that alignment it brings the fundamental frequency up to 36Hz based on the length of the front port. If I lengthened the ports I can get effectively get the fundamental frequency all the way down to 20Hz if I wanted. At the end of the day in the case of these B&C drivers, on paper they have the best of both worlds with an xmax much higher than typical pro drivers with the higher efficiency which is surely beneficial >50Hz.


In terms of a simple bass reflex, B&C recommends a ported alignment 5.3CF tuned to 35Hz. Overall in the case of a 6th order the total net system volume will be ~10cf+ (depending on how tightly to the trunk I can build the front chamber "sleeve"). Generally in series 6th order, the low-tuned rear chamber is much smaller than the front chamber so I think in terms of the design objective this is well in the real of a useable volume (~40% of what would be recommended for a standard bass-reflex requirement). This will also the design constraint so that the rear chamber can actually be fit into the trunk in one piece... LOL
The rear chamber still needs to be large enough for the drivers to extend themselves. The rear chamber typically still needs to be close to the recommended airspace. If it's too small, you're going to have a severe low end roll off. Those B&C's are definitely not like most subs, though. But the rear chamber doesn't combine with the front chamber airspace quite, because you have the rear port that individually pressurizes the rear chamber. So the rear chamber needs to be big enough. With many series 6th orders, the front chamber can widely vary. Like you don't even have to try hard on the front chamber because the rear chamber is what matters. The front chamber can be small or big have a loading wall that's tuned or literally just a wall to shrink the opening from the front chamber, or even no wall at all. The front chamber on a 6th order is very similar to how you would add vehicle's cab into the equation, except for the rear port/front port relationship. Like you wouldn't make your rear chamber too small just because you put it inside of a car.
 

Buck

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Like here's a series 6th I did for one 12. And I'm just showing this stuff in general to help you maybe make some decisions, not even saying you don't have it covered necessarily. Your box size does concern me.
 

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Buck

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Here's the inside. I will delete these if it's an issue, I'm not trying to thread jack. I'm showing you the size of successful series 6th order I've done. This box was 26.75d x 23h x 48w.
 

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Buck

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And lastly. Ok. Here's a series 6th order for 2 Fi Team 18's. Uhhhh you can clearly see the size of this monster, it's a full wall. This box is 46.5d x 35h x 48w. I was kind of upset he didn't take a good video of it because he's busy, but he said it was super loud. He's one of my favorite guys I design for because he's always like "do the most ridiculous thing you can." It's only 2 4" vc 18's on 8000 watts lol. This was a very low tuned enclosure, I think 25 hz or so on the rear chamber. Now the front chamber on this one is larger than I had to go, we just did it larger to pick up around the 45-55 hz range or so because of how low the overall box was going to play. That rear chamber was 10.4 cubic feet net iirc.
 

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recon440

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Hi Buck,

Thanks for all the examples you posted and I really appreciate the insight. Your designs look really impressive.

Eyeballing it it's almost like the front chamber in your designs is generally 200-100% the size of the front chamber. I don't saying that as I think it's a funny that most people refer to a "ratio" when designing 4ths and 6ths but it does give an rough idea of the Q of the box (amongst a myriad of the factors). With such a generalization am I correct in thinking you are typically designing for a flat free-air response to ~30Hz? Regardless, it looks like a realistic ratio I'm using here (~7CF:3.5CF) but I usually like to simulate it to get a better idea. :)

To be frank, taking into account Hoffman's Iron Law, in my case I am quite aware that total box size might be overall undersized to leverage the full low end sensitivity of two 18's (let alone PA ones) but it's pretty much the largest I believe I can feasible slot into my trunk. Yes, I can do the XLs but weight and the top end response of these B&Cs is really attractive. It's also compelling that if I want to mess around at a car meet, I can pull out the rear section out of the front section and play it as a simple vented enclosure @ 130+ @ >60+Hz which is kind of a sweet spot of bass thats audible in PA applications.

I'm not exactly targeting to get loud at 30Hz and am looking to get loud in the sweet spot ~40Hz and up. In my experience a true car subwoofer will probably be 3dB louder down low (or at least tuning peak base on QTS of a given driver) but give up a lot on the top end (~6-10dB if were talking 70Hz+). As I understand it, you need at least 4 times the power or 4 times the box to get as loud 1/2 octave lower which get unfeasible when were talking about already pretty much maxing up the trunk volume of a large sedan... LOL. I think that's a feasible trade-off I can live with for perceived loudness.

Ultimately, I am hoping to get much louder than the two Type Rs broad-band. Is there anything that concerns you in the sim here? It seems that it's picking up at least 5dB in the sweet spott in this regard watt-for-watt. Even on the low-end (<35Hz) it should be similar and should pick up at least another 3-5dB when I dump 2-3 times more power on the B&Cs. I think that the inductance of the B&C is quite low and well controlled so that should be quite accurate in regards to simming it.

As for tuning in the future, it seems like there is a lot of flexibility with the front and rear port adjustments. Despite the smaller chamber, based on port lengths it does not appear as though they are mutually exclusive in regards to having a large chamber to let the woofers "breathe". If I make the front port quite long, we can get the xmax extremely high at low frequencies. Is that something you noticed too?
 
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Buck

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Hi Buck,

Thanks for all the examples you posted and I really appreciate the insight. Your designs look really impressive.

Eyeballing it it's almost like the front chamber in your designs is generally 200-100% the size of the front chamber. I don't saying that as I think it's a funny that most people refer to a "ratio" when designing 4ths and 6ths but it does give an rough idea of the Q of the box (amongst a myriad of the factors). With such a generalization am I correct in thinking you are typically designing for a flat free-air response to ~30Hz? Regardless, it looks like a realistic ratio I'm using here (~7CF:3.5CF) but I usually like to simulate it to get a better idea. :)

To be frank, taking into account Hoffman's Iron Law, in my case I am quite aware that total box size might be overall undersized to leverage the full low end sensitivity of two 18's (let alone PA ones) but it's pretty much the largest I believe I can feasible slot into my trunk. Yes, I can do the XLs but weight and the top end response of these B&Cs is really attractive. It's also compelling that if I want to mess around at a car meet, I can pull out the rear section out of the front section and play it as a simple vented enclosure @ 130+ @ >60+Hz which is kind of a sweet spot of bass thats audible in PA applications.

I'm not exactly targeting to get loud at 30Hz and am looking to get loud in the sweet spot ~40Hz and up. In my experience a true car subwoofer will probably be 3dB louder down low (or at least tuning peak base on QTS of a given driver) but give up a lot on the top end (~6-10dB if were talking 70Hz+). As I understand it, you need at least 4 times the power or 4 times the box to get as loud 1/2 octave lower which get unfeasible when were talking about already pretty much maxing up the trunk volume of a large sedan... LOL. I think that's a feasible trade-off I can live with for perceived loudness.

Ultimately, I am hoping to get much louder than the two Type Rs broad-band. Is there anything that concerns you in the sim here? It seems that it's picking up at least 5dB in the sweet spott in this regard watt-for-watt. Even on the low-end (<35Hz) it should be similar and should pick up at least another 3-5dB when I dump 2-3 times more power on the B&Cs. I think that the inductance of the B&C is quite low and well controlled so that should be quite accurate in regards to simming it.

As for tuning in the future, it seems like there is a lot of flexibility with the front and rear port adjustments. Despite the smaller chamber, based on port lengths it does not appear as though they are mutually exclusive in regards to having a large chamber to let the woofers "breathe". If I make the front port quite long, we can get the xmax extremely high at low frequencies. Is that something you noticed too?
Usually when i do 6th orders like that it's for really low end bass. Sometimes it's not. But just the way things peak with rap, that's why I do them like I do. I put that bigger chamber in the front to boost certain ranges, depends on the vehicle and things like the Fs of the sub and how far I would expect the box to be loud at in higher frequencies,

Like I said, the rear chamber is really what matters. The front chamber is significantly less important. The ratio between the rear and front is largely irrelevant, I just look at each chamber and what I need out of it. And sometimes I have to make that front chamber a little larger just to have everything fit inside of it. But you're right in general there about what you've said, IMO.

There is quite a benefit to having a large front chamber as well, so you can have a large radiating surface without having a port a mile long. Some of my front chambers are tuned to 45 hz. That really comes into play with a 10,000 watt wall like this, notice that the front chamber almost barely qualifies as a series 6th. This front chamber is tuned higher than 45 hz, I don't remember exactly what it was. Like 55/60 hz maybe. That loading wall is basically just that, it technically is tuned very specifically, but it's purpose is more to have a collected singular radiating surface. You try to make a port work for a 10,000w wall with a tiny front chamber.....it's very difficult to do without giving the front chamber some beefy size, so much air coming through that front port:

But yeah I didn't realize you had 3.5 cubes per in the rear. That might be alright if you aren't trying to play much below 40, like you were saying. I tend not to trust modeling software or t/s parameters in general. I use them as an aid, but man I've seen too much t/s BS (lol) to know not to trust every response curve I see. See, most of my customers want to play in the mid 20's with their walls, so our designs are definitely going to look different. That's what I love about series 6th orders....you can make them do whatever you want. There's so many options.

I do need to get back into hornsrep. I was going to learn how to do hornsrep well and tapered t-lines and horns and stuff like that, just like nobody ever wants those. I've thought about my own calculations to do horns just based off certain ratios without even using a program. Just haven't had a reason to, really. So that would some of the stuff I'd actually have to learn how to do is horns and any kind of non-equal width or surface area port. I've barely played with it, so you may be ahead of me in some ways if you know how to do that.
 

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