Dual 8", In-Wall Ported Box

[hddummy]

If you saw my other thread on this topic (link) you would know that I'm trying to help my parents install a 5.1 system since they are renovating their house. I was having trouble with sub design/placement options because they are very picky customers. I was inspired by joorge's post in my thread about the parts express "boogieman" sub. I came up with something that I think will work decently well. It won't shake the house down by any stretch, but they are not audiophiles and they have hardly given much to work with.

We settled on a small space in a wall that is centrally located in the large open room. This space is about 10" wide, 11" deep and 36" tall. I plan to build a ported box to take advantage of every cubic inch they will give me. The whole thing will be flushed in the wall and finished with a cloth grill. This is not a large space, so my choices are very limited. I have selected a tang band 8" (link) from PE. I'll use two of them and a 30" folded slot port to give me a box tune of about 29Hz. This driver has great power handling, Xmax and a small VAS, which was key to this design. I'll top it off with a 500 watt BASH plate amp, but it will be running only 250W because the drivers will be wired in series for an 8ohm load.

Look for box design pictures later today.

[hddummy]

[dgilme]

Is the bottom 8 in a sealed box, or is it just a brace in between the two subs?

[Mike P.]

What is the net volume of the box?

[hddummy]

The two horizontal members are just braces. I didn't bother to draw in hidden lines for open area.

I believe the net volume is 1.15 cu ft.

port is 8.5x1.5x30.5

i will also use 1" sonic barrier on all up or down facing surfaces to combat any resonance due to the long height dimension. This is also why i compacted the port into the top...to reduce the inside height.

[hddummy]

Modeled FR

[Bent]

Any chance you've considered Soundsplinter's RL-i 8" in dual 4 ohm config?
They could be wired for a total of 4 ohms load to the amplifier, and I'm thinking would make better use of the power available.

I haven't run the sims yet, and the driver Q shows to be quite low - sow it might not be the best for a ported box.

[Mike P.]

The RL-i 8 would definitely be a better option if price wasn't an issue, but it has been out of stock for a long time, second hand is the only option to obtain them.

[hddummy]

Yes, price is an issue. There is also the obvious quality and materials upgrade on the Rl-i to consider. However, I did model the RL-i 8" and the response models nearly identically to the TB W8 at the same power levels in the same box. That being said, the RL-i 8 would handle more power and allow the amp to deliver more power due to a better impedance configuration. My models still show the TB W8 as producing plenty of SPL, so I don't think the RL-i 8 is a justifiable cost upgrade. Thanks for the suggestion though!!!

[Mike P.]

The TB's will output 105 db at 30 hz and drop off below that, not bad a a pair of 8's in 1 cu.ft.

[hddummy]

I also considered the CSS trio8. It is also a nice quality driver. I was thinking two in parallel, but it has twice the VAS of the TB W8 and I would be limited to one driver in the box size I have. It could produce the same SPLs as a single, but would be reaching excursion limits. With a pair of TB W8 drivers, I would be running much lower excursions. The plus side to the CSS is it is more efficient at the low end and can produce the same SPL as two TB W8s with half the power in the 20-80Hz region. So, the trade here is amplifier cost vs driver excursion/linearity.

[hddummy]

Mike P. wrote:

The TB's will output 105 db at 30 hz and drop off below that, not bad a a pair of 8's in 1 cu.ft.

That's what I'm saying. Also, a good value at $400 all said and done for custom built-in sub.

I am open to any suggestions for other 8" drivers. I'll be building the box soon, but I can hold off on cutting the hole for the driver until the end.

[Mike P.]

I did some modeling of the TB 8, you can get an extra 7 db at 20 hz if you lower the tuning to 27 hz and modify the HPF to 24.3 hz with 2 db of boost from 31 - 35 hz. Pink is with the default HPF and boost the amp comes with.

[Mike P.]

The TB's will output 105 db at 30 hz and drop off below that, not bad a a pair of 8's in 1 cu.ft.

That should be 108 db at 30 hz, I forgot to model in the default boost the amp comes with. The above graph shows the default HPF and boost.

[hddummy]

Good advice...thanks!! I can add port length easy enough. I does drop my volume to 1.1 cu.ft. but that shouldn't make too much difference.

[Mike P.]

Just curious, how big is the room this sub is going in?

[hddummy]

I don't have a good number becuase the area is part of a large open floor plan. If I had to guess, I would put it at about 720 sq ft and consisting of a living room, breakfast area, kitchen and foyer. If you look in this thread, the sub will go in the wall directly below the right surround speaker. I know it is not ideal, but it will have to do.

[hddummy]

Mike P. wrote:

I did some modeling of the TB 8, you can get an extra 7 db at 20 hz if you lower the tuning to 27 hz and modify the HPF to 24.3 hz with 2 db of boost from 31 - 35 hz. Pink is with the default HPF and boost the amp comes with.

Although of built a few, I'm still learning the art of subwoofer design, so could you please explain how you decided on that particular boost level and freq. range. There are a lot of options to choose from with those boost tables, so why did you choose what you did and why not something different? Thanks for your help and time....I really appreciate it.

[Mike P.]

Room gain will boost the low end so the end result will be better yet.

[Mike P.]

The default values for the amp are R17=10K and R18=60K. The closest to that from the boost tables is the bottom line of the "2db of Bass Boost" table. The HPF would be about 30 hz and add the boost. The table states the boost is 2 db from 36 - 40 hz. To add that to the modeling, add a Parametric EQ filter, input the "Center Freq" as 38 hz, the gain as 2 db and the "Q' as 1.2, as stated in the boost table. Do the same for a 24.3 hz with 2 db of boost from 31 - 35 hz. Check cone excursion with input power applied to verify the sub is within Xmax. You can try other combinations of HPF and boost to see the effect it has on the output of the sub.

[hddummy]

so you didn't have a real reason behind the values you chose other than it was one notch lower in the boost tables.

[Mike P.]

The cone excursion of the default values dictated that a lower HPF could be used. The next lower HPF was 24.3 hz.

[hddummy]

Gotcha. Anything lower and you driver it past Xmax at these power levels. By the way, what subwoofer design software is that?

[Mike P.]

WinISD Pro (version 0.50a7)

[hddummy]

Small problem. If I extend the port the additional length to get 27Hz tune, the end gets too close to the center brace. I would have to keep 29, which isn't bad or extend the port to about 39 inches total length so that it ends at the brace. That is getting too long and port resonance could come into play. I could also reduce the port cross section, which would reduce the length requirement for a given tune. The question is, what are the dimensional limits on a slotted port. It is currently 8.5 x 1.5, if I go down to 1.25, my port length goes down to 31.25" and my port air speed is about 13m/s. I think I'd be good and I get the added benefit of some extra effective volume. I just don't know if that is too thin for the slot. Any help?