To spec or not to spec?

[BassAddikt]

Alright, I'm not completely sure where this topic should have been posted at, which is why i'm posting it here. If its the wrong section, let me know. I made a post a few weeks ago asking about my box and trying to find out if its designed properly or not because of the "rattling" noise I kept hearing. And also because I read so much about the 12-16 sq in per cu ft "rule" and, based on that, my port is way too small. Turns out the screws mounting the subs were coming loose (some were broke in half) so I got that fixed and it sounds much better, but I'm still concerned about the port, which finally brings me to my question. The guy that installed the system told me not to build a new box based on the manufacturer's specs. He said that manufacturers only sort of test the drivers in a "general" fashion, and that their specs (box volume, port area, port length, etc) won't be suitable for "real life" applications. I've tried asking him how he designs boxes without going by manufacturer's specs and all he tells me is that his software tells him what to do and what everything should be. I apologize for the lengthy post but I'm getting a little aggravated with being told one thing by him and reading almost the opposite info on here (as well as other forums). I wanna believe him but I know you guys on here know what y'all are talking about. So, IS he right? Is it a bad idea to design a box strictly off of manufacturer's specs for the driver? I've already designed a box for the subs I already have and everything seems to look good and it looks like it'll work out (designed it based on specs from manufacturer's site). I'll post pics of the one I designed, as well as his. The main difference is my design has alot more port area (140sq in) compared to his (45sq in). Keep in mind I'm running 4 15s on a 3000 watt amp, but only pushing around 1500 currently (gotta upgrade electrical). Gonna stop ramblin now lol but if someone has any kind of insight on this, could you let me know? I really don't wanna blow my subs if the box I designed isn't right. Again, sorry for the long story of a post. Hopefully someone can guide my a the right direction.

[Triticum Agricolam]

So manufacturer suggested box specs are exactly that, a suggestion.  They are a guideline that should work decently well in most situations.  They aren't a requirement though.  Depending on your specific situation and what you want to do you may want to go outside the guideline.  Knowing when do that and what to do differently comes from knowledge and experience.  

As far as your box builder goes, like I said in your other thread, unfortunately that guy just doesn't know what he is doing and he built you a very poorly designed box.  Your box builder's statement that manufacturer's suggest box specs won't be suitable for "real life" applications is just ridiculous.  They may not be optimal for all (or even any) situations, but they should be at least decent for most.

My suggestion would be to rebuild your box to the proper specs, if you need help determining what the proper specs are I'd be more than happy to assist you.  Once you get your new box built and experience just how much better it performs you might want to go back to your builder, show him the difference the new box made, and ask him to explain why he designed you such a poor box in the first place.  Maybe at least then he will learn something and not screw over the next guy.

[CstrokerV]

Wait your box he built and design. For 4 15s only has 45 sq inches of port? I had more then that for 4 12s

[BassAddikt]

5 minutes ago, CstrokerV said:

Wait your box he built and design. For 4 15s only has 45 sq inches of port? I had more then that for 4 12s

Yes, that is correct. I modeled his box in sketchup so it would be easier to see. The port is 15"x3" and only 7.5" deep. He says its tuned to 29hz. After reading alot on this forum, I came to the conclusion that 45 sq in in nowhere near enough for 4 15s. The manufacturer specs say 35 sq in @ 14" long, and thats the specs for a single driver. So, I figured if you multiply that by 4, you get approx. 140 sq in or so. But here's the box he built, which is currently what is in my expedition. Don't get me wrong- it sounds pretty good and gets decently loud but I feel like they have so much more potential if they were in a properly designed box.

[BYTizzle]

I saw your other thread and answered my own initial questions. 

 

MMats specs call for 3 cubes per sub net with 35sq in per sub.  I wouldn't worry about the length spec since that will be what you "adjust" to get the tuning where you would like.  If Im calculating it correctly, their tuning suggest around 34hz as the frequency (I really wish they would just call that out instead of the bass ackwards way they suggest an enclosure).  You would need a bigger box to meet all of these specs!

This would be close but you would need to measure to see where you could squeeze more volume in.  This is 46x36x16.5, subs facing up, port facing the "hatch".  Gets you the port area around what you need for 4 of those subs, the length to tune to ~34hz but its a bit tight on net volume.

[BYTizzle]

Sorry this wouldn't post in the above post...

 

If you reverse mount the subs you can pick up .6 cubes pretty quickly!  Reverse mount and dropping the port length to 15" long puts you at 11.8 net at 34.2hz with 133.5 sq in of port area. 

[BassAddikt]

I greatly appreciate you takin the time to do that! But I do have one question- how is the net volume only 10.4cuft? I designed a box about a week ago, if you don't mind me going over what I came up with? Your dimensions are very similar to mine, and I would just like to see if my designs are good lol btw the box is going in an expedition so size really isn't a limitation. And if i miss anything, please let me know so I can understand what I did and how to fix it👍

Ext. dimensions: 46 x 33 x 18.5    Int.  44.5 x 31.5 x 17

3 cu ft per sub, so 12 cu ft total w/o displacement

sub displacement= 0.16 cu ft, so box volume would be 12.6 cu ft total

44.5 x 31.5 x 17= 23,829.75 ,so...23,829.75/1728= 13.7 cu ft (with port)

For the port, specs say 35 sq in per sub, which would be 140 sq in total

My dimensions for the port is 31.5 x 4.5   giving me 141.75 sq in

I have the length at 14", so the volume is 31.5 x 4.5 x 14= 1,984.5    1984.5/1728= 1.1 cu ft

The box volume is 12.6 cu ft after sub displacement, so the 1.1 cu ft port volume would make the total volume 13.7 cu ft, which that is exactly what I get after calculating the volume using the int. dimensions- 44.5 x 31.5 x 17= 23,829.75/1728= 13.7 cu ft.

 It seems like it would work well since I have the airspace for the subs, the displacement they need, the port area for 4 drivers, as well as the length which is what the specs recommend, and it is tuned at 34hz, which is what I want. The only other difference with my design is instead of having subs up port back, I have subs and port facing up. I'll post a pic. But is there any difference in output, sound, performance, etc. with having the port facing the rear hatch instead of facing up?

 

[BYTizzle]

Its .16/per and you remove that amount from the net volume when the magnet is inside the box.  You also need to calculate the internal volume from where the port edge is inside the box, it no longer counts towards net volume for the subs.  Hopefully this makes sense.  Im terrible with Geometry so I will say that I def rely on programs to help me lol

As far as port placement goes, Ive looked into building a tahoe/suburban several times and everything Ive read said subs up/port back is the loudest.  I have 0 personal experience with it but looking at other builds, they do it that way and certainly know more than I do about that specific application.

[BassAddikt]

3 minutes ago, BYTizzle said:

Its .16/per and you remove that amount from the net volume when the magnet is inside the box.  You also need to calculate the internal volume from where the port edge is inside the box, it no longer counts towards net volume for the subs.  Hopefully this makes sense.  Im terrible with Geometry so I will say that I def rely on programs to help me lol

Yeah, that makes sense. For some reason I was thinking that space was included in the box volume, but I see what you're talking about. As far as the displacement, the specs say 3 cuft per sub..is that before or after displacement? I assumed it was before displacement so that's how I came up with my volume   3 cuft +  0.16 = 3.16 x 4 = 12.6 cu ft? 

[BYTizzle]

But going off those dimensions, magnets inside the box (regular mounted), subs up, port back, 11.5 net @ 34hz with 133.5sqin of port area using this setup:

 

Edited because I put the port specs in backwards lol...