SVS custom sub project question???? (1 Viewer)

[Mike_Dikun]

I am building a subwoofer for my HT, using the original 12" SVS sub driver that I aquired from Patrick Sun. I also am using the Parts Express plate amp.

I am currently building my box, I have all the pieces cut and 3 of them glued together. I haven't attached the piece that holds the amp or the piece that holds the sub and the port.

I have calculated that the volume of the inside of teh box is roughly: 4 cu ft, before I add the sub and the amp. Lets say I lose .3 cu feet. So I will have about 3.7 left, and if I stuff it with polyfill that number will be right around 4 again? Correct assumption??

Next question is about my port. If I remember correctly: I think I would need a 4" port with a length of abouth 22" for that size box to tune it to around 18hz???

Question three: If I installed a 6" port, how long would it have to be to tune it to around 18hz??

Last question,(for now anyway): If I left the box sealed, approximately where would my -3db point be??

Thanks a lot

 

[Ryan Schnacke]

1) Yes you could get back to 4 cu ft with the right amount of stuffing. But what is the right amount? Maybe 10 oz per cu ft?
2)Using the software at
www.wssh.net/%7Ewattsup/audio/
I get 4.0 cu ft tuned to 18Hz requires a 4 x 23.2inch port
3) or a 6 x 54.4inch port. Remember that a 6 inch port has over twice the port area of a 4 inch port, so length must also increase to more than double.
4) Don't know for sure but I suspect F3 would be pretty high, like maybe 50Hz. But a box that big would have a low Qts and a shallow rolloff.

 

[Dan Hine]

Mike,
Typically, adding 1.5lbs of stuffing per cubic foot adds about 30% to the enclosure. It seems like you only want to add 10% more volume to the 3.7cubic feet so I'm guessing around half a pound would be good.
I'm getting a 22.75" length port for 4" diameter. But between 21-25" length the tuning frequency goes between 17hz and 19hz. So as long as you're in the middle there somewhere you should be fine.
As Ryan said, a 6" port will be waaaaay too long.
For a 4cubic foot sealed enclosure you're looking at an F3 around 40hz with a 10db downpoint right around 20hz. Depending on what you're looking for in a subwoofer and the size of your room, it may be alright. Myself, I would go with the port.

 

[Mike_Dikun]

Ryan, thanks for responding and helping me out.
I see, I didnt know that about using bigger size ports. I thought the bigger the diameter of teh port, the shorter teh length could be.
I dont have 23.5 inches inside teh box, would I be able to use 2 4" ports, each one 12" long???
Wow!!! My dropoff point would be 50hz if I went sealed, that's really high :frowning:

 

[Mike_Dikun]

Dan, thank you as well.

I am looking for this subwoofer to be mainly for HT right now. In fact, I like building stuff a lot and wouldnt mind building another subwoofer that would be more tailored to music down the road if this sub doesnt sound good with music.

So I guess i'm looking to do whatever it takes to make this a HT sub.

Thanks

 

[Mike_Dikun]

If I went with one 4" port I could make it about 17.5 inches, but the end of the port and the box would only be about an inch apart. Is that enough room?? Also, that would mean that I wouldnt get quite as low right?

 

[Ryan Schnacke]

Mike,

Dan's probably closer to the mark when he said 40Hz. But the key is shallow rolloff. The 3dB point isn't some magic number where frequency response drops off into a bottomless pit. You can get useable response down below that and sometimes room gain will compensate for the rolloff for a fairly flat in-room response.

When you go from a single 4 inch port to two 4 inch ports you're doubling the port area so you'll have to double the length of both ports to maintain the tuning point. Sorry, there's no easy way around this. More port area requires more port length.

A single 4 x 17.5 port in 4 cu ft should put the tuning at about 20.5Hz. But if the port ends that close to a box panel then the tuning may be affected. If I understand it right, the effective port length will be longer since the air is still somewhat constricted immediately surrounding the port intake. Most DIYers recommend keeping the port end at least one diameter-length from the enclosure panels, but if you're a bit adventurous and prefer a lower tuning anyway then you might try something like 2 or 2 1/2 inches.

You might try redrawing your sub with the same internal volume but with 1 long dimension and 2 medium/short dimensions to allow for long ports. This is why sonotubes work so well - tall and skinny.

Or you could consider using a 3 inch flared port. You sacrifice port area but you can get lower tuning with a shorter port and the flares will minimize turbulence. The question here is: How much air can your driver displace? (What driver are you using?)

Edit: Oh yeah, you're using the SVS driver. It might get away with a 3 inch flared but 4 inch is a safer bet, especially if you plan on driving it to max excursion.

If you can't compromise your port area, enclosure dimensions, or tuning point then its time to consider passive radiators.

 

[Dan Hine]

I can't find the link right now but I recall that Patrick Sun used PVC with an elbow joint in order to make the port fit. I can't remember if he noticed any ill effects though. You may want to get in touch with him. Or, if you want to wait until the end of the weekend, I'm building a sub for a friend using a Dayton 12" DVC with 2 3" ports and will be using elbow joints to make it fit. The PVC and MDF is cheap enough so that if it doesn't sound good then no loss. But I'm fairly confident it will be ok.

 

[Patrick Sun]

4" wide port is fine, 6" wide port is probably overkill for the SVS 12" driver.

 

[Mike_Dikun]

Well, the box is completely assembled. I used a 90 degree elbow joint to make my port.
The port ended up being around 23-24 inches long, depending on exactly where your suppose to measure
The overall outside dimensions are: 25.5" wide x 18" deep x 20" high.
The inside volume without the driver and bracing is 4 cu ft.
With the driver and bracing i'm guestimating it to be about 3.8'ish.
I calculated that if the volume was 3.5 cu ft tuned to 19hz i'd need a 24" port.
If it was 3.7 cu ft and tuned to 18.5hz i'd need a 24" port and....
If it is 4.0 cu ft and tuned to 18hz i'd need a 23.5" port.
SO, whatever the sub "thinks" the internal volume is i'll be somewhere between 18hz and 20hz. (I hope)

 

[Mike_Dikun]

Now, next up is finishing the box!!

I think i'm gonna go with a cherry veneer.

I'm also going to put a glass top on it.