Sub box help - Porting and tuning
 

Hi,

Just looking for some advice on a setup im looking at putting in my boat. Unfortunately the sub is going to be a bit of a budget one until i can afford better, however id like to get a box made that gets the best out of it until i can upgrade.

The sub is a MomoPolk DVC mm2124. http://www.polkaudio.com/products/mm2124dvc
Ive had it for a while and its surplus to requirements + waterproof so id like to use it.

I emailed polk to ask what they reccomended box wise, but they were useless and wouldnt answer any technical questions re box design (i dont think the person knew) but said 1.85 cubic feet.

David from Earmark said 2.75 so i have based my calculations on a variety of scenarios.

What i dont understand is where to put a port, or actually how to tune a box.

Any hints or tips would be good. As the shape is a bit different i dont think i can just download software to tell me how.

Thanks
Byron

I would tune the box to around 35 hz here is a calculator http://www.the12volt.com/caraudio/boxcalcs.asp to determine the length and width you need to make your port. I would also have at least 36-42 square inches of slot port.

Put it in a 1 cu ft SEALED box and call it a day. It'll sound great.

A few generalizations....
The 2.75 cu.ft. given is a rough external displacement including the main chamber, port, port wall and construction stock based on the T/S specs. The total external displacement is the logical starting point because it allows you to first locate and design the enclosure relative to the boat. Why design an enclosure based on a particular loading scheme that won't fit.
Unfortunately when manufacturers give you a simple internal number on a bass-reflex enclosure they are often unable to clarify whether that is the gross chamber, net chamber, with or without port or port wall. It's useless information without a very specific and full set of plans.
If you are going to design a quality bass-reflex enclosure then you have to start with the woofer's Thiele/Small parameters. Then you have to decide what type of enclosure you are after whether a linear and wide SQ alignment or a narrow and peaky SPL alignment or somewhere in between.
Most Box Programs are limited in the number of elements that are included in the calculation. They may not show what is really going on an octave above the tuning frequency. A Box Program may show that you can reduce the enclosure displacement, maintain the same port surface area and lengthen the port to still tune at any given frequency. However, the Box Program cannot indicate how bad that box may actually sound. There isn't really a substitute for what trial and error or experience brings to the equation.
It's nice to place the port along an outside wall of the enclosure so that you are saving the displacement of having an additional port wall. This makes the box just a bit smaller.
Port orientation really doesn't matter as long as the port and woofer radiate into the same air mass, meaning you don't want the woofer direct-radiating while the port fires into a storage compartment or the inverse.
The port mouth needs to be a sufficient distance from any obstacle or plane that could alter it's tuning.
An irregular shaped bass-reflex enclosure is fine as long as it's fairly basic. But you want to avoid elongated appendages.

David
Earmark Marine

Hi,

Appreciate all the comments.I guess the crux of the matter is that there arent the local shops in my area with the knowledge to help, Polk havent been much help, so unfortunately im going to be reliant on the experience and wisdom of others hence asking here.

It probably wont be the only sub that ever sits in my boat, hence not being pedantic about the box (Thiele/Small parameters etc), however i do want to do a reasonable job.

In terms of the listening we do, its not rock music, largely hip hop/dub so thats the type of music im into.

So based on this, is it safe to say i can use a box programme to give me a rough idea? I have always thought a port wall would be necessary?

If someone in the know with some experience of this sub could give me a breakdown of the best way to get some quality out of it, and how to suit my area, that would be appreciated.

Byron

I have the below referenced sub, also pretty cheap. I agree with putting it into the 1 ft cube SEALED box and it will sound great comment. If my boat wasn't in storage I would measure the box for you. I am guessing it is about size of below listed. One of the two probably.

http://www.crutchfield.com/S-I0jPKr9...o-DXi-124.html

http://www.crutchfield.com/p_777BB12...ml#details-tab

http://www.crutchfield.com/p_777BB12...ss-Bunker.html

I greatly prefer a properly tuned ported box to a sealed box, especially in a boat.

In a boat I prefer the subs to be tuned to a higher frequency but most of my listening is done while the boat is in motion. I would recommend 50hz over 35hz especially with that sub. Note, that sub is designed for a sealed box and wants the acoustic suspension offered with a sealed box. The power handling is not that high and the xmax (excursion) is not very high either so it's not a great option. You can probably pick up a different Polk, an Alpine Type R or other sub that would work much better for around $100. Considering all the work you will have into the box I would highly recommend a different sub.

I agree I would choose a different sub. That type r would be good, also Sundown sa12. I wouldn't tune that high if you listened to any hip hop/ rap or country because it would sound like s**t. I have 2 sa12's tuned to 32hz and sounds pretty damn good. You can use aero ports so you can play around with the tuning to see what you like best. That's what I did. Tried 40zhz ending up at 32hz. But ever application is different.

In the tuning region of a bass-reflex the excursion is much lower than that of an air suspension. More of the output comes from the port. The speaker is especially damped in this region. Generally as you tune higher and above the speaker's resonance, you get more of a peak in the response which means more peak output and less musicality. That musicality is an issue when at rest but is definitely erased at speed. The peak is welcomed at speed because hearing the bass (period) could be considered the only priority. Those are the subjective preferences person by person. Tuning at 35 Hz or for maximum linearity is low risk because there isn't much program material below the tuning frequency. Tuning higher, say at 50 or 55 Hz, requires that you have an adjustable subsonic filter on your amplifier so you can nudge that filter just under the tuning frequency.

David
Earmark Marine

Hi guys,

Thanks for your input. I live in New Zealand so getting a marine rated sub isnt quite as easy as buying something off the net and having it arrive a few days later. Cost wise its probably the equivalent of 5/6 times what you pay by the time we get it here. My boat also lives outside, so it needs to be marine rated as ive already killed a few head decks.

Unfortunately the budgetry contraints means that it will have to be this sub this summer, and i can maybe look at upgrading in a few years time. I have built plenty of boxes over the years so that isnt a problem, just getting the most out of this sub is the concern.

Can anyone reccomend an enclosure size, port layout and size for me? That would be most helpful.