About Tuning and PVC Aero ports

[ChrispyShimp]

Some Ideas I have and some I've used on aero ports, and a few how to things for flaring the pvc.

Here's a pic or two of the end result, I've done these myself, really easy.

(Looks pretty good, huh?)

A lot of beginners, myself included, have built their own box, and have completely missed the tuning they wanted.

My first box took me two weeks of sanding and swearing, but I built the port to a certain size the local shop told me to use.

That box worked just fine, and I go thinking I can build another one tuned with the same subs and volumes.

I built the second box in less than three days, painted too.

Well, tuning has nothing to do with the actual volume of air in the port.

I had such high hopes, the box was solid, painted, and the tuning was at 67 hz when I was shooting for 33-ish.

here it is, still fresh paint too. I was stoked, and when I tested it, I almost cried.

So I had this solid box I just built, and the tuning was way the hell off.

If you are in this situation, I've got a possible solution, for cheap.

A removable plate with interchangeable aero ports, or PVC tubes.

BUT FIRST! You should determine what the tuning should be.

If you aren't nerdy, it's alright, neither am I, just study the next part for a bit.

What tunes a box? First, consider what the sub is doing. It's transferring mechanical energy to the air around it.

An enclosure is a defined space that controls this mechanical energy in the air. Portions of that energy, depending on the frequency, will bounce around and push back on the sub's cone. This is called loading. It makes bass! (Note, there is such a thing as TOO MUCH loading)

Unloading is when the air bounces IN SYNC with the cone, there fore NOT pushing on it, allowing it to travel further, reaching the mechanical limits of the spiders and surround. This is typically bad, as the soft parts act almost as bump stops as the cone flies forward and backward freely, which quickly wears and tears the parts, and alignment becomes an issue as well. Not to mention distortion caused by said alignment.

Unloading is bad for just about any driver, and enclosures seek to provide an environment that controls the amount of pressure that interacts with the cone so this doesn't happen. Fortunately, unloading occurs at select frequencies that depend on the air (or enclosure) around to woofer, it's own characteristics, and the power applied.

All drivers have a certain frequency where they will resonate in open free air and unload. This is also the resonant frequency, or the Fs. This varies widely between drivers, even ones of the same size. The important part is that the enclosure is tuned to provide a lot of restriction at or closer to that frequency, Fs. This pushes the Fs of the whole sub/box assembly lower, usually to a point where it's not music anymore, just wind.

Say a random 12" driver resonates at 35Hz in free air. Just about any reasonable sized enclosure will provide some loading, but using both the volume of air it interacts with and the port's restriction to control the 'breathing' of this air will provide the cone with restriction, so loading occurs, and bass is created!

What does all this mean? The volume of air in the box, and the restriction to flow on that air, tune the enclosure.

There's a few more things to it, but that's it in a nutshell

So, this is where you either ask a local shop, or look up the TS parameters from the manufacturer.

Or ask around the forum here, there's plenty of folks here that will help, or at least offer advice.

There's plenty of calculators for the port tuning, but don't blindly punch in numbers and build it.

Do some research, chances are, some one has either built a box for the same driver, or can help you find TS for the driver.

Some shops will sell you a design. Seriously, Google it. Or Bing, whatever.

Okay, so, port tuning. Well, it's determined by cross sectional area and length of said area. Kinda like PORT SURFACE AREA, not volume.

I had two boxes with 750 cubic inches of port volume, and the tuning didn't give a damn. Turns out, I needed roughly 42-ish square inches of cross sectional area, and a tidy 11.7" something port length for two 4" round ports to tune my setup to 34-ish. Perfect!

The port for the first box had a port of exactly 3" by 12.5" by 20" long.

3x12.5= cross sectional area, or roughly 38". this resulted in a tuning of about 31 hz. Given the Fs of my subs, 26.7hz, this did great.

With the second box, I kept the VOLUME of the port the same, and had a Cross sectional area of 8" by 12.5" by 7"long.

Yeah, way too much area, and 7" long, total failure. So I went into a short lived depression, wanted to sell the whole car right then.

Got my head back on straight and visited my local shop. "Aero ports"

I went home and researched for days, learning about tuning. At first, I drowned.

Saw the weird math symbols and freaked out. But I wanted to learn. And I progressively did.

I determined the cross sectional area I needed, and searched through a bunch of calculators, and figured it out.

And if I can do it, so can you. Just research it.

So, once you have your numbers, what?

A rule of thumb, sort of, is the 1x8 rule. Say you design a port that's, well, 1"x9" in cross section. This would result in a lot of turbulence as the air is pushed in and out of the port, creating annoying not-awesome-bass noises, and also reducing output, as that energy in the air is lost to turbulence. Less Bass! So keep that in mind if you have a slot port. Not really a problem with round ports, just make sure they are large enough to let it breathe some. Some folks claim that 12-15 Sq. In. of cross section for each cubic foot, won't go into that here, but it is something to consider.

I visited my local hardware store with a notepad, and wrote a list of all the pvc stuff I could possibly think of using.

I also brought a tape measure, checking how long this was, all that. Brainstorming.

So, with prices and numbers all over the notebook, I went home and tinkered around with the calculators and Google Sketch-up, fitting the right pipes in. It occurred to me, I can get a butt connector for whichever diameter I choose, and glue it into a plate that bolts over the old port.

I went with two 4" tubes, the cross section matched what I needed, and it also was simpler than one 6" because of the length(I only had 16" from old port front to back of box, 6" would have been a side port) for the desired tuning. I only needed one 24" section of 4" and two butt connectors. Cut a board, glued the butts in, and went around the old port rim with some weather stripping. Bolt it together good and snug, and it's fixed! Now it makes big bad bass just like the first box did.

Here's what it looked like. And yes, that wiring was ugly, it wasn't like that for long. You can see a bit of the weather stripping, too.

Don't forget to also calculate how much of the box's internal air the port(s) take up, or in my case, frees up. This is pretty easy in Sketch-up, just make it a group and pull up entity info, it normally tells you the volume of the group, unless it's not really square or off or incomplete.

If you are frustrated with using Sketch-up, here's a link that should really help.

How did I flare the ends of the PVC?

Cheapy heat gun and a perfectly shaped brass bell. I had the bell nailed to a stump, laid the heat gun where it blew on the last inch of the PVC. You might have to use a small bowl or cup, anything that you can find that will spread the pipe when it's hot.

BE CAREFUL NOT TO OVER HEAT THE PIPE! Or to heat it too fast, it can crack. Take your time, rotate the pipe A LOT. PRESS GENTLY, and heat it evenly, you'l feel when it's soft, it won't spin as easy, it'll grip a little. Never force it to bend, it's patience here. Also, some types of PVC will hardly bend at all, even under heat. And have an open bottle of water nearby to dump in the pipe when you want it to hold the shape you've bent it to.

A little singe might occur on the outer lip of the flare, but I found that this can easily be sanded off. If you want a video, there's quite a few of them, some are not the best, but the idea is the same.

THERE IS NOT A ONE SIZE FITS ALL ENCLOSURE! One enclosure may work extremely well for a particular driver, but that doesn't make it the worlds best enclosure. On top of that, each make and model of vehicle will affect the output of the enclosure, regardless of type, as the vehicle itself acts as ANOTHER ENCLOSURE! If a 6th order bandpass is totally nuts in the back of someone's truck, that doesn't mean it'll in a SUV or car, or even a similar truck.

Each vehicle bounces the mechanical energy around differently, so even if you had an 'Ideally Tuned Enclosure', it wouldn't perform Ideally in a vehicle, as the vehicle acts as a larger enclosure, and chances are it's not an '"Ideally Tuned Enclosure"' for your 'Ideally Tuned Enclosure'.

If you give up completely and are fine with paying for a ready made box built just for you, could I suggest PWK designs? (Google it!)

Pete totally blows me away in terms of speaker knowledge epicness. He sure as hell knows how to build a box.

Any input from those with greater knowledge is openly welcomed,

including correcting me if I'm wrong.

Congrats if you've read the whole thing! Hope this helped.

[Joe X]

Honestly there's only a few pieces of information that are really necessary and those are the external dimensions of your box, the diameter and center line length of your ports, what amp are you using, diameter of the subs and what is exactly your complaint with how it sounds, what music plays well, bad, etc.

[alaskanzx5]

Haven't read it all yet but going to, just don't have much time left on lunch right now