Port plug effects

[hondobub25]

. Built a center console box for a 2000 regular cab silverado. Box is 3/4 MDF 31x12.25x13.5 port is 12x3x34.25 36hz Skar evl10 d2 with Skar 1500.1 amp. Wired to 1 ohm.

I used winisd and was never convinced the box would be correct. Ended up snaking the port at one end. Port takes up too much space and I realize this was probably just a bad way to build this box. Net volume is 1.2ish. But I had some size constraints.

It plays, it's peaky and lows were kind of non existent. Never heard it bottom out or surpass xmax. Put a plug in the port just to see how it would respond. What I dont understand is why the lows, like 20 to 30 are so much stronger. I'd say almost twice as strong. And it met max excursion for a moment. Dosent a sealed box keep the sub from moving as much? It was crossed over at 100hz. Kenwood DMX 908S. 1 gauge cable throughout. Voltage at the amp is consistent at 14.7 volts. Deck is clean to max volume and gains were set well before clipping with oscilloscope. Any input would be appreciated. 

[Joe X]

From your measurements I get like 1.37 cubic feet tuned to 41.75 Hz and a port area of 36 square inches.

 

Port area is twice to 3 times what is needed, tuning is sky high, no wonder you are getting the results you

describe. And yes only plugging the port can fix that box.

 

Back to the drawing board I guess.

[Kyblack76]

Dude just described a prefab box.  Tuning in the fucking 40's. Just a big ass hole (way short, and big vent, because wood cost money, air doesn't) for the port.

[hondobub25]

I'm not even sure what i did now and that computer is gone. Maybe a Linkwitz transform filter plugged in wrong? whatever.

If I built the wedge box attatched below, and flipped it on its side, so height 1 would be the bottom and height 2 would be the top, and  mounted the sub on the angled end, facing the back of the cab. A straight 12x2 slot port at 24 inches on the bottom facing the front of the cab I come up with 36hz, and a net volume of 1.75. Or Do i need to do something different with port placement?

1500 watts puts the port velocity at 42.5 fps and 1000 watts says 34.5 fps.

Again, any input is appreciated.

 

[Joe X]

Follow the manufacturer's specs except for tuning, keep tuning 32-35Hz if you want to listen to all the low tuned music,

 

A decent calculator is Torres which you can find for PC.

 

Solve a few design samples, I have posted many and when you get similar results you know your calculator use is correct.

 

 

[hondobub25]

This is what I've come up with. I plugged it into Winisd and the only difference I can see is Winisd calls for another inch or so of port length. what is the threshold for port velocity? 17 m/s seems low to me. This model here is twice that, but thats with a 1250 watt signal. I wont be able to hear the chuffing at that volume right?

 

[Joe X]

The first thing to bear in mind is that the linkwitz transform filter will fix your gain plot (for measurements inside a car) but it will make other curves plot incorrectly so for example your vent airspeed plot is showing an erroneously high airspeed, disable the filter for viewing that plot. For a slot port your vent airspeed should stay below 30 m/s.

 

The port length that TORRES shows is correct, as WinISD is incompatible with slot ports.

 

The 27 square inches of port area are not needed, 22 should be fine to avoid making your box larger than it needs to be.

 

[hondobub25]

I had no idea Winisd isn't compatible with a slot port design. Does it give any usable information with a slot port selected? with the changes i made its now telling me that my port velocity is 38 m/s with no filters applied. if that's bad info, is there a formula to solve for port velocity? If its good info, what would be the first thing you would do to address the 38 m/s?

This port volume seems to be about as close as I can get while keeping the dimensions simple and easy to build. Thanks again, not many people are willing to help much. I will pay it forward.

[Joe X]

I have already entered the model for that sub in bass box pro ( same as winISD but pro grade and made for car audio use) and everything checks out, when the manufacturer gives you box specs you can be sure you are OK at least to the RMS of the sub, also, use torres to determine the box dimensions, and ignore WinISD which is made for non slotted ports like pipe ports. WinISD will let you know about sub excursion, frequency response and everything else that you know.

 

If necessary I could show you my plots but at this point you should make sure that what you are building corresponds to the Torres you have made.

[hondobub25]

Ok, so I'm gonna go with this last model at 1.7 cf. I do have another question regarding the design. My plan is to point the sub toward the rear of the cab, at an angle. The easiest place to put the port would be on the opposite end, on the bottom of the box. I thought about putting on the same end as the sub but it would be kinda tight. I don't know if there is a benefit either way.