Port Chirping?

[GRNDPNDR]

First of all, Hello I've read the boards many times, done much looking here, but now I've decided to register and post. Just to give people some background I'm an electromechanical engineering student (30, not a youngin) and have dabbled with car audio a bit in the past.

I've built a few boxes, gone through the design work and have a fairly decent understanding of audio/boxes, and a pretty good background in electrical.

That being said, it's been a few years since I've dabbled with audio so I'm a bit rusty on ..... probably a lot lol.

I've always wanted that truly kick ass system, but life has not taken me down a path of riches, and being a student I'm pretty god damn poor, so recently I caved and picked up a used, prefab, ported, dual 12" speaker box to just get a system in my car so I can finally have some decent sound.

I've had the Insignia NS-P2000 sitting around for probably 2 years or more now, and just wanted to use it (I HATE selling stuff that I bought)

I got the amp and everything finally installed this past week, and for a budget system I am absolutly amazed at the sound and sound quality of these subwoofers. Especially for only about 500W. I'm running the amp bridged, the speakers are 8Ohm and running in parallel so receiving 250W each.

There is almost no T/S params provided by the manufacturer and the product has long since been discontinued. The manual suggest a box of 48.6L presumably for EACH subwoofer.

My box is roughly 3 cubic feet, and the port is 4"x9.25". If I'm not mistaken it's tuned to around 30Hz.

Here is the problem (finally); At higher volume levels I can hear a chirping of sorts coming from the port, slightly high pitch, sounds kind of like a cat purring. I can stick my arm in the port and it will stop chirping.

As I understand it, this sound is possibly the sound of the air moving through the port too fast/port tuned too low?????

Is there anyway to correct this easily without pulling the box out and re-working a bunch of stuff, and without changing the tuning of the port? I like it tuned to 30Hz.

[GRNDPNDR]

so, is turbulance my problem?

[One Way]

Seems like you might not have enough port area fOr the subs.

Shitty prefab boxes tend to be tuned with little port area.

Edited June 29, 2012 by One Way

[Sonic]

If the port exit dosent have a little radius, (which in most cases with prefabs) then putting one might help reduce the noise.

[GRNDPNDR]

The port itself is just a cardboard tube. I'm considering replacing it with some 4" PVC or something.

Would making the port longer do anything of benefit? or would it need to be larger in diameter? I can't really make the hole bigger but I could maybe add a second port somewhere.

[Sonic]

so, is turbulance my problem?

Hmm here an idea, you can calculate it (fluid dynamics). just kidding that's alot of work I have tried it ..lol..., easier to build a proper box. lol...

[Sonic]

The port itself is just a cardboard tube. I'm considering replacing it with some 4" PVC or something.

Would making the port longer do anything of benefit? or would it need to be larger in diameter? I can't really make the hole bigger but I could maybe add a second port somewhere.

Replace it with this...http://www.parts-express.com/pe/showdetl.cfm?Partnumber=268-352

[Sonic]

[GRNDPNDR]

I might be able to pick something like that up at our local surplus store....which carries a bunch of car audio stuff, but it's mostly all crap like Pyle or Pyramid.

IIRC I have seen ports there though, they might have the right size for mine.

[Sanitarium]

you've got 12 square inches of port total,

for that box/woofers you want at least 27 square inches of port.

in my experience, tuning a smaller box at 30 hz cannot be done correctly with anything shorter than 24" of port length (in general) ,. i didn't even need to do to the math to tell you that 9" of length giving 30 hz was ENTIRELY too small.