port chirp?

So I wasn't sure if this is the correct forum to ask this question but it's about sub enclosures so I thought it would fit. I have 2 12" subs in a ported box that is 3 Cubic Feet, with a port that is 4" x 9.25" long. I believe this is tuned to 30Hz. At higher levels there is an odd chirp that comes form the port. If I shove my arm into the port (thus making it smaller) the chirp goes away. I think it's the sound of air moving through the port too fast? or the port is incorrectly sized/tuned? Can anyone shed some light on what this noise might be and how I could possibly fix it?

Is it port whistle? I don't have a lot of experience with ported boxes but I have seen aeroports that are designed to eliminate port whistle. How does the box sound? If it sounds good then changing port size/length shouldn't be a fix.Kevin Pierson

Well the box does generally sound good.... to me anyway. I guess it would be port whistling but it's choppy like a cats purr. If the box sounds good why wouldn't changing port size/length be a fix?

Because you'll change the tune of the box. If it is tuned at 30hz now changing the port will change that.Kevin Pierson

What kind of port are you using now?Kevin Pierson

It's just a cardboard tube that came with the box. like a carpet tube. the hole is 4.5", the tube is 4" Id, so it has 1/4" walls. I also don't think the port has 4" of space between the end of the tube and the box.

It could be port noise: but generally that is a dull low frequency sound or a "chuff" sound, not a chirp.  Is the sound a higher frequency?  If so, it could be mechanical noise from your woofers which indicates you are driving them too hard or that your amp is clipping.Support the12volt.com

It doesn't sound like a sub being over-driven, and if it was why would the sound go away when I tnsert my arm into the hole (not entirely blocking the port)

I had a pair of pro audio speakers in their original cabinet. Those cabinets are built pretty poorly. The old holes that were the location of the hanging bolts went straight into the enclosure and with them gone, the enclosure made the noise you're talking about.      Port is probably to small a diameter for the output volume of your speaker(s)/amplifier(s). Making it larger will drive up your tuning frequency if you don't lengthen your port, but it will eliminate the noise.      Putting your hand in there is messing with the airflow and making it so that the noise can't form. The problem is still there though. Its kind of like a resonant tube. The high speed airflow at that length of tube creates that high pitch resonance.      I'll check this enclosure out in BassBox in a little while. Trying to wire up an alternator right now....typically, I just run whatever I randomly pick up off the floor.

1995 Ford Ranger Supercab

MECA member

Team CSS

You definitely remember your numbers pretty well. With a pair of some older Type-R's, you'd be hitting around 34 Hz. Tuning frequency is 30 Hz.      I'm not sure what voltage your amps are at, but if you're chirping, then the port needs to be larger. If you took it up to a 6" without increasing the length, then you'd hit around 48 Hz. Too high for me, but not for most.      I'd just figure out what the largest enclosure size is that you're willing to put in the vehicle and then build it. You can nip this port chirp in the butt before it even shows up....typically, I just run whatever I randomly pick up off the floor.

1995 Ford Ranger Supercab

MECA member

Team CSS