Port Midrange Leakage

[jimangie1973]

I have 1.2 cu ft ported enclosures tuned to about 34 Hz. The port in each is 3 inches diameter by 10 inches long. The internal box dimensions are 20x8x13 inches. There is 4 inches of free space between the end of the port and the interior of the front baffle (1" MDF). The box is rear ported with the port 1" below vertical center, to allow for a brace.

I measured the port output and found a substantial midrange resonance centered at approximately 540 Hz, with high Q. The peak was only about 5dB below the max port output at 40 Hz. Are these midrange resonances avoidable? Also, are there any mathmatical formulas to model this behavior?

I also measured the port output of a pair of Paradigm Studio 40 V2s that I have. These also have a midrange resonance, at around 850 Hz, but not quite as high in amplitude.

Thanks for any info.

Jim

[GordonW]

The only way to deal with such noises are:

a) Move the port opening inside the enclosure, to hopefully find a "dead node" for the frequency in question. Sometimes, this requires right-angle ports... once you do that, you have more options on "aiming" the opening.

b) Damp the heck out of the inside of the cabinet, to ABSORB the midrange "noise" before it gets to the port. This can require damping material "curtains", cabinet wall-damping materials such as Black Hole, and the like.

c) The ultimate solution? IMHO, a passive radiator. Almost impermeable at midrange frequencies...

Regards,
Gordon.

[freddi]

wonder how much port leakage I had from this 18" reflex?

[Cal Weldon]

Jim,

Put a sock in it!

Honestly, aperiodic loading of the port will help to calm the nasties. Start with a little stuffing and continue to add until you are happy. Use fiberfill not fiberglass as you don't want the stuff chuffing around your room.

Old socks work well also

[Dave Bullet]

Old socks work well also

Don't forget to wash 'em first though

[AKN]

Don't forget to wash 'em first though

Dirty ones may have better absorbing qualities.


jimangie1973,

Do you have direct sight between cone and inner end of port tube?
If so and at low angle will be troublesome. Some bracing or simular in the direct path will damp mid leakage.

[jimangie1973]

I tried stuffing the port with some polyfill just as an experiment. That weakened the deeper bass only slightly, but the midrange resonance was still present. I also jammed an old cotton shirt in each port. It killed the port resonance and unfortunately the bass response too. This is a solution if I add a sub.

The addition of a right angle attachment to move the interior port entrance is a possibility, as well as improving the internal damping. I thought I read to avoid the port angle attachments, but I can't find any comments about that now.

Since the speakers are rear ported, it's not too big of an issue. Thanks for the responses thus far.

[jimangie1973]

Originally posted by 4fun


Dirty ones may have better absorbing qualities.


jimangie1973,

Do you have direct sight between cone and inner end of port tube?
If so and at low angle will be troublesome. Some bracing or simular in the direct path will damp mid leakage.

I'll see if I can add some bracing in the path. It's an MTM so the tweeter is directly in front of the port. The angle is maybe 30 to 45 degrees to the lower woofer. The upper woofer is slightly farther away, and behind bracing.

[Pit Hinder]

Jim,
easy solution, and I've been the lone preacher in the desert since times immerorable.
Make the port 1.4 times as long. Drill half a dozen 8-10mm holes at half length. Problem gone. A bassreflex port behaves like a TL, and right where you don't need the standing wave, so kill it.

Pit

[lndm]

Pit Hinder, your idea sounds plausible. Do you suspect this is similar to flaring the port?