Port vs. PR - Midrange quality

[noah katz]

I'm working on the design for a 2-way using the Eighteensound 12ND710 and BMS 4552ND on XT1086 horn/waveguide.

The woofer has a very low Q and the only way to get useful low end is by using porting or PR.

At first I assumed a PR would be less problematic than a port as far as midrange coloration, since the latter gives a direct path for sound to escape from the cabinet.

Upon further thought, the PR is not opaque acoustically and has a much larger area. Fb will be about 55 Hz, so the PR will have a regular cone.

Given how much sound passes through a solid wall, I imagine a cone lets a lot through.

Does anyone know of any studies where a port or PR was mikede for the midrange freq?

The only data I've seen was in LDC, where Dickason covered only blips in FR due to "mutual coupling", whatever that means.

Thanks

[joecarrow]

Whichever you pick, one way that I think would work well is the style of cabinet that's tall and skinny with a port. A lot of people have built these based on Martin King's simulations (other sources may be available, but his website is quarter-wave.com), and it seems to me like a fairly good way to make bass.

The idea is that the length of the cabinet supports a quarter wavelength resonance that is lower than the length of the cabinet would suggest, thanks to the mass loading of the port. The cabinet is stuffed with poly-fill to deaden the upper harmonics of pipe resonance, and has the added benefit of being a great way to kill the high frequencies inside the cabinet.

Martin's simulations are no longer free, but he sells the MathCad files at what I consider to be a reasonable rate. The folks in this forum may know other simulation packages that do this kind of cabinet- I think it's a winner for preventing midrange from leaking out of the port.

[noah katz]

Joe,

"The idea is that the length of the cabinet supports a quarter wavelength resonance that is lower than the length of the cabinet would suggest, thanks to the mass loading of the port."

Not sure what that has to do with midrange coming out of the port.

[BobEllis]

The MLTL as described will be stuffed lightly with polyfill, at least the upper half. this will help suppress midrange that could reach the port. Also, the sheer distance between port and driver will further suppress midrange.

[noah katz]

Distance and stuffing I get

[Rudy Jakubin]

It doesn't have to be tall and narrow. It could also be folded. I made these so the MTM's can be placed on top.
1/4 wave bass is hard to describe. I haven't heard an Infinite Baffle bass but the 1/4 wave is supposed to really come close. I got 2 - RS225's in this box which is about 96" long. On top are the Natalie P's in a sealed box.
One thing I noticed with MJK's worksheet was the infinte baffle line which you're supposed to match with the driver you selected or come close. Dave (planet10) suggested one day I try an Aperiodic 1/4 wave box for my midbass drivers with a Qts. of 0.59.
So I tryed one (second pic). It's 122" long fully stuffed and when you place a midbass driver in it playing at 95db, the midrange coming from the port is below 50db.
I don't like ported box bass, but the 1/4 wave box is a totally different experience. I think the best way to describe it is smooth and clean with no audible midrange coming out the port.

[Dennis H]

Upon further thought, the PR is not opaque acoustically and has a much larger area. Fb will be about 55 Hz, so the PR will have a regular cone.

Bingo. If the driver is playing up into the midrange, the PR will have to be pretty light and won't serve as much of a sound barrier. So, you'll still have to do the same sorts of things to damp the midrange from the back side, e.g. lining the cabinet walls with absorbing material and aiming the port/PR away from the listener.

[noah katz]

"1/4 wave bass is hard to describe."

Guys, I'm not asking about the bass. These are compact LCR's to be used down to 60 or 80 Hz.

"Bingo..."

I looked at some Stereophile measurement graphs; even designs with the ports right next to the woofer (though I don't know what was done inside) show port output 20-25 dB down above a few hundred Hz, so it looks like it's not an issue.

[joecarrow]

Hi Noah,

Sorry if that got this side-tracked to a bass discussion; I really did mean it just for midrange clarity. If you look at the B&W Nautilus, or Sigfried Linkwitz's Pluto speaker, you can see some instances where the undesired part of the midrange is effectively killed with a tube stuffed with fiber.

In addition to preventing the sound from making its way out of the port, a properly executed tube can prevent the sound from re-radiating through the driver itself. However you do it, it sounds like you found enough information and suggestions to make a good choice and find the sound you need.

[Rudy Jakubin]

That Aperiodic TL pictured above was strictly for midrange. The backwave has to travel 112" through stuffing and the port puts just enough pressure to load the midrange driver.

[BobEllis]

I find cone breakups that are suppressed only 25 dB quite audible. I'd shoot for more than that much - through rear port placement, and possibly MLTL.

Now you've got me curious - I am going to have to measure my ports' midrange output.

[noah katz]

"I find cone breakups that are suppressed only 25 dB quite audible."

Not talking about breakup, just backside radiation that finds its way out of the port; but maybe it's all the same.

I'm going to try separating the woofer from the ports with a 1.5" thick foam partition(s); it should be very absorptive in the midrange but tranparent at Fb.

[BobEllis]

I realize that you were speaking about backside radiation. I just used breakups as an example to point out that just because a sound is suppressed 20-25 dB doesn't mean that it is inaudible. YMMV, etc.

The foam idea sounds interesting. It would be nice if you could measure with and without to see if it does what it seems like it should.