How to calculate port size for MLTL speaker?

[Brewski]

I'm working on the buyout MTM's from my brother and read a week or so back that tall narrow towers functioned more like an MlTL speakers than base reflex for the purposes of tuning. What I'm trying to figure out is how do I modify the expected bass reflex tuning port size in that case?

The speakers I'm working on have a volume of ~32L and I'd modeled a tuning of 49 Hz so I was planning on 2 (2"d X 4.5"l ports). What I don't know is how much that port size needs to change based on the speakers functioning more like MLTL speakers than bass reflex.

The internal cabinet dimensions are ~ 6.5w" X 38.5h" X 8.25d". For stuffing I was thinking on the top behind the speakers I'd line the back wall with bed matteress/egg crate foam plus moderately stuffed with pollyfill until ~19" down. At that point I'd have a brace that air could pass through seperating the top speakers half from the crossovers, ports, speaker terminal in the bottom half. I've not glued the internal bracing so this could shift up or down as recommended if ithis needed to be changed as part of getting the tuning right.

Thanks for the advice
Jay

[Paul K.]

Re: How to calculate port size for MLTL speaker?

It's really not possible to determine what the correct port size should be without actually modeling it, at least I can't. The 1/4-wavelength resonant frequency established by the internal height of the enclosure, which becomes the line's "length", contributes part of the system tuning, with the other part of the tuning provided by the ports. If you provide me T/S values for the midwoofers and the location of the tweeter's center from the inside top of the enclosure, I'd be happy to model it (the tweeter's center is the design center for the TL).
Paul

I'm working on the buyout MTM's from my brother and read a week or so back that tall narrow towers functioned more like an MlTL speakers than base reflex for the purposes of tuning. What I'm trying to figure out is how do I modify the expected bass reflex tuning port size in that case?

The speakers I'm working on have a volume of ~32L and I'd modeled a tuning of 49 Hz so I was planning on 2 (2"d X 4.5"l ports). What I don't know is how much that port size needs to change based on the speakers functioning more like MLTL speakers than bass reflex.

The internal cabinet dimensions are ~ 6.5w" X 38.5h" X 8.25d". For stuffing I was thinking on the top behind the speakers I'd line the back wall with bed matteress/egg crate foam plus moderately stuffed with pollyfill until ~19" down. At that point I'd have a brace that air could pass through seperating the top speakers half from the crossovers, ports, speaker terminal in the bottom half. I've not glued the internal bracing so this could shift up or down as recommended if ithis needed to be changed as part of getting the tuning right.

Thanks for the advice
Jay

[Brewski]

Re: How to calculate port size for MLTL speaker?

Paul thanks for your time it greatly appericated. The tweeter is centered at 8.5" down from the internal measurements 9.25 via external measurements. The parameters for the Peerless 6.5" as measured by NEO Dan are:

Re 3.5784
Fs 45.20428
Zmax ohms 56.69876
Qes 0.32504
Qms 4.83006
Qts 0.30446
Le 0.64
Dia 135 mm
Sd 14313.88220 mm2
Vas 21.14758
BL 6.73692

There are additional measurements for the woofer in the following thread.

http://techtalk.parts-express.com/sh...ss+measurement

Post number 28

Thanks for modeling the speaker
Take it easy
Jay

[Paul K.]

Re: How to calculate port size for MLTL speaker?

Am I correct to assume the two midwoofers will be wired in series?
Paul

Paul thanks for your time it greatly appericated. The tweeter is centered at 8.5" down from the internal measurements 9.25 via external measurements. The parameters for the Peerless 6.5" as measured by NEO Dan are:

Re 3.5784
Fs 45.20428
Zmax ohms 56.69876
Qes 0.32504
Qms 4.83006
Qts 0.30446
Le 0.64
Dia 135 mm
Sd 14313.88220 mm2
Vas 21.14758
BL 6.73692

There are additional measurements for the woofer in the following thread.

http://techtalk.parts-express.com/sh...ss+measurement

Post number 28

Thanks for modeling the speaker
Take it easy
Jay

[Brewski]

Re: How to calculate port size for MLTL speaker?

Yeah they're wired in series going for an ~8 ohm load until the hand off to the tweeter.

Thanks
Jay

[Brewski]

Re: How to calculate port size for MLTL speaker?

Paul,

What are you using to do the modeling? I was searching a bit last night and saw some information on mathcad but didn't know if there was a free tool that was available to model with in the future?

Thanks
Jay

[Paul K.]

Re: How to calculate port size for MLTL speaker?

Okay, I finished modeling your cabinet as an ML-TL. I modeled with the ports' centers 2.5" above the internal bottom of the cabinet, keeping their diameters at 2" each. The ports can be mounted on the baffle or on the rear panel, whichever would be better for the cabinets' placement. I modeled with a stuffing density of 0.75 lb/cu.ft. needing a total of 7 to 7-1/2 ounces of stuffing uniformly distributed in the top 19-1/2" of the cabinet. I've attached three system response graphs (the red lines in the graphs). The first is how I would tune the system if it were for me, at 51 Hz, while the second shows a system tuning frequency of 49 Hz as you originally had planned. For 51 Hz, the ports' lengths need to be 1.75", and for 49 Hz, their lengths are 2.5". And, just for comparison, I show a third graph which would be how the system response would have turned out if you had kept the ports' lengths as you originally planned. If you want other graphs showing different aspects of the design, like impedance or ports' air velocity, I can do that (I'll keep the model up in case you do).
Paul

Yeah they're wired in series going for an ~8 ohm load until the hand off to the tweeter.

Thanks
Jay

[Brewski]

Re: How to calculate port size for MLTL speaker?

Paul,

If you don't mind posting the other graphs for the 51 hz I'd be currious to see the impedance and port air speeds.

Thanks again for your time
Jay

[Paul K.]

Re: How to calculate port size for MLTL speaker?

The attached two graphs show the impedance curve (red line) and air velocity as a percentage (shown numerically as a decimal) of the speed of sound. To answer your other question, I use the free version of MathCad which I downloaded from a link on Martin King's website, and I used one of Martin's worksheet for this modeling, of course. Oh, I also arbitraritly modeled with 0.2 ohms in series with the drivers, and I also looked at the modeling with 0.5 ohms in series; the only difference was absolute magnitude of the system response (some drivers are very sensitive to series resistance, ending up with deformed response curves that need addressing, but these were not). And, all of the graphs represent the predicted result with a 2.83-volt input.
Paul

Paul,

If you don't mind posting the other graphs for the 51 hz I'd be currious to see the impedance and port air speeds.

Thanks again for your time
Jay

[Brewski]

Re: How to calculate port size for MLTL speaker?

Thanks again for the help

Jay

[Paul K.]

Re: How to calculate port size for MLTL speaker?

You're quite welcome and I hope you like the results.
Paul

Thanks again for the help

Jay

[buzzy]

Re: How to calculate port size for MLTL speaker?

Paul, if you would be so kind...I have looked at the three graphs that you were nice enough to post. The difference between the three, I really don't comprehend. Could you, or anyone else, offer a brief description 'for dummies' as to why you choose the 51hz tuning as opposed to the 49hz or the original ports. Thank you very much....some day these things may make sense to me. Trying to learn something.:rolleyes:

[Paul K.]

Re: How to calculate port size for MLTL speaker?

The difference is the output from 100 Hz down to the "knee" in the curve, afterwhich the output drops more rapidly. So in the roughly 45-100 Hz region, the higher tuning frequency of 51 Hz allows more output, a more flat response. There isn't much difference between the first two graphs, but quite a bit when you look at the third graph.
Paul

Paul, if you would be so kind...I have looked at the three graphs that you were nice enough to post. The difference between the three, I really don't comprehend. Could you, or anyone else, offer a brief description 'for dummies' as to why you choose the 51hz tuning as opposed to the 49hz or the original ports. Thank you very much....some day these things may make sense to me. Trying to learn something.:rolleyes:

[Pete Schumacher]

Re: How to calculate port size for MLTL speaker?

Paul, if you would be so kind...I have looked at the three graphs that you were nice enough to post. The difference between the three, I really don't comprehend. Could you, or anyone else, offer a brief description 'for dummies' as to why you choose the 51hz tuning as opposed to the 49hz or the original ports. Thank you very much....some day these things may make sense to me. Trying to learn something.:rolleyes:

Its a matter of how the response is shaped near cut off. Look at the 60Hz line. The tuning as Paul recommends has a couple more dB output at 60Hz than the lower tuning. 3dB is quite significant, and will be audibly different than the lower tuned version. Tonal balance will be affected.

[buzzy]

Re: How to calculate port size for MLTL speaker?

WOW ! you guys aren't only GOOD...you are also QUICK ! I see the difference now. Thanks to both of you gentlemen, you have made the blind see. :eek: