Mass Loaded transmission Line Port Location

[Geoff Millar]

Hello from Oz

I have a two way project with a Peerless SDS160 and Vifa BC25TG on my ever growing 'to do list'. A couple of years ago I did a test build in a BR cabinet which was getting there sound-wise, but the crossover wasn't quite right. The Peerless needs a large cabinet unless you do it sealed, but that doesn't provide the required bass extension.


I stumbled across another two way design for this woofer, but it's a MLTL with the port in the bottom of the cabinet; tweeter is a SEAS.

I hope I'm not treading on toes by posting the diagram, but it's readily viewable on line:

I'm interested to know what you think about the position of the port in terms of its being a MLTL design, any thoughts welcome!

Thank you

Geoff

[Brian Steele]

The position should be fine. I've seen it used a few times (I think Polk used that arrangement for some of their commercial speakers). It's not an offset vent, so you don't get the advantages of that type of build, but with how it's positioned you shouldn't get any issues with audible out of band vent resonances anyway.

[Geoff Millar]

Thanks Brian

Obviously I'll need to toss up between the two enclosures; they're roughly the same size although the BR is 4cm wider, so I may need to re-think the size of the inductor if I go with the MLTL. The crossover for that speaker specifies 1.5mH and the BR, 1.8mH.

Geoff

[Paul K.]

The location of the down-firing port is not a problem in or of itself. The more (most) important point is where the internal entrance of the port is along the line's length and relative to the location of the woofer. Usually, in designs like you've shown, the port's internal entrance is optimum when it's located 3-4" from the bottom of the cabinet. If you have TL-modeling software that takes into account the cabinet's dimensions (like the 1/4-wavelenght resonant frequency of the cabinet's height) and component locations, not just the cabinet volume, you're able to find that "perfect" location, one that gives the smoothest overall system bass response shape.
Paul

Hello from Oz

I have a two way project with a Peerless SDS160 and Vifa BC25TG on my ever growing 'to do list'. A couple of years ago I did a test build in a BR cabinet which was getting there sound-wise, but the crossover wasn't quite right. The Peerless needs a large cabinet unless you do it sealed, but that doesn't provide the required bass extension.


I stumbled across another two way design for this woofer, but it's a MLTL with the port in the bottom of the cabinet; tweeter is a SEAS.

I hope I'm not treading on toes by posting the diagram, but it's readily viewable on line:

I'm interested to know what you think about the position of the port in terms of its being a MLTL design, any thoughts welcome!

Thank you

Geoff

[Geoff Millar]

Thanks Paul

This is a Dutch project, I'm using Google translate to work my way through the write-up.

Geoff

[Paul K.]

I did some ML-TL modeling to illustrate what I said in Post #3.
Paul
Port location in an MLTL.doc

[Geoff Millar]

Thanks for going to the trouble Paul, that's a neat illustration of the issue.

Geoff

[Audio Dude]

The Qts of this driver really begs for a TQWt design imho

[Geoff Millar]

Do you mean the Scanspeak or the Peerless, please?

Measured Qts of the Peerless used in that project is 0.537, the Scanspeak is 0.36 (spec). Measured Fs of the Peerless is 62.4Hz compared with 28 for the Scanspeak.

One problem with the SDS160 is that the specifications can be quite different from actual measurements, e.g. the stated Fs is 52Hz. My pair could well measure differently.

Geoff

[Paul K.]

The modeling I did used actual T/S measurements of my SS 18W/8531 mid-woofers where their Qts averaged 0.41 (and because I modeled with 0.5 ohms in series with the driver, the Qts became 0.45). And while their were differences between specified and measured values for the other T/S parameters, plugging them into the same model makes very minor overall differences, primarily in the optimum tuning frequency and essentially likely inaudible.
Paul

[Geoff Millar]

Thanks Paul

That's interesting: in the case of the Peerless, the difference in Fs is 10.4 (52 compared with 62.4), which is about 20%. I assume that if the measured and specified Qts differed by so much, that would be significant in terms of the enclosure design, or at least what sort of enclosure would best suit?

Unfortunately the Scanspeak is way out of our budget here - A$299, ouch.

By the way, I was very pleased with the results of my test build of the SB16 MLTL; again, it needed some work on the crossover but it sounded nice. Apart from the shape - now discontinued - it's a nice little mid-woofer.

Cheers

Geoff

[Paul K.]

Here's the thing about T/S values. Look at and calculate the ratio of fs/Qts and the product of (Vas times fs squared) for a driver twice, once using the published specifications and again with actual measurements you have. If the measured values of fs and Qts are both higher or lower by about the same percentage as those published, their ratios will also be about the same. And if Vas changes appropriately in the opposite direction that fs changed, the design for a TL using one set of T/S values will be either correct or essentially so for the other set of T/S values. Whether a specific driver will work well in a TL depends on Qts mostly. I've found that drivers with a Qts in the 0.35 to 0.50 range to almost always work well, whereas drivers with lower (or really low) Qts values don't, and the same applies to drivers with really high Qts values. I used the SS driver for my example just because I've used it twice in personal builds and really, really like it. If you can get Dayton Audio drivers readily and without paying an arm and a leg for them, I can happily recommend the RS180P-4 and RS180-4 for TL usage.
Paul

Thanks Paul

That's interesting: in the case of the Peerless, the difference in Fs is 10.4 (52 compared with 62.4), which is about 20%. I assume that if the measured and specified Qts differed by so much, that would be significant in terms of the enclosure design, or at least what sort of enclosure would best suit?

Unfortunately the Scanspeak is way out of our budget here - A$299, ouch.

By the way, I was very pleased with the results of my test build of the SB16 MLTL; again, it needed some work on the crossover but it sounded nice. Apart from the shape - now discontinued - it's a nice little mid-woofer.

Cheers

Geoff

[Geoff Millar]

Thanks Paul, that's very helpful

I have a spare pair of the RS180P-8s; of course they don't play as loud as the 4 ohm version which you used in the "Brillante" (?). The RS180P is a really nice sounding driver, looks great too. Used in Curt's 'Slapshot' MTM, we love them. A$60 each at the time of purchase, $110 now - but still heaps cheaper than the Scanspeak.

I was going to use the spares for a build along the lines of the 'SuperNova Minimus' is in the PE Project Gallery but used the tweeters in another project.

Geoff

[JRT]

... overall system bass response shape.

Paul, have you ever tried designing an MLTL midwoofer alignment that tracks an LR4 acoustic highpass (q=0.5), for perhaps easier cross to a subwoofer exhibiting LR4 acoustic low pass?

[Paul K.]

No, I haven't. Within whatever cabinet size i might want, I always try for an f3 no higher than 40 Hz and usually no lower than 30 Hz because that's what I want and need. I tune the system, no matter what kind of TL it might be, to create as flat of an overall (modeled) response shape from the knee in the curve on up, then whatever the roll-off below the knee results, is what it is. And even though I've modeled lots and lots and lots of TLs for others, no one has ever asked for anything beyond an f3 range and cabinet size or shape restrictions.
Paul

Paul, have you ever tried designing an MLTL midwoofer alignment that tracks an LR4 acoustic highpass (q=0.5), for perhaps easier cross to a subwoofer exhibiting LR4 acoustic low pass?