PMC TL Stuffing

[pkitt]

FWIW, I've found best overall results (response smoothness and F3) for an ML-TL is to stuff the first half of the line's length at 0.75 lb/ft3. For a tapered TL, stuffing the first 60-67% of the line's length is best, with the stuffing density dependent on the taper ratio (the higher this ratio, the more it helps to smooth out the overall response and the lower the stuffing density required). I would much rather end up with an F3 in the low- to upper-30s with an acceptably smooth response above the knee in the curve, than worry about about there being two impedance peaks. For the record I build only floor-standers and use only polyester fiber for the stuffing.
Paul

 

[haf]

Isn't this a lot more informative than arguing about Bailey's paper vs whatever?

A great and informative Simulation. Thanks xrk971! Are you willing to share the Akabak script? I would love to play around with it by myself. I currently build a floor-standing TL based on a Akabak simulation (really simple simulation of a two 8'' chassis TL), so I'm curious how experts simulate TL via Akabak. regards, Armin

 

[xrk971]

Prototype PMC style TL with DC130A

48in long TL with 5:4:3:1.5 ratio and DC130A-8 driver and DC28F-8 tweeter.
Some progress in foam core... still need to do capping operation of gluing final side on.

 

[xrk971]

Model of above TL

Here is the model of the above front firing vent TL for a loosely and moderately stuffed case. This is a Harsch XO at about 2kHz (BW4 on woofer and BES2 on tweeter).

Moderate stuffing freq response:

Group delay for modearate stuffing (about 9ms at 50Hz):

Dense stuffing in tail section:

Group delay for densely stuffed tail:

I am debating which way to go, although once can change amount of stuffing in tail ven easiy enough.

Impedance for moderate stuffing (not quite aperiodic yet):

 

[xrk971]

More progress...

Added the twin terminal cups for an active 2-way, added moderate stuffing in the TL except last leg where model shows with/without switches to aperiodic TL. Also some open cell foam behind driver to reduce reflections back out of cone.

Stuffing and foam installed:

Capping operations - heavy boxes and waiting for Liquid Nails (low VOC) to dry:

 

[xrk971]

TL monitor build completed

Completed TL monitor:

Measured raw response of woofer in moderately stuffed TL running full range:

Active 2-way with miniDSP and rough-cut Harsch-like XO at 1350Hz (electrical 1st order HPF at 2kHz for tweeter, electrical 4th order BW LPF at 1000Hz for woofer) - XO plot:

Measured system HD:

Measured Phase (note characteristic rise of phase at XO point for Harsch-like XO's) :

Measured Impulse Response:

Measured Group Delay (with 1/2oct smoothing for clarity) about 6.4ms at 50Hz:

 

[BP1Fanatic]

How does sound to you?

 

[xrk971]

I have started off a new thread for the PMC inspired TL speaker here:

http://www.diyaudio.com/forums/multi-way/281778-low-cost-pmc-inspired-tl-monitor-dc130a-dc28f.html

The listening impressions are in post 1 of the above thread. But I will summarize it for you again here:

The TL sounds very nice - the transients are well aligned in time between the two drivers, the bass is clear and precise without the muddiness of a reflex, the mids are articulate and pleasing, and the highs of the DC28F are excellent - very smooth, transparent, engaging, with no sibillance or harshness. I still need to do more work to refine the XO with careful measurements and simulation in PCD, then transfer to miniDSP rather than doing the XO "by eye". I think I might move the XO up towards the 2kHz point but see that there is additional HD from the woofer there to avoid. If I have time, I may even try to design my first passive XO to see how that looks. Of course it won't be transient perfect given the flush baffle.

If I like a speaker, I will build a stereo pair and this one deserves a stereo pair. I really like the sound - completely different than a bass reflex with the same extension. The group delay is very low - comparable to a sealed cabinet so the sound is very coherent and the bass is not floppy sounding. If you haven't hear a damped TL before, this is a good chance to try - it will make you a believer.

 

[Sonce]

If you haven't hear a damped TL before, this is a good chance to try - it will make you a believer.

This is what I was talking about before - properly damped TL! Properly designed and damped TL can have slightly lower extension than a comparable sealed box, with almost all good sides of a sealed box, and a slightly "drier", more precise sound.
xrk971 once again generously gives great contribution to the diyaudio community.

 

[xrk971]

This is what I was talking about before - properly damped TL! Properly designed and damped TL can have slightly lower extension than a comparable sealed box, with almost all good sides of a sealed box, and a slightly "drier", more precise sound.
xrk971 once again generously gives great contribution to the diyaudio community.

You are welcome. Once I modeled it and saw that it had group delay as good as a sealed box but with more extension and a shallower fall-off it seemed to provide the advantages of a Linkwitz transform on a sealed box but without the electrical complexit, I had to try it. And the measurements pretty much confirm the model's low group delay and impedance response. You need to look for a squashed double hump. If single hump too much and you should just use a sealed box. Two sharp peaks and you have large GD like a bass reflex. Tuning the damping to the right amount is critical and I got lucky sort of. I have a feel for how much to put in to match the damping specifications that I used in the model.

The thing that impressed me was how "dry" the bass is - it is not floppy and is clean like a LT sealed box. The box is substantially bigger than a little 7liter sealed like I would have gone with but the size is not unmanageable. Having a two way with a good silk dome on top gives a nice sound. I might try these with a swap to my AC130F1's to see how that sounds.

 

[Sonce]

the measurements pretty much confirm the model's low group delay and impedance response. You need to look for a squashed double hump. If single hump too much and you should just use a sealed box. Two sharp peaks and you have large GD like a bass reflex. Tuning the damping to the right amount is critical

That is exactly my experience - only one impedance peak result in a overdamped design. Well attenuated first impedance peak is an optimal design. Two impedance peaks design (like all PMC designs) is not much different than a vented box.

 

[planet10]

Two impedance peaks design (like all PMC designs) is not much different than a vented box.

But it is. Martin King clearly showed the difference with his published ANSYS sim.

dave

 

[xrk971]

That is exactly my experience - only one impedance peak result in a overdamped design. Well attenuated first impedance peak is an optimal design. Two impedance peaks design (like all PMC designs) is not much different than a vented box.

Which is first peak? Lower or higher frequency peak? Mine has the lower peak more attenuated than the higher frequency peak.

 

[jphutchins]

Reticulated Foam? Reticulated.

Hello, this thread has been a great help since I recently got a pair of PMC LB1s with the foam at the mouth of the TLs deteriorated. Without the foam in place I could confirm that the internal lining seemed to have held up. Without the foam in place my initial tests by ear showed an alarming amount of harmonic distortion at lower frequencies. Clearly audible harmonics appeared when playing sine waves below 60Hz. Disgusting. To my eyes looked like too much excursion for the volume yet even at low level (with no visible excursion) the harmonics persisted. Inspecting the original dried out foam I could not see how it served any acoustic purpose... My best guess is that it restricted air flow so I put my faith in PMC and ordered the replacement foam.

The PMC replacement foam I adhered with rubber cement which seems to be similar to what they originally used. The harmonic distortion is gone and to my ears I have some well performing monitors for cheap. Apparently they manufacture the foam in house - it is not cheap and it takes a long time to ship. Got to give them credit for having replacement parts for a 25 year old speaker though!

I also replaced the tweeters during this process - one of them was labeled Vifa DT27TG 35 06 and the other Vifa DT27TG 75 06. They appeared identical. At any rate I replaced them with the new stock of Vifa DT27TG 35 06 because that's what's available - I did not order from PMC - the only compelling reason to do so would be to see if they somehow have stock of the Vifas from before the company was bought. Here's a list that I compiled of speakers that would take this specific Vifa as a drop in replacement, IMO:

AB1, AB2, AB2S, BB5-P, BB5SE, MB2-P, DB1+, DB1 Gold, MB2S XBD, DB1S+, DB1S-A, DB1S-All, OB1, FB1+, GB1+, IB1, IB1S, IB2, IB2S, LB1, MB1-P, TB2+, TB2S+, TB2S-A, TB2S-All

Some of these older PMCs could be quite a bargain!

Bottom line - the foam at the mouth of the transmission line is critical to the design in my limited and not very scientific experience.

Cheers,
J.P.

 

[JanZoo]

Hey all,

It's been a while since I've been on here! I need some help from someone better versed in the TL design than me.

I bought a pair of PMC TB1s super cheap on eBay to add to my studio. I've been struggling with a big 50hz null, which I thought was the room, but after measuring closer to the speaker it looks like it might be the stuffing in the TL They had some black foam covering the rear ports, which was starting to crumble, and completely fell out when I pushed it.

I've experimented with an RTA mic 6cm from the center of the woofer. I took 3 measurements, one without the foam, one with the original foam (which did very little in it's current form) and one with a rolled up tea towel stuffed in the port, quite loosely but enough that it's fairly sealed.

They sound much better stuffed, although I feel like they've lost some "life". Could anyone advise me on a better way of going about this? What materials should I look at? Is this port supposed to be fairly open, in which case is this a driver issue? I noticed that my LEAK 3090 TLs only have a bit of speaker grill across the ports, although they are front firing.

Many thanks,
Blindmelon6

So hello guys !!!

I had hard times treating my home studio room and trying to make my PMC TB2s (the model with the soft dome black tweeter) to work.

Basically it's the same issue is this poster's. My pseudo-anechoic close graph looks similar, and the foam in the speaker itself is loose and it's crumbling when I touch it.

I found this thread, so I tried to stuff a pair of socks in the port, just to see what's going to happen with the graph, but seems that I didn't put enough in it, because well, nothing happened, the graph stayed the same.

Pink line is without socks, and yellow is with, basically no changes.

Could it be the speaker response got altered so much because the foam in it decayed ? Should I stuff some more material in the port to get more smooth roll off ?

With this 50-80hz dip is impossible to work at all.

Thanks

 

[JanZoo]

So, I wasn't lazy, and I measured what's coming out of the rear transmission line port. And this is the result.

I don't know the physics behind it, I'm not a speaker constructor, I'm just trying to get the best of it if possible, but if those two graphs augment each other, than it's no wonder why I perceive them as a "ringing honk festival".

 

[JanZoo]

So, yeah, stuffing the port with basic foam for speakers made them smooth.

Now how the hell was PMC advertising this as +/- 3db flat to 40hz... They are +/- 3db until 100hz or so... My Adams A7x, that are also 6.5 inch drivers, are going flat till 60hz and then starting to roll off... These things (TB2s) are rolling off from 120hz or so...