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PPSL hit box

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Posted on July 26, 2014 at 07:42:22
Scholl
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Posts: 1358
Joined: March 8, 2001
 I guys!

While experimenting with my PPSL IB 15s I find the manifold limits their upper frequency to 225hz. Running them wide open I find the "hit" is excellent but vocals are nasely due to manifold standing waves so I cross for a clean 100hz and below. I'm thinking PPSL would excel if it could be built with smaller woofers and operated below 400hz.

I'm thinking about experimenting with the 10" woofers below in a 10.5 by 10.5 by 4.2" manifold and 2 cubic foot box.

What high frequency rolloff could be expected?

Any comments?

Scott



 

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RE: PPSL hit box, posted on July 26, 2014 at 14:22:46
weltersys
Industry Professional

Posts: 685
Location: FL
Joined: September 28, 2004
 Scott,

As a reasonably educated guess, the rolloff for the PPSL 10" would be around 1/3 higher than your 15", around 330-350 Hz.

You could model it in Hornresp to get a more specific idea of the plenum resonance and upper rolloff, though one driver being reversed does change things slightly from the predictions, since drivers are modeled as flat pistons.

Art

 

RE: PPSL hit box, posted on July 26, 2014 at 15:59:38
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 just a comment why not install some 30 ppi open cell (or close to that)foam inside the manifold between the two drivers. You should be able to anchor the foam to the cavity and to the magnet structure. That should go a long way to dealing with cavity resonances. Best regards Moray James.
moray james

 

RE: PPSL hit box, posted on July 27, 2014 at 12:18:33
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001
 Just this week I completed my 3rd in a series of ppsl's. In each case I learn more. My most recent used the 12" bigger brother Dayton PA-310.
This is what, a $65.00 driver? My previous (and still existing) uses Eminence Definimax 4012's
They Dayton version was built in a previous form as a ppsl bi-pole.
I did not like the rearward radiation, so I flipped the baffle inside out, so it effectively became a ppsl. The only thing is the wiring is now exposed, but no big deal as this was also an experiment to hear whether loading into a sonotube as a common back volume gave me different results than when each driver is loaded into an individual sonotube.
Well, my one question was answered: it is now confirmed it is better to share a common back volume AND to orient the drivers such that the back wave does not load into a "tunnel" but rather firing against the curved wall.Sorry that I do not have pictures taken at this time as they would go a long way towards illustration. Another difference is the Eminence version uses a triangular shaped slot for the top and bottom, but the sides (or baffle boards) are still parallel. The Dayton is a more conventional shallow rectangle. I can for SURE hear this "box effect", but the THWACK (or hit!!!) aspect is almost as good as with the Eminence drivers. To me, this is the FORTE of this design; it must be the reaction cancelling aspect. As far as the useful upper limit, I strongly believe this is a function of the construction/size of the plenum. Both builds have problems as they approach 250Hz, but lessor so with the triangular version. I suppose the next step would be to build the plenum with all four walls non-parallel, and this, of course makes the build even more complicated. I am thinking of re-building the Eminence version by replacing the opposing sonotubes with a conventional enclosure, and as Moray suggests, lining as much of one wall of the plenum with acoustic even though I know it's effectiveness is limited as the frequencies descend. It couldn't hurt though, could it?
As I close I must reiterate that the THWACK !! has never been duplicated by any other speaker I have heard. A good test source is
the Sheffield "James Newton Howard and Friends". Just listen to the DRUMS on a ppsl and you will be as sold on the design as I am (we are).
The design just needs some tweaking, that's all !

 

RE: PPSL hit box, posted on July 27, 2014 at 12:31:45
weltersys
Industry Professional

Posts: 685
Location: FL
Joined: September 28, 2004
 "just a comment why not install some 30 ppi open cell (or close to that)foam inside the manifold between the two drivers."

That type of foam is acoustically transparent in the 250 Hz range, it would have virtually no effect on cavity resonances.

 

RE: PPSL hit box, posted on July 27, 2014 at 14:47:17
Scholl
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Posts: 1358
Joined: March 8, 2001
 Thanks for the input.

I think this is an area where we have to experiment with the manifold. I would expect the high limit is determined by a quarter wavelength on the manifold dimensions. A 10" woofer with 10.5" by 10.5" manifold would be 320hz.

 

RE: PPSL hit box, posted on July 27, 2014 at 15:20:49
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 that being the case (and thanks for reminding me I should have realized this myself)I am sure that there is some form of damping which could be included into the cavity and on the driver basket which would go a long way to help it's just a matter of experimenting. One could install some 1/4" thick S.A.E. rated F-11 acoustical felt on the plenum walls on the basket and motor of the one exposed driver they ought to help a lot. I am sure some significant improvement could be found. Best regards Moray James.
moray james

 

Great thread, posted on July 28, 2014 at 06:01:38
JoshK
Audiophile

Posts: 820
Location: NJ/NYC
Joined: August 3, 2001
 I have nothing useful to contribute, but I found this thread very interesting. I plan to build some PPSL boxes with my JBL 2226s.
----------------
"When Khruschev said "we will bury you" I don't think he meant with surplus parts." zacster

 

RE: PPSL hit box, posted on July 28, 2014 at 12:56:24
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 Scott: look forward to a photo or three. thanks for posting and congrats on your success with these. Best regards Moray James.
moray james

 

Dampening materials, posted on July 28, 2014 at 13:29:12
weltersys
Industry Professional

Posts: 685
Location: FL
Joined: September 28, 2004
 "I am sure that there is some form of damping which could be included into the cavity and on the driver basket which would go a long way to help it's just a matter of experimenting. I am sure some significant improvement could be found. "

I have done extensive testing with a wide variety of materials in varying thickness: acoustical felt, cotton, fiberglass, polyfill, lamb's wool.

None of them provide a "significant improvement", all of them are capable of reducing a peak around 170 Hz, but also reduce the output level below that by a similar amount, net result is just less output, with a similar peak.

And a day's measurement work, and a lot of cleanup after...

 

RE: PPSL hit box (pictures), posted on July 28, 2014 at 13:35:17
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001



I'm going to try to upload 2 pics. If I can't do it in one post, I'll add
another.
The 1st attempt was a "reaction cancelling" but this proved to be a disaster. I did not know why (yet).
The 2nd attempt was a ppsl, but I made the plenum walls TOO FAR apart.
My thinking was [that] I needed 2xSD for an opening. That was not the case;
as a matter of fact the slot opening should equal one driver's SD for a ratio of 2:1
Please do not make your plenum 10.5 x 10.5, as I already went through this.
Pretty much wasted one summer's worth of woodworking, and I was so disgusted with myself that I ended up building the next one during the winter, waiting for some sunny days on my back porch. The one that worked best has the plenum walls 5 inches apart. I'm referencing 12 inch woofers.

 

RE: PPSL hit box (pictures), posted on July 28, 2014 at 13:37:43
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001



It seems only one picture at a time....

So here's the ppsl version that had the plenum walls too far apart:

 

RE: PPSL hit box (pictures), posted on July 28, 2014 at 13:43:11
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001



Finally, here's a close up view of the beginning construction.
This is the version with the back of the plenum built in a triangular fashion

 

RE: Dampening materials, posted on July 28, 2014 at 14:00:24
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 Thanks Art: It's hard to imagine that the there is much ability to resonate within the plenum with a cone basket and a motor stuffed into the middle of it but it obviously has to resonate at some frequency and as you measured it does. Your observation that the damping you experimented with also caused a drop in output below 170 Hz is to me very interesting indeed. Perhaps that suggests that the resistive drag on the air flow is one possible issue within the plenum? Maybe attempts to breakup the resonances with reflectors or diffraction devices would be a more effective approach to stop the resonances while at the same time allowing free air flow within the cavity to. so the goal would be to have minimum air turbulence inside of the plenum. Tough to do but the first thing that comes to mind is to eliminate and minimize corners inside the plenum.
Thanks again Art for posting this info as it is so very interesting. Best regards Moray James.
moray james

 

RE: Dampening materials, posted on July 28, 2014 at 14:06:30
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001
 Last night I lowered the crossover point of the low-pass section to my current ppsl's to 125Hz. It was 250hz before. This has minimized the cavity resonances quite a bit, so, I can only estimate they are somewhere in the area of around 200hz + -

This supports my earlier theory that these things (ppsl) are best served up as woofer duty only.

 

RE: PPSL hit box (pictures), posted on July 28, 2014 at 14:59:35
Scholl
Distributor or Rep

Posts: 1358
Joined: March 8, 2001
 the 10.5 by 10.5 was depth and hight. I'd do 4.2" wide for 2.5:1 ratio.

How did this one sound?

 

RE: PPSL hit box (pictures), posted on July 29, 2014 at 12:35:02
Scott L
Audiophile

Posts: 353
Location: Knoxville
Joined: February 2, 2001
 It's by far the best of the bunch, but I still feel like improvements can be made with this design, by the use of non-parallel plenum side walls as well.

 

FH1, posted on July 30, 2014 at 12:34:13
Scholl
Distributor or Rep

Posts: 1358
Joined: March 8, 2001
 I noticed you posted info on a DIY Peavey FH1. How did you like that box from 100-400hz?

 

RE: FH1, posted on July 30, 2014 at 14:17:04
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 I do have the designs for the FH1 but I bought a pair too cheap to pass on. I like the FH1 better than a LaScala its got a slightly larger mouth is a little longer and has no dead spot like the LaScala does, it is really a better Belle. In the end it was just too large for me but it was hard to give up on the great efficiency so hard to go backwards when it comes to that. I think the FH1 is probably the go to design in this band, short of going to a straight horn it is very likely as good as it gets with just one 90 degree bend. Crossover ought to be between 800 - 100 Hz at the bottom. Claude J has played with and modeled a number of better drivers to help extend the top response. corner reflectors will help a LaScala top end same as in the FH1 the LaScala won't go as low as an FH1 does. I think the small added size of an FH1 over a LaScala is well justified. I am sorry I could not be of more help I ran the FH1 for only a few months. Best regards Moray James.
moray james

 

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