Experience with PT Wave Guides

[Zilch]

So, Earl, you posit that the zero-crossing null between the two resonance peaks in the impedance curve correlates with F3 of the driver/waveguide combination, and their loading.

The Fmin is easily ascertained by WT2, either via Fs or Vented Box tests, and the F3 may be found from CLIO MLS measurement.

Here they are for the same 2431H driver on PT-H1010 and PT-F1010 waveguides, an interesting case, since they are so similar. Can your theory discriminate between these two?

Indeed.



PT-H:

Fmin = 1393 Hz
F3 = 1208 Hz (cursors)

PT-F:

Fmin = 1162 Hz
F3 = 1099 Hz

PT-H impedance (red, phase is blue,) top, PT-F next. Note that the FR curves (bottom) nicely track those Mr. Widget did a year ago, above....

[Zilch]

How does the AM crossover "work" on the frequency responses of this driver on these waveguides?

I've normalized to the 2 kHz - 4 kHz area where the filter basically does nothing; the violet follows the red, and the blue follows the black in this region.

Am I operating "loaded" with these combinations?

[Earl K]

Hi

How does the AM crossover "work" on these frequency responses?

I've normalized to the 2 kHz - 4 kHz area where it basically does nothing:

- I'm not clear about what you're actually asking here .
- I had to research ( just now ) what the AM crossover is. It seems to me that you , Guido and Giskard mapped all this out, a year ago. So, I'm confused regarding your need to query. Anyways ;

- I'll look at the schematic , then later I'll offer a verbal analysis , element by element .

[Zilch]

I'm not clear about what you're actually asking here .

You stated in #7, above:

- the PT-H, 100° x 100°, looks to have a loading limit of around 1500 hz.

I'm seeking to refine that determination, as several of us are trying to use them closer to 1 kHz.

Toward that end, I have provided new quantitative information about the impedance and response curves, and have also shown how the filter I'm using is "working" (interacting) with those, in actual use.

You've recommended crossing down to the load limit of the horn/driver combination. I'm trying to ascertain more precisely where that is for these PT waveguides with 243x drivers. It seems to me it may be as much as 200 Hz lower than I am presently operating them.

The "How does the AM crossover 'work'" question was rhetorical, answered through presentation of the curves. I'm not asking what you thought I was asking here (sorry for confusing,) but any suggestions for improvement would certainly be welcome, as well as comments on the topology. It incorporates a series resonant LCR notch, which you have recently mentioned elsewhere (Re: Dawson).

[Looks like it could use another one @ ~13.7 kHz for 2431H, actually. ]

The question is whether or not I'm operating "loaded" under these conditions....

[Robh3606]

You've recommended crossing down to the load limit of the horn/driver combination. I'm trying to ascertain more precisely where that is for these PT waveguides with 243x drivers. It seems to me it may be as much as 200 Hz lower than I am presently operating them.

I will be trying them out at 1.5K with 24db slopes. JBL runs them down to 1.1K the PTH that is. Let you know what happens. Take a look at this quote I grabed from Earl about his OS waveguide. The link takes you to the thread.

Length isn't a factor, but mouth area is. Down to 800-900 Hz, with absolute directivity control the mouth would have to be about 24" in diameter maybe a little more if you want to be safe. Now with that knowledge in hand, what is the angular coverage that you want, because that will then determine the depth. Wider coverage is a shorter waveguide. Larger throats also have shorter waveguides.

Horns and Waveguides operate from such completely differnt points of view that its hard to compare them dimensionally because each case is different. There is also the fact that there is no such thing as "cutoff" in a waveguide - so this entire concept is out the window. But, much to my delite, the OS the coverage angle is explicitly stated in the design equations. This then tells me directly what the tradoffs are for coverage angles and throat sizes etc. This does not exist in horn theory, since angle independence is assumed.

Horn theory deals with rates of change of areas, while waveguide theory is concerned with rates of change of the boundaries. The later gives you the former, but not the other way arround.
Earl Geddes

http://www.audioasylum.com/forums/hu...es/107845.html


Rob

[Zilch]

Hi, Rob.

Several of us have looked at and worked with them now, and I'd say that you're in good territory crossing at 1500 Hz.

My own experience is that they sound better crossed at 1200 Hz than they do at 1 kHz, and I have made no serious attempt at using them below that. JBL uses them down to 1.1 kHz, yes, but that's in SR, and may not stand up to Hi-Fi listening criteria.

Mr. Widget has listened to them here at 1 kHz, and categorizes them as "PA horns" used that way. From our first measurements, he concluded 1200 Hz was the lower limit.

Bottom line at this point is we each simply have to try them. To me, the PT-F sounds better than PT-H, generally, and I think some of the data supports that.

Admittedly, I have not conscientiously controlled the variables (slopes, filter curves, etc.,) but those are my impressions.

From Geddes, I conclude that "cut-off" does not compute with "Waveguides," and perhaps not "loading," either, but who knows WHAT these actually are (the design theory upon which they are based,) and how they relate to conventional horn theory?

[Robh3606]

Hello Zilch

Yeah I hear you. I use the 2344 up there as well. I tend to be real conservative and don't push horns towards the lowend of their range. I thought Earls Geddes comments were interesting as they confirm the different depths on the PT wavequides and also felt that his comment about the lack of a cut off frequency compared to horns was something new to me at least. I like to keep as far away from the lowest recomended crossover point as makes sense. I am curious about how the 2431's sound around that peak in their response. I would like to see what the coated diaphrams look like up there to see if the coating helps that peak at all. I know in the new Array Series they crossover to a tweeter so that peak could get burried and notched and simply not be an issue in that application. Might do the same if they sound ragged up there.

Rob

[Robh3606]

Oh well I got my horns from JBL and guess what!! They are the wrong ones So off I go tomorrow to give them a call. It took me a while to figure out what they where. They are the 75X50 352606 from the SRX722's. I ordered the PTH1010's. So there a bit different. I am actually glad cause I wanted to try a horn out in this geometry. This is the first time I can actually listen to the 2431's and they sound like they could be quite nice. I have not done any comps just a series cap and a notch to get them smoothed a bit. Zilch I think you looked at there little brother. These are decent size. They are aluminum and ring like hells bells un mounted.

Rob

[Zilch]

Zilch I think you looked at there little brother.

Yup. Power response was poor. Above 10 kHz, they crashed off-axis.

Sounded good, tho, if I stayed put.

Yours, a narrower pattern, may behave better, tho.

CLIO'll tell the tale.

http://audioheritage.org/vbulletin/s...5528#post65528

[Pleasingly crispy-sounding, as I recall.... ]

[Robh3606]

Well I got the right horns finally and a pair for 2435 diaphrams I plan on using in the 2431's. Have any of you compared the diaphrams on these, I got guite a surprise I expected them to be the same except for the materail. There not at least the examples I have. The 2431's are flat with no halfroll like the 2435?? I am wondering if I have the older version in these. I don't seem to have the peak at 14KHz as well??? The red and Green are the drivers on the my other horns. The dip at 3Khz is a measurement anomally. The violet is on the PH1010 with the notch filter across the driver. Attached are a couple of photo's of the 2435 and 2431 diaphrams as well as the phase plug on the 2431H.

[Zilch]

According to the specs, 2431H is "A version of the 2430 with a narrower coil milling and flat surround for extended HF response."

http://audioheritage.org/vbulletin/s...&postcount=402

Red and green are what drivers on what horns, please, Rob?

[Robh3606]

Hello Zilch

Those are 2431 on the 90X50 SR horns with a make shift network I threw on them while testing.

Rob

[Zilch]

Thanks, Rob.

Though I suspect you're moving on past the SRX horns now, if you try to EQ them flat on axis, I found they'll still collapse above 12 kHz or so.

I put considerable effort into making them and 2332 work, with poor results.

It's noteworthy that, while JBL shows essentially the same result as you found in the on-axis FR curve, they omit DI and beamwidth data above that frequency range.

http://www.jblpro.com/srx700/PDF/JBL.SRX722.pdf

Bottom line: they cannot be effectively EQ'd over the specified horizontal coverage, in my experience working with them.

[They'd make good dinner bells, tho.... ]

[Robh3606]

I installed the 2435 diaphrams today and was surprised to find they wouldn't fit . I laid the 2431 and 2435 side by side to see that the 2431 had part of the terminal block and rivet removed. I went to the grinder and took the edge off the terminal block to match the 2431's and they fit. I used the provided filter paper to remove the old ferrofluid and cleaned the gaps. Then put in the new fluid. That was fun as the gap flux sucked it right out of the tube!! I lined up the diaphrams and reinstalled the covers. Did a quick sweep at home and then tried some measurements with a preliminary network on the PTH1010 horns. To my surprise they looked very good and had much more extension up top then I thought I was going to get from them. Matter of fact a little too much which really surprised me looking at Widgets horn study and Project May Data. Attached are the Sine files with harmonic distortion both 2nd and 3rd and an overlay done with MLS to see how well they match up. I have not listened to them yet or finalized the network so I still have a bit of work to do.

Rob

[Zilch]

I used the provided filter paper to remove the old ferrofluid and cleaned the gaps. Then put in the new fluid.

Well, there's how ignorant I am -- didn't know 2431Hs had ferrofluid in them. I've never pulled a diaphragm on one.

To my surprise they looked very good and had much more extension up top then I thought I was going to get from them. Matter of fact a little too much which really surprised me looking at Widgets horn study and Project May Data.

Same surprise here:

http://audioheritage.org/vbulletin/s...&postcount=115

They measure better on PT waveguides than the specs would dictate.

Attached are a couple of photo's of the 2435 and 2431 diaphrams as well as the phase plug on the 2431H.

Didya see what was "two-stage" about it?