...and it's also nice to know that the drivers are actually working right before you test them, to get any meaningful results, i.e. no rubs or buzzes.
Edgewound
...and it's also nice to know that the drivers are actually working right before you test them, to get any meaningful results, i.e. no rubs or buzzes.
Edgewound
WW,Originally Posted by Mr. Widget
1) A PWT is relatively easy to make. See AES documents [1] & [2] for details. Presuming you already have a microphone and test gear, the cost of materials to make it is less than $100 and it should take no more than a weekend to fabricate. While the acoustics of each tube will be slightly different, that difference unlike the horn, will not materially affect measurement validity, particularly when such data is used for the purposes of comparative analysis.
2) All comparisons are intrinsically “relative”, and in this case, need to be based on reliable and objective data. There is a difference between prerequisite and “ideal”; for valid comparisons to be made, first reliable measurements must be obtained. When using a horn as a test jig, this is not happening; e.g., horn artifacts, at the bottom end, are manifesting themselves as distortion products due to the driver being operated below the horn’s effective pass band, which begins at least one to two octaves above horn [fc].
Regards,
WHG
References:
[1] Title: Plane-Wave Tubes: Design and Practice
Author: AES Standards Committee
Publication: AES-ID, No. 1id-1991, Rev. Jun-2003
URL: http://www.aes.org/standards/b_pub/a...1991-r2003.pdf
Abstract: In section 2.2.1 of the "AES Recommended Practice — Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement" calls for the use of plane-wave tube to perform measurements of horn drivers. Because many variations and results are possible, depending on the details of construction of plane-wave tubes, this document discusses those variations for the purpose of encouraging further experimentation.
[2] File: ASAJ047-0596
Title: Building a Plane-Wave Tube: Experimental and Theoretical Aspects
Author: Roberto Magalotti
Author: Carlo Zuccatti
Author: Paolo Pasini 596
Publication: AES-J, Vol. 47, No. 7/8, p. 596, Jul/Aug-1999
Abstract: The primary purpose of this report is to describe the building and testing of a plane-wave tube for measuring compression drivers in an audio laboratory. A new method for testing the tube for undesired reflections was devised, with and without sound absorbing materials. The experimental results show good agreement with the predicted behavior outlined in the AES document on plane-wave tubes, and confirm the predictions on usable bandwidth limits.
"When using a horn as a test jig, this is not happening; e.g., horn artifacts, at the bottom end, are manifesting themselves as distortion products due to the driver being operated below the horn’s effective pass band, which begins at least one to two octaves above horn [fc]."
I thought we discussed this already. The point is, those artifacts will affect all of these drivers in the same manner. Since our purpose wasn't to determine the absolute value for each driver but rather the relative performance between them, I think you are going over board. Of course if you would like to make us a PWT, I'll be happy to use it in future tests. It would be nice to have one for 1", 1.5", and 2" drivers.
Widget
BTW: your link didn't work... I did try to check it out.
It is not work for me too...
after manys try on site to pass in other way I realize this information is sale on this site...
(...)
Jean.
BTW this very interesting informations...
I have been experimenting with plane wave tubes for the past few months. I was unhampered by knowledge in my first attempt, and stretched 8' lengths of 2" plastic plumbing pipe all across the back yard. Despite progressive stuffing in the line, there were a series of monster peaks and dips in the response.
Next a friend obtained information from a U.S. manufacturer on their plane wave tube construction techniques. They recommended a 6' tube, mic hole very near the driver, and a long tapered wedge of open cell foam stuffing beginning immediately past the microphone. This advice yielded a tube that works very well.
More recently I read the AES article, and now understand more about how these devices work. The article recommends that the smallest diameter tube be used to push a transverse mode cancellation to the highest frequency possible, extending the reliable bandwidth of the measurements.
Plane wave tube measurements can tell you a lot about driver performance in a short time, without the flattery of the high frequency beaming of most horns. The measurements in a tube flatter the low end response, though, as the loading is so much better than in any real world horn. Most compression drivers look like awesome subwoofer drivers on a PWT, with response down only 6dB at 10Hz. or similar.
OK, and then?
We need a Horn with all its flattery high frequency beaming. To tweak the crossovers we now need to measure the horn/driver combo. Bingo, back where we started.
I see your point Guido, but the desirability of high frequency beaming is open to debate. A horn that steers into a progressively narrower beam at high frequencies will sound and measure well directly on axis, but the off axis radiation that largely creates the reverberant field will be noticeably dull in comparison. I believe that our brains can detect this descrepancy easily, adding another artifact that creates a reproduced, rather than live sound. This may be one reason why conical flare horns tend to sound so uncolored, as they vary little in response over their entire pattern.
Plane wave testing is useful to isolate out the driver's power response from horn effects, making it possible to compare drivers on an even footing. I agree completely that it is also necessary to test a given driver on the horn it will be used with, as we don't listen to them on plane wave tubes!
A point I'd like to add is that the conical wave guides you speak of are not the conically shaped horns like the JBL HL91 (2307/2308). These horns are exponential and do progressively beam higher and higher frequencies, as do dome and cone transducers.
The conical wave guides you are discussing are similar to JBL's Bi-radials in that they have a falling on axis response that achieves (in theory at least) an even power distribution and will have flat frequency response off axis as well as on axis if EQ is added. Unlike JBL's Bi-radials the conicals do not have the secondary acoustic center and are therefore claimed to have a better sense of realism and imaging.
Widget
Okay, since I'm about to go offline for about a month or so, I thought I better reiterate my sincere "Thank-You" to Mr. Widget, Zilch , Johnaec, and all the rest of the crowd who participate in putting these newer ( & older ) JBL Pro products through their paces .
For those not aware; these R & D efforts are done at these fellows own expense ( time and money ) , and I for one, really appreciate the huge efforts being made to peel back the mystery surrounding these products. These guys unselfishly publish their findings and I want it known , I really appreciate it !
Since I find myself as a user of JBLs' "off the shelf" products / what these guys have done is "spot on" for my needs ( and I would guess for most of the people who frequent this forum ) .
Again ; Thanks Guys ! <. Earl K
To be precise, it's the DRIVER that has the falling power response, not the horn, and the horn geometry either "corrects" it by differential steering, as in exponentials, or it does not, and must be EQ'd flat, as in most Biradials and other constant directivity designs, or some intermediate compromise in between.
The little OASR horn we studied is termed a "conical waveguide" by JBL. Is that an example of the conical under discussion here? Looking again, there's no apparent throat constriction to perform diffusion; the shape itself confers constant directivity. If so, I'm gonna hook it up and listen some more.
[I NEED to pay better attention, quite obviously....]
You and everyone are welcome, of course. We're "driven" by relentless inquisition is what....
Yeah, yeah, sure, sure... I was talking in terms of them working as a system, but yes, you are correct.
I have to admit, I haven't paid much attention to the OASRs, but yes, if JBL is calling them conical wave guides, then yes that is what Steve is referring to. Do they make any with a lower cut off? These had a pretty limiting bandwidth.
Widget
They make some rectangular versions that may qualify. Lemme look....
2352 (630 - 18 kHz) and 2392 (300 - 4 kHz) series, looks like:
http://www.jblpro.com/pub/technote/tn_v1n21.pdf
http://www.jblpro.com/pages/components/23525354.htm
http://www.jblpro.com/pages/components/23929394.htm
They hybrid Biradials, looks like.
Somebody else'll have to play with them. I'm doin' PT waveguides....
Widget, that was an important clarification. By "conical", I refer to any horn with straight walls rather than curved. A conical horn can be axisymmetric (round), square, rectangular or some other shape.
So far my experimental conical horns have been rectangular... and large. Because of the lack of low frequency loading that curved throat horns achieve, a conical horn must be quite long to achieve decent loading on the low end. Usually, conicals are a case of "measures bad, sounds good". Due to the lack of l.f. loading and broad dispersion of the highs, an on-axis response curve will typically look like Humpty Dumpty. What is not told in the measurement is the wide open clarity of sound of these horns. If one can employ some EQ to flatten the response, and can tolerate the low Spouse Acceptance Factor of really large (say 24" by 32" by 36" long for 300Hz. cutoff) horns, I think it is hard to do better.
A.J. May, who worked in loudspeaker development for decades at RCA, built many experimental conical horns about 50 years ago. He was enthusiastic about their broad, even and controllable dispersion vs. frequency. RCA never produced any conicals though, because they were too large for a given cutoff frequency.
Zilch, I agree; a conical horn will measure close to the plane wave tube response for a given driver both on and off axis, down to the horn's cutoff.
Steve,
I was reading the paper on the 2344 horn a while ago and in it was discussed the pros and cons of various horns and geometry and in some ways the 2344 is hyprid using differaction and a certain mathematical expression for the curve.
Have you ever considered a conical/ differaction hyprid horn?
Ian
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