Plane Wave Tube Construction

[1audiohack]

Hi All;

I have decided to build a plane wave tube. For a start I have the AES papers on them. What I would like to do is to duplicate the one used at JBL and or Brush Wellman.

In the transducer library the JBL 2435, 2450 and 2452 were all measured with TEF, the same system I primarily use. The closer I can stay to the JBL set up the more comfortable I will be.

Is there anyone here that has access to this information? Any help will be greatly appreciated!!!

[Mr. Widget]

I recently toured the Brush Wellman factory and testing facility... both were impressive and their attention to QC very impressive, but I do not think they use a plane wave tube. I'll find out and let you know more.


Widget

[yggdrasil]

Here's something I have found somewhere on the internet:

rho = density of air
c = speed of sound
d = diameter
Upper limit = 1,22 * c / d
lower imit = c / 4 / length of tube

Now let's talk about horns driven by compression drivers. One of the handy pieces of information that used to be included with compression drivers (but is not always now) is the plane wave tube response. A plane wave tube is a tube the same size as the exit of the driver (or smaller or larger and including an adaptor to have a smooth transition from the driver's throat to the size of the tube). There is absorbing material in the tube which is designed to provide a certain load to the driver - a load of rho * c * diameter of the tube, where rho is the density of air and c is the speed of sound. This is usually called a "rho c" load; rho * c is the specific impedance of air. This load is supposed to be constant at all frequencies and allow easy comparison of different drivers, but in reality the size of the tube determines the frequency range measurements are useful in. The upper limit of the tube is set by the diameter. 1.22 * c / d is usually given as the upper limit. There will typically be a notch at this frequency and other notches above it. The lower limit is c / 4 / length of the tube. So only the response between these frequencies should really be looked at. With high frequency horns, you can typically build them large enough to obtain a rho * c * some constant input impedance that is relatively flat above the cutoff frequency (depending on the type of horn). This means in theory the frequency response on the horn will be the same as on the plane wave tube (although the sensitivity will be different - compression drivers' sensitivity on a PWT is very high). The only caveat here is that many high frequency horns have a polar response that varies with frequency. In other words, at low frequencies the horn is not big enough to the control the sound radiated by it, so the sound spreads out over a wide angle. At high frequencies, the horn becomes acoustically large compared to the sound waves being radiated, and the sound is confined into a narrower angle. This means that at higher frequencies more power is concentrated over a given area, so the sound pressure level is higher in that area (but lower outside it). This is effectively an acoustic equalizer, so this effect needs to be added to the plane wave tube response to come up with the on-axis response of the horn / driver combination. This also explains why typical horn design programs do not predict the on axis response of high frequency horns very well - they don't typically include this factor.

Attached is a spreadsheet for calculation of PWT's.

[JeffW]

Google Books has a limited preview of John Eargle's "Loudspeaker Handbook" with formulas and tube sizes/diameters for various compression drivers, pp 180~181

[grumpy]

I've pondered doing this... found clear tube source... bought much cheaper opaque
1.5" and 2" tubes (won't be able to see/monitor absorbent "wedge")... read
about sensitivity of mic port placement and repeatability of mounting and sealing
the drivers under test (precision and stability of mount plate)...
end result: tubes are still in the garage

Hoping you have more drive/desire to see this through

[Allanvh5150]

Hi All;

I have decided to build a plane wave tube. For a start I have the AES papers on them. What I would like to do is to duplicate the one used at JBL and or Brush Wellman.

In the transducer library the JBL 2435, 2450 and 2452 were all measured with TEF, the same system I primarily use. The closer I can stay to the JBL set up the more comfortable I will be.

Is there anyone here that has access to this information? Any help will be greatly appreciated!!!

I think this piece of gear was used in the Manhattan project. You really want to split atoms?

[1audiohack]

I think this piece of gear was used in the Manhattan project. You really want to split atoms?

No, I want to split hairs. I don't want to build that thing, well, of course I do but,,,

Fortunately information about PWT's is prevalent, I am hoping to build identical to JBL's units, if the information to do so is not available then I will do the math and build a suitable unit. I am simply hopeful that someone here knows the dimensions of the JBL PWT's.

Thanks all,
Barry.

[JeffW]

The JBL model that 1audiohack wants to replicate is one driver on one tube. He just posted that pic for fun, that's not precisely what he's going to build.

[1audiohack]

Wow someone did some serious house cleaning here! Thank you.

The published JBL measurements are somewhat condensed as are most others I have been able to find for reference. I do however like the fact that the charts shown for the 435Al, 2450 and 2452 are shown with the 2nd and 3rd harmonic traces in actual place as compared to the fundamental tone, I think it gives a better picture to what's real going on.

What is missing is the phase response. Unless the responses are averaged, TEF will provide the phase response of not only the fundamental but the harmonics as well. This is I believe important, a harmonic in a reproducer is of of of course not desirable but a harmonic that is also out of phase (time) is nothing but total noise. Some of the magnitude traces have the second harmonics within 20dB SPL of the fundamental. I have duplicated these measurements very closely and the phase response falls thousands of degrees in reference to the fundamental in the frequency range that is said to be usable. These drivers used this way would not even qualify for use in the Bell Labs Symposium on Auditory perspective published in 1934!

A primer to what I am looking at;

The newer drivers (that I can get my paws on) used in the high-performance audio applications have very good magnitude and phase responses in the minimum phase passband and have markedly low 2nd and 3rd harmonics that also are phase coherent, so the signal to noise ratio is very good but on the ends these drivers loose it big. The trade is improved performance in a more limited band width for a more musical compression driver. The goal here for me is not to design a driver, rather to know how to best use what's available with the fewest compromises in driver integration.

Looking at (measuring) the newer drivers with all of the different diaphragms available has revealed some interesting details to me. I know what I am doing has been done before and is known to some. I fully intend to share when I believe I can defend what I am finding. In short I believe I can now describe many of the subjective differences I hear with objective measurements. Time will tell and some of you will be the first "peer review." Just remember this is a (my) hobby, not my profession and I am just trying to learn.

Now I just have to build the tube assemblies.

I haven't researched or sourced the PWT foam yet, has anyone already done that? Grumpy?

Thanks again all,
Barry

[grumpy]

I was going to use fiberglass, inserted as a long wedge shape. Then measure and rearrange as necessary... Hence my search for clear tubes and a place to put the thing so it would not be disturbed. I expect other materials would hold shape and position better... with less fiddly reassembly.

If I can remember the articles that sort of described several pwt's that were used at JBL, I'll post the links... One was curved for compactness, iirc. You may already have this info.
There was at least one aes paper that described some of the issues involved with building and using pwt for audio testing.

[Allanvh5150]

All interesting stuff. One thing I have never really understood though and no one may have the answer, why did JBL always test drivers into a 1" tube? Surely this would upset the load that the 2" throat driver sees? Please enlighten.

Allan.

[grumpy]

Iirc, for comparison's sake... most mfgs convert their readings to values as though they were taken on a 1" pwt. There are reasons why using just one tube for extended freq measurements might not be adequate (e.g. Hf limits using a large tube).

[Allanvh5150]

Iirc, for comparison's sake... most mfgs convert their readings to values as though they were taken on a 1" pwt. There are reasons why using just one tube for extended freq measurements might not be adequate (e.g. Hf limits using a large tube).

Thanks Grumpy. I thought it would have been a better idea to spec the driver on the horn or horns they were meant for. Real world specs. Do you know of the reason why they tested in this way?

Allan.

[1audiohack]

A plane wave tube is a way to eliminate some of the variables. Without doubt what happens when a horn is installed is most important, that's where the rubber meets the road.

I have swept these things in every configurable way available to me from hanging in space with the rear caps off and nothing bolted to the throat to caps on with the throat stuffed and a plate bolted over it's throat in attempt to discern what features of the impedance trace are connected to the driver it's self and what features are horn related.

It is said that a knowledgable teacher only saves an eager student time. This is surely true, many of the tests I have done most likely serve no real purpose besides narrowing the scope of the search. Getting down to asking the right questions and spending time and energy on the ones that really matter is the process I am still in.

Thanks again for all the help!

[JeffW]

Thanks Grumpy. I thought it would have been a better idea to spec the driver on the horn or horns they were meant for. Real world specs. Do you know of the reason why they tested in this way?

Allan.

Read the link to the Eargle book I posted upthread, it goes into a little detail about using different sized tubes.