Hi all
Is anyone of you experienced with making plane wave tube measures for horndrivers?
Id like to build a home setup comparable to industry standards.
Whats important / tricky?
Just any tube 25,4/50,4mm metric inner core diameter tube?
Recommended to put the tube in a deadening tunnel (e.g. sand) ?
How perfect the inner dia. has to fit to the driver mouth (half/10th/100rd of mm)?
How long has the tube to be?
Can the mic be 90° to the driver or axial?
Is the mic at the end or somewhere half way?
Is the end to be dampened?
Etc etc
cheers
Josh
Is anyone of you experienced with making plane wave tube measures for horndrivers?
Id like to build a home setup comparable to industry standards.
Whats important / tricky?
Just any tube 25,4/50,4mm metric inner core diameter tube?
Recommended to put the tube in a deadening tunnel (e.g. sand) ?
How perfect the inner dia. has to fit to the driver mouth (half/10th/100rd of mm)?
How long has the tube to be?
Can the mic be 90° to the driver or axial?
Is the mic at the end or somewhere half way?
Is the end to be dampened?
Etc etc
cheers
Josh
Journal Audio Engineering Society has articles on this.
AES E-Library >> Building a Plane-Wave Tube: Experimental and Theoretical Aspects
Best wishes
David
There is some information in the subject here.
Poor man's plane-wave tube?
The transition from driver to tube should be smooth and exact.
The mic should be perpendicular, close to the driver and functionally flush to the inner surface of the tube.
The tube should be dampened so that there is no difference if the end is open or capped. This is the real tricky part to get right.
See what you think. I will be glad to pick this up again
Mine is still very much in use.
Barry.
Poor man's plane-wave tube?
The transition from driver to tube should be smooth and exact.
The mic should be perpendicular, close to the driver and functionally flush to the inner surface of the tube.
The tube should be dampened so that there is no difference if the end is open or capped. This is the real tricky part to get right.
See what you think. I will be glad to pick this up again
Mine is still very much in use.
Barry.
Hi Dave. I see they want cash for the poor man i found a nutshell version:
http://pcfarina.eng.unipr.it/Public/Standing-Wave/palnewt.pdf
Thank you Barry!!
Its a lot different than what I had in mind
Guess ill come back with a sketch for your approval
Josh
for the poor man i found a nutshell version:http://pcfarina.eng.unipr.it/Public/Standing-Wave/palnewt.pdf
Thank you Barry!!
Its a lot different than what I had in mind
Guess ill come back with a sketch for your approval
Josh
Hi Dave. I see they want cash
I read the JAES paper by Magalotti at the library, no cash required.
It doesn't add much to the standard except for use of time domain window to check the tube absorption, already mentioned in Barry's (1audihack) thread IIRC.
There is also a conference paper by M Buck, I haven't read this yet because my library doesn't have it.
If Barry has read it perhaps he can tell us if it's worth a search?
Best wishes
David
My memory sucks! If I have it, it is on my work desktop. I will try to remember to check Monday.
Barry.
The ones we used at JBL years ago were clear plexiglass or plastic of some kind. Typically about 4 to 6 feet long. Should be reasonably rigid. They had a long wedge of fiberglass that stopped maybe a foot from the compression driver mounting flange. The mic was put in flush from a side hole so it was at 90 degrees to the length of the tube. (It doesn't really matter as the mic is being driven in pressure rather than free field.) Note that the mic pressure response is unrelated to any calibration curve you are likely to have. Mic was maybe 6" from mounting flange.
In spite of the fiberglass wedge there was a visible standing wave ripple in the curve (1/2 wavelength?) but it wasn't too bad and was always there so easy to ignore.
In spite of the fiberglass wedge there was a visible standing wave ripple in the curve (1/2 wavelength?) but it wasn't too bad and was always there so easy to ignore.
Any luck with this?
Was this due to reflection between the speaker to microphone or the speaker to start of the absorber?
Any new information in >this< Buck paper? (I expect the US compression driver measurement experts must be a bit of a clique!).
Best wishes
David
The ripple I remember was due to the length of the tube and from the internal wedge being an imperfect absorber. In our case it was in the low hundreds but it is tied to the length of your tube. Buck suggests gating but I don't think your low end extension would be that good unless your tube was really long.
I haven't fully read the Buck paper but it looks like very good info. Having the mic close to the compression driver exit seems best.
B&K always published response curves for their mics with 0 degree incidence and pressure response (at least for the free field mics. Pressure response is also what the electrostatic actuators/calibrators would measure). From their data it looks like a half inch mic that is free field flat at 0 degrees will have a pressure response that rolls down gradually from around 10kHz (is about 3 dB down at 10kHz). Since this is totally due to diffraction then it represents the pressure build up on the face of a 1//2" cylinder and should be similar for any mic of that dimension.
Regards,
David
I haven't fully read the Buck paper but it looks like very good info. Having the mic close to the compression driver exit seems best.
B&K always published response curves for their mics with 0 degree incidence and pressure response (at least for the free field mics. Pressure response is also what the electrostatic actuators/calibrators would measure). From their data it looks like a half inch mic that is free field flat at 0 degrees will have a pressure response that rolls down gradually from around 10kHz (is about 3 dB down at 10kHz). Since this is totally due to diffraction then it represents the pressure build up on the face of a 1//2" cylinder and should be similar for any mic of that dimension.
Regards,
David
Sorry to be slow. Busy as heck at work.
I do have the Marshall Buck paper and did re-read it last night. I didn’t take the time to read the others completely but I think the Buck paper has the most information helpful make a workable PWT.
I remember be because it set me on a wild goose chase for the Scott Paper company that sold their foam manufacturing division, to a company that sold it to another, and over and over,
Barry.
Gracias guys! Sound knowledge here...
Did anyone get curious to improve the ripples?
Id think of smoothening the edges of the mic hole on the tube.
Maybe smoothen the continuity from mic to tube by plasticine shaping around its edge.
What if i make the tube double length, more space for damping?
Progressive damping from soft cottonballs, into sheep wool, into felt, into firm foam...
Cheers, josh
Did anyone get curious to improve the ripples?
Id think of smoothening the edges of the mic hole on the tube.
Maybe smoothen the continuity from mic to tube by plasticine shaping around its edge.
What if i make the tube double length, more space for damping?
Progressive damping from soft cottonballs, into sheep wool, into felt, into firm foam...
Cheers, josh
You should just build up a tube and see what it looks like.
I really doubt that you will find length related artifacts to be that bad.
http://www.jblpro.com/pub/components/2445J.pdf
Shows comparisons of a 2445 on a tube and on a horn. Lower frequency wiggles are what we are talking about. Not a big effect and will be there on every curve so you will quickly learn to ignore them.
Take a look at some of the later compression drivers here to get a sense of how horn response compares to tube response. On the early CD horns the upper range of response was a great match to the tube response (i.e. directivity was constant!)
Others here:
Vintage
Double length for the tube is similar to the box stuffing irony: double length, standing wave drops an octave, stuffing becomes less effective, double depth of treatment required, repeat...
I really doubt that you will find length related artifacts to be that bad.
http://www.jblpro.com/pub/components/2445J.pdf
Shows comparisons of a 2445 on a tube and on a horn. Lower frequency wiggles are what we are talking about. Not a big effect and will be there on every curve so you will quickly learn to ignore them.
Take a look at some of the later compression drivers here to get a sense of how horn response compares to tube response. On the early CD horns the upper range of response was a great match to the tube response (i.e. directivity was constant!)
Others here:
Vintage
Double length for the tube is similar to the box stuffing irony: double length, standing wave drops an octave, stuffing becomes less effective, double depth of treatment required, repeat...
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