Exploding wire
The impulse frequency content from an exploding wire depends on the current draw verses wire cross-section verses acoustic load (amoung other things).
You are better off with a spark...but beware, nature doesn't really believe in 'pink' noise. The low frequency content will be a Fourier component, and will be almost non-existent.
The impulse frequency content from an exploding wire depends on the current draw verses wire cross-section verses acoustic load (amoung other things).
You are better off with a spark...but beware, nature doesn't really believe in 'pink' noise. The low frequency content will be a Fourier component, and will be almost non-existent.
Back in the '70s, KEF reprinted a bunch of older papers about various types of speaker enclosures. In one on transmission lines (I think), the guy used exploding wires to test for resonances in his cabinet designs. As I recall, it worked very well. If you're interested, I can try to dredge up the reprint and get the name(s). Perhaps Google will be more helpful if its pump is primed first.
Grey
Grey
Scott, you're a wild man! What a great topic. I did some searching and found all kinds of interesting things, although not the succinct answer you'd like. Here it is in random order:
1) A high speed movie (1000 fps) of an exploding wire can be found at:
http://www.redremote.co.uk/electricstuff/fastexplode.html
Please also be sure to read the commentary on the still frames that are extracted from the movie, as they are very helpful interpreting things.
2) Some rough parameters regarding what sort of capacitor energy is required can be found at:
http://web.mit.edu/Edgerton/www/ExplodingWire.html
They also point out that the bulk of the energy comes in the form of a plasma discharge, similar to lightning.
3) For a measly $24 you can purchase a study of exploding wires that points out, "With the exception of a nuclear explosion itself, the highest energy density that can be obtained at a given point for a short time results from a wire explosion when a sudden large pulse of electrical current is passed through it."
http://www.stormingmedia.us/71/7195/A719593.html
It's not clear if this paper has the information you're looking for, but it may.
4) Some interesting practical aspects of your project can be studied at:
http://home.earthlink.net/~jimlux/artlight.htm
5) Downloadable movie clips of exploding wires with soundtracks can be found at:
http://homepage.ntlworld.com/electricstuff/uktesw99.html
6) Look at the first reference here:
http://www.eeel.nist.gov/811/811_pubs_hv.html
The spectra referred to in the paper's title is almost certainly optical and not acoustic. However, please note that there is both an annual conference on exploding wires as well as a book on exploding wires!
7) While the following site is probably more entertaining than useful, it does point out that, "the sound from lightning may not simply
be from thermal transient air expansion, but from unexplored plasma dynamics which produce shock waves via motor effects. Perhaps this is the source of the intense sound from the [exploding wire] discharge.
http://www.amasci.com/amateur/capexpt.html
8) Some interesting excerpts and references to the actual mechanism of the expansion created by the exploding wire can be found at:
http://www.energyscience.org.uk/notes/rn9715.htm
(It may be helpful to use the "Find" function in your browser to search on "exploding".)
9) One experimenter describes "a 350v 20,000uf (1200 joule) discharge system" controlled with "110 Amp SCRs".
http://www.imagineeringezine.com/e-zine/cproj-list.html
Although sparse on details, it sounds like you can contact the constructor for further information.
10) There is a professor at the University of Florida that can almost certainly help you out. His home page is at:
http://plaza.ufl.edu/rakov/index.htm
He has a bibliography page that for example lists a paper, "Acoustical spectra of electrical discharges".
http://plaza.ufl.edu/rakov/LPE/Ch11.htm
11) Good food for thought can be found on the last question of the test posted here:
http://www.its.caltech.edu/~oom/handouts/final03.pdf
You can figure out the voltage required to generate a super-sonic shockwave, which is probably what you want.
12) It may also be helpful to understand the traditional calibration techniques used for condensor microphones. (I don't know what type of microphone you are testing.)
a) An electrostatic actuator uses external electric fields to move the diaphgram. Correction curves must then be applied to account for the acoustic aberrations caused by the microphone itself, as well as its protective grid (removed for the measurement). Of course, it leaves you wondering exactly how they came up with the correction curve...
b) Reciprocity techniques use two identical microphones closely coupled, with one acting as a speaker and the other as a microphone.
c) Some calibrators use microphones with a known frequency response in a feedback loop to control the output level of the actuating transducer.
Have fun!
Charles Hansen
1) A high speed movie (1000 fps) of an exploding wire can be found at:
http://www.redremote.co.uk/electricstuff/fastexplode.html
Please also be sure to read the commentary on the still frames that are extracted from the movie, as they are very helpful interpreting things.
2) Some rough parameters regarding what sort of capacitor energy is required can be found at:
http://web.mit.edu/Edgerton/www/ExplodingWire.html
They also point out that the bulk of the energy comes in the form of a plasma discharge, similar to lightning.
3) For a measly $24 you can purchase a study of exploding wires that points out, "With the exception of a nuclear explosion itself, the highest energy density that can be obtained at a given point for a short time results from a wire explosion when a sudden large pulse of electrical current is passed through it."
http://www.stormingmedia.us/71/7195/A719593.html
It's not clear if this paper has the information you're looking for, but it may.
4) Some interesting practical aspects of your project can be studied at:
http://home.earthlink.net/~jimlux/artlight.htm
5) Downloadable movie clips of exploding wires with soundtracks can be found at:
http://homepage.ntlworld.com/electricstuff/uktesw99.html
6) Look at the first reference here:
http://www.eeel.nist.gov/811/811_pubs_hv.html
The spectra referred to in the paper's title is almost certainly optical and not acoustic. However, please note that there is both an annual conference on exploding wires as well as a book on exploding wires!
7) While the following site is probably more entertaining than useful, it does point out that, "the sound from lightning may not simply
be from thermal transient air expansion, but from unexplored plasma dynamics which produce shock waves via motor effects. Perhaps this is the source of the intense sound from the [exploding wire] discharge.
http://www.amasci.com/amateur/capexpt.html
8) Some interesting excerpts and references to the actual mechanism of the expansion created by the exploding wire can be found at:
http://www.energyscience.org.uk/notes/rn9715.htm
(It may be helpful to use the "Find" function in your browser to search on "exploding".)
9) One experimenter describes "a 350v 20,000uf (1200 joule) discharge system" controlled with "110 Amp SCRs".
http://www.imagineeringezine.com/e-zine/cproj-list.html
Although sparse on details, it sounds like you can contact the constructor for further information.
10) There is a professor at the University of Florida that can almost certainly help you out. His home page is at:
http://plaza.ufl.edu/rakov/index.htm
He has a bibliography page that for example lists a paper, "Acoustical spectra of electrical discharges".
http://plaza.ufl.edu/rakov/LPE/Ch11.htm
11) Good food for thought can be found on the last question of the test posted here:
http://www.its.caltech.edu/~oom/handouts/final03.pdf
You can figure out the voltage required to generate a super-sonic shockwave, which is probably what you want.
12) It may also be helpful to understand the traditional calibration techniques used for condensor microphones. (I don't know what type of microphone you are testing.)
a) An electrostatic actuator uses external electric fields to move the diaphgram. Correction curves must then be applied to account for the acoustic aberrations caused by the microphone itself, as well as its protective grid (removed for the measurement). Of course, it leaves you wondering exactly how they came up with the correction curve...
b) Reciprocity techniques use two identical microphones closely coupled, with one acting as a speaker and the other as a microphone.
c) Some calibrators use microphones with a known frequency response in a feedback loop to control the output level of the actuating transducer.
Have fun!
Charles Hansen
Grey is right about his memory of KEF projects. There is a debate about what constitutes a tansmission line or labirynth (I can't spell) but the recent interest (30-40yrs)was started by the work of Arthur Bailey who published a design for a T/line using a KEF B139 and a Celestion HF1300. It was in the old "Wireless World" and must have been originally produced in the late 1960's. He makes reference to using an exploding wire to do impulse testing. I think they removed the bass unit and put a board over the speaker appeture with the wire on the inside. Discharged a capacitor thru' it. Worked a treat apparently. For the trivia buffs, I think the model was marketed commercially by Radford.
scott wurcer said:
In the olden days electrets were made from paraffin and wouldn't hold a charge very well. Modern electrets will hold a charge quite well (although I would imagine that elevated temperatures would tend to discharge things).
Even the best electrets are not as stable as externally polarized microphones, which is why the externally polarized ones are used for critical measurements. I think a good electret may lose a dB or two of sensitivity after 20 years if not abused.
You probably don't even care so much about a few dB of sensitivity, but rather large changes in the frequency response. Even if the electret discharged significantly, it would only affect the output level and not the frequency response. The only thing that would affect the frequency response is if the tension of the diaphragm changed. Again, I don't think it would be a problem at all unless the mic were exposed to elevated temperatures.
Good luck!
update
Ok, so I tried this believe it or not , and got some very encouraging results. I took a gas grill starter from HD and connected it to two sewing needles to make a spark gap. It turns out a spark generates a doublet in the time domain so a 6dB per octave correction is needed on the sampled result. I got totally clean response plots with no averaging at all. I'll try to post some results eventually because I think that any mic could be calibrated to +- a dB or so with this technique.
Ok, so I tried this believe it or not , and got some very encouraging results. I took a gas grill starter from HD and connected it to two sewing needles to make a spark gap. It turns out a spark generates a doublet in the time domain so a 6dB per octave correction is needed on the sampled result. I got totally clean response plots with no averaging at all. I'll try to post some results eventually because I think that any mic could be calibrated to +- a dB or so with this technique.
I tried about 1 foot and 2 feet, the ignition spark is very sharp (1 sample) and occurs at exactly the right time (2ms or so ahead in the later case) so it's a great speed of sound demo too. I put the needles on the end of a pair of chopstcks pointed at each other and kept all reflecting surfaces far enough away so I can window the pulse nicely. The most annoying thing is the clicker. Results are good from 500Hz or so on up, most mics behave pretty much theoretically below that anyway. I don't have access to a lab mic like an 1/8th inch B&K so I can't reallty verify the absolute quality of the impulse (actually doublet) but it looks pretty ideal.
I found a paper on using a focused laser to create a super spark in air. It had a 180dB peak amplitude and excited nonlinear acoustic behavior but qualitively I see the same things.
This thing would also be useful to quickly get the absolute phase of your mics or setup stereo pairs just right too. Or maybe check the transmission properties of grill cloths and such, which is what the laser pulses are used for on grosser things.
I found a paper on using a focused laser to create a super spark in air. It had a 180dB peak amplitude and excited nonlinear acoustic behavior but qualitively I see the same things.
This thing would also be useful to quickly get the absolute phase of your mics or setup stereo pairs just right too. Or maybe check the transmission properties of grill cloths and such, which is what the laser pulses are used for on grosser things.
scott wurcer said:
How 'bout one of those hand-held stun guns? Far as I'm aware they all electronic which would eliminate the mechanical noise of the clicker.
Best Stun Gun has a 600,000 volt handheld on sale for just $59.95. And it has that second set of electrodes between the main electrodes that's used as a spark gap for test/demo/showing off.
Apparently sales of 600,000 and 900,000 volt stun guns are restricted in MA but I'm sure Stuart could help you get around that.
se
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