How good an acoustic impulse is an exploding wire?

[scott wurcer]

I was surprised that I could not find a reference on the web. I was thinking of a quickway to check out a microphone for flatness without using any expensive gear. Discharging a big cap through some thin wire was the idea.

[Mr Evil]

Clapping is often used as an easy way to generate an impulse for quick tests of audio gear.

[TwoSpoons]

An exploding wire can in fact be used as a detonator for explosives, so it must be capable of producing a pretty good shockwave.

[tiroth]

If there is arcing, won't that screw things up? Maybe a single spark through a spark gap?

I would still think the LF response would be quite poor.

[brandzip]

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.

[GRollins]

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

[Charles Hansen]

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/electrics...stexplode.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/electri.../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

[Jonathan Bright]

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]

Thanks for all the replies. It seems the peak SPL might be a problem with the wire by overloading the mic. Maybe a small spark gap would be a better choice after all. I just got my old Nakamichi mic up and running after 20 years and wanted to see if the electret has degraded with age.

[pinkmouse]

Someone once mentioned high pressure compressed air as a pretty good source of pink noise. Anyone got a calibrated mic they could check it with?