Need a microphone that can survive high temperatures (~500F)

August 5, 2009 7:12 PM

I need to amplify the sound of a carbon arc in a WW2 era searchlight. The mic will be inside the drum of the searchlight, about 25" away from the arc. The temperatures at that distance should be 500F or less. Are there any microphones that will operate happily at that kind of temperature?

This is for an art project! I plan to plug it into a standard PA system, and would prefer not to have to supply phantom power.

posted by gribbly to science & nature (9 comments total)

Any reason you can't just synthesize the sound?

There are all sorts of incredibly programmable soft synths available for free. My favorite is ZynAddSubFX.
posted by Netzapper at 7:17 PM on August 5

Why does the mic need to be inside? Use a parabolic dish and grab the sound from a safe distance.
posted by seanmpuckett at 7:22 PM on August 5

I don't think anything with a membrane would withstand that temperature for long. I know there are microphones rated for high temperatures, but they are mostly very specialized toys for industrial use and probably cost lots of money.
I'd start by looking into piezoelectric transducers, maybe mounted (they pick up sound/vibration by contact) on a cold (-ish) part of the searchlight (most soldering tin/lead alloys melt around 350-400F). The simpler ones should set you back just a few dollars apiece and are basically a round metal plate with a ceramic disk and a couple wires coming out, like this. The two pieces are glued/mounted together in various ways, and the glue could probably fail at high temperature, but it might still be worth a shot.
posted by _dario at 9:38 PM on August 5

Could you use a laser microphone and target a suitable part of the light? (using an angle that minimises interference from the emitted light and maybe adding a filter)
posted by malevolent at 3:40 AM on August 6

The arc is inside a glass casing, right? The air/plasma inside the casing will be loud, but the casing itself might not vibrate much, which means that parabolic or laser mics pointed at it will not necessarily give you much signal, but it will be a fairly realistic representation of what someone standing next to the light might hear.

Assuming you have the light and can modify it and what you really want to hear is the hissing of the arc, you could put a small metal tube into the casing, facing the arc and leading out of the casing for a distance of a couple of feet, then seal the mic onto the end of the tube. As long as the tube is long enough and doesn't conduct heat very well, the mic will be safe and the sound will carry quite well down the tube. I'd go with half-inch copper plumbing pipe (commonly available, cheap in a short length like that) and put a 90 degree bend in it somewhere so the mic doesn't get radiant heat from the arc. If the mic end is still too hot, solder some big copper fins onto the tube to dissipate heat from it.
posted by polyglot at 6:00 AM on August 6

And by "seal the mic onto the end", I mean you glue a half-inch condenser element into the end of the copper pipe.
posted by polyglot at 6:01 AM on August 6

This might work well, since it can go up to 300 C, but the website doesn't say where one can purchase it. I'm guessing that it it is pretty expensive.
posted by double block and bleed at 6:10 AM on August 6

You could also put a contact mic on the housing. Or a guitar pickup. I used those in a band with a percussionist who played on gas tanks, heating ducts, etc. Worked like a charm.
posted by jdfan at 6:53 AM on August 6

Thanks for the answers everyone. The one thing I forgot to mention is that the searchlight will be moving (pivoting in place), so the arrangement needs to account for that too. Might make the laser/parabolic options tough.

@Netzapper - good idea, but it would go against the spirit of the project. We want to amplify the arc itself.

@fdfan - guitar pickup is an interesting idea. I'll look into that.
posted by gribbly at 8:34 AM on August 6