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Wind Sensor As Hot Wire Microphone

Posted: Sun Sep 07, 2014 2:49 pm
by Mikrotopia
Hi All,

I'm a sound artist and am trying to recreate a DIY Hot Wire Microphone for a WW1 anniversary project.
The idea is to create a simple device that people can put together in a workshop setting (experimenting with technology inspired by WW1 tech) and then use it to make recordings of low frequency sounds.

I know there are other probably simpler ways of detecting low frequency sound but i want to stay true to the original and have the hot wire as part of the design

I wanted to use the Modern Devices Wind Sensor as a starting point. Can anyone give me advice on whether this can be done, how to proceed or how the MD Wind Sensor could be hacked?

Here's some more information on Hot Wire Microphone's:

"The Hot-Wire Microphone

Image

The hot-wire microphone ... does not reproduce sound pressure variations electrically, but is more of a detector of sound and an indicator of its energy. Since the name does crop up from time to time, we'll describe it here for completeness. It is specifically used for low frequencies and for infrasonic signals. It was developed during the 1914-1918 war as a sound ranging device, for acoustic location of artillery to aid counterbattery fire. After the war, Tucker and Paris perfected the hot-wire microphone for infrasonic detection, publishing their results in 1921.

An example of a hot-wire microphone is shown at the right. It consists of a very fine platinum wire placed over the neck of a Helmholtz resonator and heated by a current passed through it. The wire is supported by a thin glass rod and a disc of mica. The disc is clamped between silver rings that make the contacts. When a sound wave of the resonant frequency arrives, air rushes in and out of the neck of the resonator at that frequency. This air flow cools the wire by forced convection, so its resistance decreases. The resistance decrease is easily detected by a Wheatstone bridge. The hot wire of a typical device is 6 μm in diameter, with a resistance of 350Ω and requiring about 30 mA to heat.

A Helmholtz resonator consists of a volume V and a neck of length L and cross-sectional area A. Its resonant frequency is given by the formula in the diagram, where c is the speed of sound. A 125-ml Florence flask makes a good Helmholtz resonator. I measured L = 5.5 cm and A = 1.54 cm2, which gave f = 256 Hz (the physicist's middle C). The actual resonance was an A, or 220 Hz, on the musician's scale, not far off. Without the resonator, the sensitivity of the hot-wire microphone is very low, so practical devices are all resonant. The microphone can be applied to frequencies as high as 512 Hz.

In addition to the DC change in resistance, it is also possible to detect AC variations in the hot-wire resistance. These variations are at twice the sound frequency, since the air blows alternately in and out, and the cooling does not depend on the direction of the air velocity. The hot-wire microphone is, accordingly, not applicable to speech or music. As its use in acoustic ranging indicates, it has a rather quick response. It is useful in a frequency range where other microphones are unresponsive."

Information borrowed from this website:
https://mysite.du.edu/~jcalvert/tech/microph.htm

Any help or advice on this would be great!

Re: Wind Sensor As Hot Wire Microphone

Posted: Sat Sep 27, 2014 1:28 pm
by paul
Mikrotopia,

Sorry about the really slow reply.
You probably figured out by now that the modern device wind sensor is completely inadequate for sound sensing.
The thermal time constant (you may have to Google that) makes it way too slow to detect anything but perhaps 3-4 hz, so not much fun.

It is possible to do this, but you need .0003 " platinum, tungsten or iron-nickel wire, which is really hard to come by, and is also very pricey.
That dimension is NOT 3 mils (.003") BTW but 3 tenths of a mil wire for this. You could also break various light bulbs for some finer filament.
You can't really solder to tungsten (reports vary - some people using acid flux) have reported success. Another technique people use is just
to break a fine grain of wheat light bulb, leaving the supports intact, and connect to the light bulb base. Then put enough current through the filament to heat it substantially
without burning out the filament (buy four or five bulbs for some various losses).

Since you don't really care about the DC voltage, your job is a lot easier.
After you've got the bulb (sans glass) hooked up to some wires, measure the resistance with an ohmmeter. Obtain a resistor in about the same range, which may be only 6-30 ohms, the specific value is unimportant. Hook up a capacitor say 10uf to the junction of the resistor and the hot wire. The top of the resistor goes to V+, the bottom of the hot wire goes to GND. Then it's a matter of hooking up a high enough gain op amp and seeing what you get.

If you haven't given up on the project - or our forum, write to support@ you know where and I'll send you a schematic

Paul

Hoo


It's possible you could use the rev P wind sensor as a circuit - you would need to alter the balance on the bridge - but you could put in a pot for that.
The hardest