Microphone to track down inductor noise? - Page 1
Forum section: electronics / testgear

author: Pack34
author_rank: Frequent Contributor
timestamp: April 03, 2018, 09:50:06 pm
content: I have a legacy board that's being phased out that has a whine on every dozen or so boards. There are a dozen inductors on this board and I just can't tell which one is making the noise when putting it up to my ear.Is there a decent microphone that someone could suggest that I can wave over the board and watch the decibels go up and down on my computer?

author: Performa01
author_rank: Super Contributor
timestamp: April 03, 2018, 10:03:52 pm
content: There's a simple(r) solution. Just use a piece of (preferably rubber) insulating sleeve, about 8-10mm in diameter. Put one end (in)to your ear and use the other end for "probing" the inductors.

author: dcarr
author_rank: Regular Contributor
timestamp: April 03, 2018, 10:24:05 pm
content: You could also try a mechanic's stethoscope.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2018, 10:29:51 pm
content: Or even a long screwdriver against your ear will carry to sound well.

author: DaJMasta
author_rank: Super Contributor
timestamp: April 03, 2018, 11:59:49 pm
content: You can also just apply a little mechanical pressure and see if it changes/stops - you don't necessarily need to tell by where the sound is coming from if you can figure out what you can poke to make it stop.Something like a headset mic or one of those long computer mics would probably do what you're describing, though. If you wanted to be fancy, a lavalier mic and a dowel or something.

author: CopperCone
author_rank: Super Contributor
timestamp: April 04, 2018, 04:20:05 pm
content: I built something like this. I got atiny smd microphone, listed as ultrasonic on digikey (however later reading the datasheet i got confused, but i think the one i gothad a good frq responce to 60khz.It had a built in unity gain amplifier and took 5v power. It was aabout 2x3x1mm in dimensions.I cut a tetris shaped piece into the end of a shiskabob skewer/dowel, to fit the microphone, a smd ferrite bead and a100nf capacitor all against each other using superglue. I soldered the components together using 30 gauge or smaller wire.I routed this wire approximately 3.5 inches down the shiskabob skewer, where it was connected to a larger 22uf tantalum cap and 100nf cap for bypassing, the intent was to make the probe bit fairly thin so it can be inserted in small holes. May have been series lc filter on power and ferrite 1nf on signal, forgotI used some zip ties to attach a heavy shielded 4 conductor cable to the region where the tantalum cap was.I soldered wires for the 5v, power gnd and signal.I then gave everything a thin coat of epoxy to insulate it.After the epoxy dried i covered everything with copper tape up to the braid of the wire. The body of the smd microphone is aluminum, i made sure to clean the glue off the copper tape on the very tip and adhese the tape to the microphone body using ultra conductive silver epoxy. I made sure the microphone hole was not covered. I soldered the tape to the braid.I then gave it another coat of epoxy to eliminate the possibility of it shorting anything out. I also put heat shrink over the heavy cable that was zip tied to the stick and filled the void with epoxy to make a fairly robust handleI put a bnc connector on the signal conductor pair, and grounded the shield to the signal ground. It is intended to be used with my wavetek adjustable analog filter that goes to 10MHz or an oscilloscope.The 5v power is provided by a floating battery pack to prevent ground loop from forming in the shield loop. I did not want a earth ground loop antenna.It is thin enough to insert into your ear and touch your eardrum lol. Good wax pick barffffI want to make another one thats better engineered using a thin brass tube rather then wires routed on top of a dowel covered with super hlue. I wanted to make the next version the same dimensions as the microphone but extended in the long axis. The version i have now looks shitty.My only concern about it for your application is that the magnetic fields may be high near the bodies of the inductors and a high h field may appear on an oscilloscope as mechanical noise despite being nonmechanical, so you may want something like mu metal shielding, or to use a extension tube as mentioned in this thread to isolate the deviceI want to get/make an audio/ultrasonic power amplifier so i can pump some magnetic fields out of an inductor or helmholtz coil to test the microphone probes suceptability to magnetic fields. I feel pretty good about the efield shielding.I do believe however that you can get a better frequency responce from these tiny mics by drilling their hole to be a bit bigger. You can probobly enlarge the thin material by drilling larger and larger holes with sharp carbide drills despite how thin the body is

author: BradC
author_rank: Super Contributor
timestamp: April 04, 2018, 04:42:19 pm
content: Quote from: Performa01 on April 03, 2018, 10:03:52 pmEven vinyl tube (available at hardware stores) works very well in this situation. I've used considerable lengths of it to narrow down a noise.I based it on the old tube-based entertainment headsets used on the 747 in the 80's, and it has served me well ever since. They had relatively comfortable foam ear-pieces though, I just shove the tube in my ear.