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Picked up a handful of electret microphones from a local component store, and store owner claims them to be generic. He calls all components generic for which there is no data-sheet or specific part number.

Wondering, if there is any way to determine if these are uni-directional mics, or omni-directional ones ? Doing some search to see if there are any distinguishing physical characteristics, I came across pictures almost the same, just that few seem to have 3 leads, and most have 2 leads.

Edit: This is an alternate approach I am trying, by replacing the electret microphone that came with an el-cheapo baby-monitor, which is definitely omni-directional, and currently results in pretty terrible howling, when the 2-way talk feature is activated. The other approach is in-discussion (as some might have seen/noticed) in this question.

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In terms of actual directionality, you'll probably just have to test them by measuring the response to a source at different angles. Normally this would be done in an anechoic chamber, but you might be able to accomplish something by supporting the microphone on a post well off the ground in the center of a large carpeted room and walking around it with the source, or outside on a calm day. You could tie a string to the post to measure a consistent radial test distance.

Directionality will probably be somewhat different at extremes of frequency.

Since your source probably wouldn't be omni-directional, try to be consistent for example always aiming it at the microphone.

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  • \$\begingroup\$ Thanks @Chris. Seems like a nice and easy way, although it seems like something that might be a problem unless it is a sound-proof room. Would 2 feet be a good radius for the string ? \$\endgroup\$ – icarus74 Jan 3 '12 at 17:14
  • \$\begingroup\$ @icarus74 - you want to insure that any reflected path has substantially more attenuation than the direct path your are trying to measure. The solution to that tends to be either non-reflective surfaces, or great distance to any reflective ones, with either an absence of extraneous noise or filtering methods (such as measuring for a long time). 2' feels short, but it might be good to have the source closer than the distance to floor or ceiling. Don't forget you yourself can be a reflective surface. \$\endgroup\$ – Chris Stratton Jan 3 '12 at 17:22
  • \$\begingroup\$ This does seem harder than what it is worth :( but shall give this a try never-the-less, should be instructional. \$\endgroup\$ – icarus74 Jan 3 '12 at 17:36
  • \$\begingroup\$ If you need something specific, you should probably just go buy it by specification. \$\endgroup\$ – Chris Stratton Jan 3 '12 at 17:39
  • \$\begingroup\$ @icarus: A exponential horn or even just a cone over a omi-directional mic can make it directional. Of course that takes more space. Multiple omni-directional mics on a flat surface averaged together also makes a direction mic within some restrictions. That's basically the phased array approach at directionality. \$\endgroup\$ – Olin Lathrop Jan 3 '12 at 18:14
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It is relatively easy to tell if an electret mic is a cardioid (directional) or omnidirectional.

Look at the back side.

A cardioid microphone has holes in the back of it.

Cardioid:

enter image description here

An omnidirectional mic doesn't have holes.

Omnidirectional:

enter image description here

The cardioid might have just one hole instead of several.
The little dimples on the omindirectional mic don't go into the microphone, they are vias connecting the top and bottom of the circuit board.

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There is no such thing as a "generic" electret mic. The store owner is just being lazy or got a box of parts that fell off a truck and doesn't know the model. While there is some commonality between electret mics, the voltage and impedance they want for best operation varies. The allowable range and recommended values are of couse in the datasheet. Electrets are cheap and available from many sources with the right documentation, so I would leave these on the shelf.

Since you've already bought them, you'll have to experiment. Start out with 10 kΩ to 3V and see what happens. This is unlikely to damage anything. If that drops more than a volt, use a lower resistor. I've seen some specified for 2 kΩ recommended operation. 10 kΩ is actually high, but it makes sense to start with conservative values and work your way down. Higher values may give you a bit more voltage out but also cut off high frequencies more.

Continuing:

My internet connection went down while typing this answer. When the submit seemed to hang I managed to copy the text in the edit window before it went away altogether. When things came back up, it appeared to have saved what I typed, so I just re-submitted it. I see now that the last two paragraphs didn't make it. Fortunately those were included in my manual save:

As for directionality, there is no way to know. Just try it.

A basic electret is a two-leaded device. Some have a FET or something else built in, so have a power, ground, and output lead. Those can vary considerably in how you are supposed to handle them. If you have 3 leaded versions I would go back to the store and rerturn them unless the owner can supply the proper datasheet. It sounds shady and a bad place to buy parts anyway.

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    \$\begingroup\$ Sounds like a perfectly normal surplus situation to me - sometimes you get data sheets, sometimes you don't - that factor is part of the deal when you shop such a source. \$\endgroup\$ – Chris Stratton Jan 3 '12 at 16:08
  • \$\begingroup\$ @Chris: Right, which is why such things are often not bargains. Electrets are cheap and available to begin with, so there is little advantage to getting them a little cheaper. For well under $1 with a datasheet, what's the point? \$\endgroup\$ – Olin Lathrop Jan 3 '12 at 16:26
  • \$\begingroup\$ Thanks @Olin. I was actually in a bit of hurry, trying to replace the mic of a project which I've partly torn down. Indeed, these were pretty cheap, so if they do not turn out to be directional, will probably return those and order online. \$\endgroup\$ – icarus74 Jan 3 '12 at 17:17
  • \$\begingroup\$ @OlinLathrop - Glad you were able to copy the text out of the edit window, but as an FYI, the site saves drafts for you. That link will give you more details, but, to summarize, the site saves what you've entered every 45 seconds. This is indicated by the words "draft saved" at the lower left: i.stack.imgur.com/pblPY.png \$\endgroup\$ – Kevin Vermeer Jan 3 '12 at 19:24
  • \$\begingroup\$ @Kevin: The site did save part of what I wrote. Apparently my internet connection went down while I was typing, so of course it couldn't save anything after that. \$\endgroup\$ – Olin Lathrop Jan 3 '12 at 20:02

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