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| Ultrasonic iPod Mic (Linux) |
| Written by Scott Mitchell | ||||
| Friday, 01 September 2006 | ||||
 Combine this cheap and easy hack with iPodLinux and get a wide-frequency mic for your iPod. With Linux installed the iPod will record at sample rates up to 96 KHz. This gives the iPod a theoretical frequency response of about 48 KHz (half the sample rate). This is well into the ultrasonic range; the average human can't perceive much above 18 KHz (there's a lot of discussing on the web regarding how these high frequency sounds affect the listening environment but that's a little beside the point here, this mod isn't going to provide you with an audiophile experience). This mic is not particularly loud or very good at picking up detailed sound but it will record into the ultrasonic range. First, load iPodLinux onto your iPod. This is a really simple process, it doesn’t affect the iPod’s original operation and it’s free! I got my Linux installer from here (http://ipodlinuxinstl.sourceforge.net/), naturally it’s for a Mac (there are PC ones if you’re that way inclined).  Next you’ll need a standard 3.5” audio connector and a piezo buzzer. I tried a couple of different buzzers, from my experience the bigger ones are better, but be careful a really large one may have the potential to damage your iPod. This is the one I used.   The buzzer is the sort of thing you find in cheap alarms, I bought mine new from the local electronics store for $3.25, obviously it’s designed to produce sound but like many things it also works in reverse and produces small amounts of electricity from noise. Connect the buzzer’s red lead to the audio jack’s centre terminal and the black lead to the plugs ground (the big outer connector); the third audio terminal (normally used for the right channel) should be left unconnected.   Now seal it up, plug it in, and boot into iPodLinux. Go to the ‘Recording’ menu and set the ‘Sample Rate’ to 96kHz.  Now go and record something.  If you’re looking for ultrasonic noise around the home then fluorescent lights are always a safe bet. An interesting feature of this mod is that the mic will also act as a speaker (this is after all the buzzers intended function). You may have to hold the mic to your ear; it will be quite soft. Off course, unless you perform some sort of pitch shift on the recording you're still not going to hear anything ultrasonic noise; currently there is not Linux application to do a shift pitch on the iPod but once you transfer the file to your computer there’s a whole bunch of options. The following image is a sonogram of my lamp recording.  You can see the ultrasonic spike when I turn the light on and the operating frequency (about 26kHz) that continues until I turn the light off at about the 10 sec mark. One thing to note, if you record at 96 KHz and then drag this file onto GarageBand the frequency rate is halved (to 44.1KHz) and the file is automatically stretched to twice its length. I think this is a bug in GarageBand and not a problem with the way the iPod is recording the sound file. What I really want to find out is will this thing record bats? 
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