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Auditory Epiphany

Posted by Tom Benedict on 24/07/2015

Earlier in the week I went to pick my daughter up from class, only to find I’d arrived an hour too early. I didn’t have anywhere else to be, so I grabbed my recording gear and proceeded to play. I tested my hot glue filled mono mic against the newer designs with the screw-on backplates (no difference). I tested them against the built-ins to see if the lack of bass response I was getting in my prototype pseudo-SASS array was inherent in the mic body design, or if it came from the SASS (it came from the SASS). Then I just clipped my lavalier mics up in a tree and kicked back to record various bugs, frogs, and birds until my daughter’s class ended.

When I got home I loaded the last track into Audacity to see what I could see. Or hear, rather. I switched it over to spectral view – what would be considered a sideways waterfall view from a radio standpoint – and looked and listened. That’s when the epiphany took hold.

While I was making the recording, I could tell the audio was dominated by a bunch of crickets. What I saw in Audacity was that there were three species of cricket, all going on slightly different frequencies. I could hear them start and stop their call in the audio, identify which one it was from where it appeared in stereo space, and see which frequency it was calling on in the display. WOW!

Then I went back and listened to the birds. Then the frogs. That’s when I saw the signature of the coqui frog.

Coqui Signature

Coqui frogs are endemic to Puerto Rico. They were introduced into the Hawaii ecosystem several years ago, and have thrived ever since. From what I understand, coquis are a point of pride in Puerto Rico similar to the mockingbird in Texas. But in Hawaii they’re considered a pest. In the densely populated areas on the wet side of the islands, their nighttime calls can be deafening.

This stretch of audio has very few bird calls, but you can see the crickets around 6000Hz. I think only two species are going in this plot. The frogs show up between about 1500Hz and 4000Hz. The coqui frogs show up as a pair of comma/apostrophe marks, the first around 1500Hz, followed by a second that starts around 2500Hz and finishes at 3000Hz.

All of this, of course, is old hat to an amphibian biologist, just as my entire epiphany is old hat to anyone who’s been doing sound for any amount of time. But to me both of these were new.

I’m enjoying this foray into sound. I can’t wait to finish the other ground-based experiments I’m doing and put this to the test in the air.

– Tom


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