The View Up Here

Random scribblings about kites, photography, machining, and anything else

Jammers, Wings, and Email Lists

Posted by Tom Benedict on 17/09/2015

The fake fur I ordered a while back came in last week, but I’ve had a rotten time making a usable windjammer for my SASS array. I think I know why:

Sound is a pressure wave moving through a medium. When a sound wave strikes an acoustically opaque object the velocity goes to zero, but the pressure doesn’t. It increases. It’s that increase in pressure that provides the 6db of gain that boundary mics get over conventional mics.

Wind is motion of air molecules at an average velocity. Again, when wind impacts a surface like a wall or a kite, that velocity drops. But the pressure doesn’t. It increases. A lot. This is what makes kites fly at high angles of attack. But it also means that no matter how much fuzzy fur I stick in front of the mics on my SASS, the kind of wind I need to do KAP will almost certainly overwhelm the pressure limits of the mics. RATS!

I’m still pursuing a good windjammer for my SASS because I know it’s been done and I like using it on the ground, too. But for kite aerial sound work I’m moving my focus toward another design: Curt Olson’s Wing. A couple of things about his wing design make it attractive for KAP. First, it’s a lot easier to build a windjammer for an Olson Wing than it is for an SASS. The SASS was designed for use indoors. Curt’s a field recordist, so outdoor use was part of his design work. (You can see the cages for a windjammer on his page.) Second, the design includes a big flat plate that can be faced into the wind to help protect the mics from the direct wind blast. This means only recording subjects directly below or downrange of the kite, but it’s a start.

While trying to figure out the problems I was having with the windjammer, I ran across Trevor Owen deClercq’s master’s thesis titled “A More Realistic View of Mid/Side Stereophony”. The thesis itself focuses on a stereo recording technique pioneered at E.M.I. called Mid/Side Recording, as the title suggests, but the introductory material also discusses psychoacoustics: how humans process sound. At low frequencies we use phase differences to locate sound. At higher frequencies our heads act as a partial baffle, which attenuates sound coming from the far side of the head from the ear in question. This lets us use differences in sound level to locate sound. At mid frequencies we use a combination. (deClerq states that we don’t do it particularly well, but that we do do it.)

Later on I ran across this study of boundary microphones which echoed deClercq’s introductory statements in a section on the Crown SASS and its derivatives. The mic spacing of the SASS let it pick up phase differences at low frequencies, and the foam block acts as a partial baffle at higher frequencies, which results in different sound levels reaching each of the mics, depending on the direction of the sound source. This explains why the stereo image created by the SASS works so well: it’s how we were designed to hear.

That study also touches on some of Curt Olson’s earlier spaced boundary mic designs, though I think it pre-dates his wing design. Now I’m curious if it’s possible to add a foam block to his wing design to help it behave more like an SASS without incurring the penalty of having to make a windjammer that’ll survive kite-flying wind speeds. As soon as I finish my wing and fit it out with a proper windjammer, I’ll give this a try.

In an attempt to answer this and other questions I joined two Yahoo email lists over the last week. The micbuilders list focuses on microphone design and construction. The naturerecordists list focuses more on field recording equipment and techniques. There is, needless to say, a great deal of overlap between the two. And to my delight, between the two lists I’ve run across posts by Vicki Powys and Curt Olson, both of whom I’ve mentioned several times, along with David Brinicombe, inventor of the fuzzy windjammer, Zach Poff, whose web site started me off on the whole Primo BT-EM172 based microphones, Gene from Frogloggers, the place I ordered my mics from, and Klas Strandberg, owner of Telinga Microphones, whose EM-172 based parabolics I’ve been drooling over since I dove into this whole sound thing. I’m still soaking it all in. I haven’t posted a word to either group.

So despite my utter failure at making a KAP-ready windjammer for my SASS, it’s been a pretty good week. But man that learning curve still looks like a sheer wall.

– Tom

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