sound performances with Sonar or Radar

[Philip Curtis <pcurt@xxxxxxxxxxx>]

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Hi,
 
I'm doing research on the use of sonar or radar in sound installations/performances. Specifically, I'm interested in a piece of David Tudor's, entitled "Line and Reflections," or "9 Lines Reflected," which used radar.  But I've also read a little about a piece of Alvin Lucier called "Vespers," and a Lucinda Childs piece called "Vehicle," both of which were older pieces that used sonar, and which Tudor had some connection to.
 
What I want to know is how the sonar or radar were used to control or produce audio. In Tudor's piece, he was sending radar to mobile sculptures that moved and created a doppler effect (I assume a very tiny doppler effect) and using the returning radar in some way. I've found this bit in an interview with Tudor, but find it a little lacking in detail: 
 
TH: How does the radar function in this piece? 
DT: It transmits the frequency out into space. The same device also receives the frequencies back and any movement in the path between the transmission and the reception causes a shift in the frequency and it's as simple as that. These days, for instance, if you protect your house, you can also use radar or you can use ultrasound, it's the same principle: you get a signal back which can trigger an alarm, but in my case I take audio frequencies from the devices and those I use to trigger modulating equipment. In a sense it's very much like the principle of altering the output to disguise it. 
 
Does anyone get this? It seems that he is using differences in the frequecies being transmitted and recieved to modulate something (what: pitch? amplitude?). How would you do that exactly? 
 
Feel free to contact me off list if you have an idea. Or if you know about other pieces and how they work.
 
Thanks,
Phil Curtis

 

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