I learned that from Toole's book, if you have access to it there's an interesting illustration of the weird directional characteristics of violins in Fig. 3.3.
Here's the paper Toole references, but they want $25 for it:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN00005100006B001994000001&idtype=cvips&gifs=yes&ref=no
Also an AES paper called "Tonal Effects of Classical Music Microphone Placement":
http://www.aes.org/e-lib/browse.cfm?elib=11768
Here's the paper Toole references, but they want $25 for it:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN00005100006B001994000001&idtype=cvips&gifs=yes&ref=no
Also an AES paper called "Tonal Effects of Classical Music Microphone Placement":
http://www.aes.org/e-lib/browse.cfm?elib=11768