Re: can sonic interferometry provide an acoustic 'image'?

Dear James,

A very good question. Interferometry, as you have probably learned in class, is the use of interfering waves from two different coherent signal sources to determine something about an object. The interference is caused by the peaks and valleys of the waves as they combine to either double in size or cancel each other out, or do something in between. Interferometry detects things of a size on the order of the wavelength of the signal source. It is truly a 3-dimensional imaging technique, in that you are measuring the distance to the object. Any source of waves can be used for interferometry, including light, sound, atomic particles, and maybe even gravity(?). The important thing to remember about interferometry is that it requires two sources that are coherent, meaning that they are operating at the same frequency and that they are synchronized. An array of "multiple spaced microphones" alone cannot provide interferometry, as a coherent source of signal is needed. Be sure not to confuse interferometry with acoustic imaging, which is used in SONAR and in medical ultrasound, as well as other applications and does, indeed, use such arrays of microphones. I am not aware of any acoustic interferometry systems being used for imaging, but they certainly may exist. I know that some new techniques in medical ultrasound operate in 3D, but I’m not sure if they are based on interferometry. The key to interferometry is something called the "interferometric baseline", which is the distance between the two signal sources (in this case speakers). The baseline must be large enough to achieve the amount of resolution/accuracy desired in your measurement. You might try finding the equations for determining the size of the baseline and then calculating it for a given frequency of sound to see what resolutions are feasible. I would encourage you to look into this since I found very little information in my short search, but would expect there may be applications for it. It would certainly make a fine science project! I found one web site that discusses the possibility of using acoustic interferometry for measuring the ocean floor at:

http://itri.loyola.edu/subseafe/c2_poi.htm.

I would expect that if this is feasible, then the Navy is probably using it, but I couldn’t find any references to such. A couple of other sites with minor references are at:

www.lanl.gov/quarterly/q_fall02/sound.shtml

http://spot.colorado.edu/~spetzler/dac.html (link defcunt as of 8/23/2006)

http://www.keelynet.com/interact/arc_1_98-7_98/00001762.htm.

Regards,

Todd Jamison