Re Toby's question on "feedback" I'm going to venture an opinion of this from an engineering view point [ I am a retired member of that profession]
All "structures" have a natural vibrating frequency. They can be very destructive in such things as buildings and bridges or just troublesome in musical instruments. The most famous destructive example is the the Tacoma Narrows Bridge [Washington State?] where the bridge self distracted because the geographic area, prevailing wind speed, etc., combined to produce a frequency which was the same as that of the bridge. It is quite dramatic to see [its all on film] six foot "I" beams twist like a ribbon in the wind. Musical instruments like other structures also have a natural frequency at which the wood vibrates. An acoustic instrument, I think, would be more susceptible to this than would say a solid body guitar because the latter's natural frequency would tend to be out of range of much or most of the musical frequency spectrum than would the former. When the instruments natural frequency is amplified it "feeds back" that frequency to the instrument causing it to vibrate more and more and would self destruct like the Tacoma Narrows Bridge were there sufficient sustained energy. In recognition of this problem some amps are equipped with a "notch filter" which allows you to find your instruments natural frequency and then to suppress it. Usually this can be done without too adversely affecting the overall sound. Another helpful feature in some amps is phase reversal which doesn't suppress the frequency but causes them to be unsynchronized. Re your friend's modifying his instrument. Any modification will change its natural frequency. That will eliminate the troublesome frequency but not necessarily the problem. The natural frequency will shift to another frequency which could be just as much of a problem. It would also change the overall sound of the instrument, particularily a violin which has so many intense harmonics all of which would be influenced by this. I hope this is helpful, Toby Alexander Mac Donald Posted to Scots-L - The Traditional Scottish Music & Culture List - To subscribe/unsubscribe, point your browser to: http://www.tullochgorm.com/lists.html
