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Natural Science Forum / Physics / Acoustics / November 2005Request for Acoustic Feedback Information (Larsen Effect)
Does anyone know of any good articles/papers/books with good information on the physics behind acoustic feedback (The Larsen Effect)? I would specifically be interested in feedback in relation of guitars to amplifiers. Any help would be appreciated, thanks in advance. Stew :o) >Does anyone know of any good articles/papers/books with good >information on the physics behind acoustic feedback (The Larsen >Effect)? I would specifically be interested in feedback in relation of >guitars to amplifiers. Any help would be appreciated, thanks in >advance. >Stew :o) The physics is very simple. Net phase shift either zero or an integer multiple of 360 degrees, and loop gain greater than unity. That is all there is to it. I'm not sure why this merits a special name, feedback is just feedback and it has been known about for many years. If you want to go deeper, look at the individual phase and amplitude contributions of strings, body, pickup, amplifier, speaker and the distance between the guitar and cabinet. Add all of these together over the appropriate range of frequencies, and you can predict what sort of feedback howl you can expect. Deeper still - every amplifier will have slightly different behaviour under limiting conditions, which will happen with feedback, unless it is the speaker unit that limits, in which case you need to analyse the mechanics of the driver. You can take this as far as you like, really. d Pearce Consulting http://www.pearce.uk.com Stew, > I would specifically be interested in feedback in relation of guitars to amplifiers. < Besides the good advice Don already gave you, and I'll add this: Guitars have three basic feedback mechanisms. An acoustic guitar with a piezo or other acoustic pickup will feed back the same as a microphone will. An electric guitar can feed back "musically" as the string vibrations are reinforced by the amplified sound. But electric guitars can also feedback "non-musically" if the windings are loose and they vibrate mechanically. This gives an out of control squealing sound that does not stop when you damp the strings with your hand. 30+ years ago I used to dunk guitar pickups in a can of heated candle wax to fix this. Today I believe pickups are manufactured that way to ensure the windings don't vibrate. --Ethan > Stew, > [quoted text clipped - 16 lines] > > --Ethan Treating the windings with Superglue works well too, and the Superglue wicks into the coils better than wax does. Porky, > the Superglue wicks into the coils better than wax does. < Very cool, great idea. Though I didn't have Superglue back in the '1960s! --Ethan > Treating the windings with Superglue works well too, and the Superglue > wicks into the coils better than wax does. That is an excellent idea.. I just used superglue for one or wo household jobs, and I noticed its very fluid behavior, having low viscosity when fresh.. about that of kerosene or water. The resulting mass, after setup is ver well bound together, and should make the wire wrap loading very solid, and will behave as a single mass. In my opinion that string have lower damping and provide louder and more sustained tones in comparison with unglued wrapped strings. In that situation, sustained sound emission from a single note strike will be limited only by sound radiation loss into the air sound waves and damping by the wood frame, if any. Angelo Campanella > > Treating the windings with Superglue works well too, and the Superglue > > wicks into the coils better than wax does. [quoted text clipped - 11 lines] > > Angelo Campanella Hmmm, I'm lost. I thought he was talking about supergluing the coils in the pickup rather than the wrapped strings. Later... Ron Capik -- Ron, > I thought he was talking about supergluing the coils in the pickup rather than the wrapped strings. < Yes, we were talking about binding the pickup coil windings, not the string windings. Maybe the latter is useful too, but on all the guitar strings I've seen the winding is already bound pretty tightly to the center core. --Ethan > Ron, > [quoted text clipped - 8 lines] > > --Ethan The problem with using super glue on the wound strings is that, though super glue has very high tensile strength, its shear strength is quite low. String vibration would shear the glue bond and it would turn to powder and fall off the string. Porky, > String vibration would shear the glue bond and it would turn to powder and fall off the string. < Yeah, that sounds about right. -Ethan >>Yes, we were talking about binding the pickup coil windings, not the string >>windings. Maybe the latter is useful too, but on all the guitar strings [quoted text clipped - 3 lines] > String vibration would shear the glue bond and it would turn to powder and > fall off the string. Super glue if reasonably tough. Most of the string, except for where the pick plucks it, will not experience vibration and shock sufficient to break the adhesion or break down the plastic structure. Whether the increased mass and reduced damping, if any, are musically significant is another story. Perhaps it is best advised to be used where coil wrapping can become loose or undone. Angelo Campanella. >>>Yes, we were talking about binding the pickup coil windings, not the >>>string [quoted text clipped - 14 lines] > > Angelo Campanella. I was going to change strings on my guitar anyway, so I tried it on the wound E string. The string sounded dead, kind of like old flat-wound strings, and the glue quickly broke down and started flaking off Must sound like hell on a drop B Tremonti for death metal. :-) > >>>Yes, we were talking about binding the pickup coil windings, not the > >>>string [quoted text clipped - 18 lines] > wound E string. The string sounded dead, kind of like old flat-wound > strings, and the glue quickly broke down and started flaking off |
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