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Surface Acoustic Waves Explained

What are Surface Acoustic Waves?     

     "Surface Acoustic Waves" ("SAWS") are minute physical wave deformations (tiny "ripples") on surfaces which behave much like waves on an ocean or lake.  The biggest difference between SAWS and ocean waves is their extremely small size and weak energy, at least on musical instruments.  (But certainly not earthquake waves which are also surface waves.)

     Surface acoustic waves on musical instruments are simply too small to be seen except for the trace of their maximum/minimum positions of their "standing waves" on strings.  You can see them between the instrument's "bridge" and the instrument's "nut", (or the player's tinger tip position, or "fret").  This trace is usually visible in good lighting when viewed from the side because the string's vertical velocity is zero at the moment it is changing directions at the top and bottom of its trace.  

     Surface acoustic waves also cannot be felt except for very low frequencies of standing waves on surfaces of the instrument which the player feels as "vibrations".  (A person in a boat with his or her eyes closed will also feel waves as vibrations, but with eyes open will see the row of waves passing beneath the boat and moving on into the distance, and so will understand what has actually occirred.) 

     Surface acoustic waves also cannot be heard since our ears can hear only waves of pressure in air, not wave deformations on surfaces.  

     Given these difficulties of seeing, feeling and hearing surface acoustic waves, it is not surprising that many musicians, musical instrument manufacturers and craftsmen have never even heard of surface acoustic waves, much less know of their potential to easily improve the volume and sound quality of musical instruments.

     Fortunately, we now know from years of experiments with banjos, acoustic guitars, violins and mandolins, that surface acoustic waves not only exist on stringed acoustic  musical instruments but can be controlled using very simple, inexpensive, temporary, movable devices which can be attached to the instrument and improve the volume and sound quality from the instrument.  

     This is possible primarily because of a well-known physics principle related to all waves called, "Constructive Interference", explained below.  Significantly, we also know from experiments that the surface acoustic waves on acoustic stringed musical instruments are an accurate representation of the music being played on the instrument.  We know this because we know how to amplify the harmonics of surface acoustic waves which improve sound quality, and how to convert the inaudible surface acoustic waves into audible sound in the instrument's sound chamber.   Once in the sound chamber this newly created audible sound mixes with the sound already there, allowing us to hear this newly created musical sound and note its improved volume and sound quality.

     Because the surface acoustic wave device which accomplishes this improvement, or its surface wave input component, is movable by the player, the appropriate portion of the device can be easily moved about on the instrument to achieve maximum performance in the opinion of the player.

The General Wave Principle of "Constructive Interference" Explained. 

     This principle posits that when waves in air, on a surface of a musical instrument, or on an ocean or lake, meet coming from different directions, their behavior will depend on the relative frequencies of the different waves before they meet.  If similar waves have the same or similar frequencies (timing between wave peaks) when they meet their heights (called amplitudes) will add algebraically.  If they have different frequencies the waves simply pass through each other, "ghost-like", without changing either wave.

     This characteristic of wave algebraic addition between waves having similar frequencies, and an ability to remain free from distortions when the waves meet and they have different frequencies, is ideal for control of sound volume and quality.  This is because waves having many different frequencies on musical instruments almost always share the same surfaces.  More important, amplification of waves, which is so important in achieving high quality sound by amplifying harmonics, is as simple as causing waves of the same or similar frequencies to meet coming from different directions!  This means that putting a random bunch of different surface acoustic waves on a 3/8" x 1" thin steel rectangle will amplify ALL similar frequencies on the rectangle!  All frequencies will amplify, (or, more correctly, "add algebraically"), because all will reflect from edges to meet themselves and/or similar frequencies at an angle, and so be amplifyied by the principle of "Constructive Inteference".  And they will not be changed by waves with other frequencies but will pass through them without changing either wave!  

     The principle of "Constructive Interference" almost seems to be heaven sent for musical instruments!  How could we have not known about this behavior and then exploited its possibilities before today?  (Perhaps a collective lack of imagination?)  Does this mean that every rectangle which can conduct surface acoustic waves is an exquisitely simple surface acoustic wave amplifier?  Answer:  Yes, and so is a standard steel staple!

     The points of a staple in a sheet of paper touch the paper very close to each other on the back side of the paper.  A surface acoustic wave traveling on that side of the paper will enter both points of the staple essentially at the same time, travel on the staple in opposite directions (away from each point) and meet on the staple's flat on the opposite side of the paper as two separate waves, "head on" and having the same frequency.  They will add algebraically (amplify) when they meet by the "Principle of Constructive Interference".  Since the staple flat is in firm contact with the paper the new, larger wave will flow out on the side of the paper with the staple flat.

     The important point to be made here is that it is incredibly easy and cheap to make a sound amplifier by stapling a piece of paper.  Now all we need is a cheap and easy way to make the inaudible surface acoustic wave audible.  - Something like a mechanical loudspeaker.