In fact, the same string can be used with frets to obtain the fundamental of the next note up, D, and all its harmonics. With 2 notes and their harmonics per string, only 4 strings would be required to obtain 4 complete octaves. If there is anything C-ish about the musical note C,- it must be the the note's fundamental frequency and its attendant wavelength.

The relationship between frequency and wavelength for both light and sound is the same: V = lf   For sound, V = 313 meters/sec, while for light V = C = 300 Thousand Kilometers/sec . In both cases, f is in Hertz and l is in meters.

The Visual Spectrum's longest wavelength should be the Fundamental wavelength of Red not the 1mm mark where Infrared light and Microwaves meet. A Fundamental wavelength for Red of 0.7168 mm will not quite reach down into the Microwave region, while a Fundamental wavelength of 1.4336 mm will allow the Visual Spectrum to overlap the Microwave domain a little.

I have chosen the wavelength of 1.4336 millimeters for the fundamental wavelength of Red. This is 11 octaves below Visible Red so that the overlap with Microwaves will occur. Thus there will be no Infrared gap between the Visual Spectrum and Microwaves.

The Visual Spectrum now begins with a wavelength of 1.4336 millimeters. Lower harmonics of Red do exist, but are excluded from the range of Reds within the Visual Spectrum.

The Visual Spectrum's shortest wavelength should also be a harmonic of Red to complete its octave, and fall near the cusp between Ultra-Violet and X-rays which ocurrs at a wavelength of 1 nanometer. The 10th harmonic of Visible Red fits the bill with a wavelength of 683.6 picometers, just inside the domain of X-Rays. The Visual Spectrum now ends at a wavelength of 683.6 picometers.