The early 1970’s saw the first serious explorations of the acoustical anatomy of stringed instruments. This work began with the study of violin acoustics in the 1960’s by Carleen Hutchins and her Catgut Acoustical Society. Hutchins et al achieved some success in the identification of particular features of a violin associated with superior tone, which could be “dialed in” by the builder.  When applied to the guitar, the same imaging techniques generated entrancing images like those shown above, called laser profilometry. These images show the top and bottom of an acoustic guitar excited with a pure tone at two different resonances, here at 390 Hz and 460 Hz, where the contour lines delineate surface excursion, just as they do on a 3-dimensional map.

The 390 Hz resonance involves a primary diagonal rocking motion of the soundboard, with the “node”, or still line, running through the bridge. This asymmetry is due to an asymmetric bracing pattern. The asymmetry of the rocking causes the soundhole to fall within an “anti-node”, or area of excursion marked by a contour line pattern. The geometry of the soundboard motion induces a similar asymmetric motion in diminished fashion in the back of the guitar, showing that the heart of the resonance is in the soundboard.

The 460Hz resonance on the same instrument can be seen to be more symmetrical on the face, but when you look at the back you a much greater excursion and symmetry of motion can be seen, indicating that the driver of this resonance is the back, with the face weakly reflecting the motion, somewhat distorted by its asymmetric bracing pattern.

Laser profilometry has come a long way in the last 40 years. Here you can see a yet higher-frequency resonance of a guitar soundboard, with the nodal lines being the boundaries between the excursion “islands”.  This pattern is consistent with a highly symmetric bracing pattern typically used in classical guitars like that shown here. It appears that there is some asymmetric element in the bracing design towards the lower part of the face.

An acoustic guitar’s lowest frequency resonances are separated by approximately a full octave. However, they occur closer and closer in frequency as the frequency rises, with the incidence of resonances increasing as the square of the frequency and exhibiting every more complex patterns.

All of these resonances shown are below the frequency range of the highest sensitivity of human hearing, from approximately 1500 Hz – 4000 Hz, which encompasses the range of the “voice formants”. Voice formants are the three or four discrete dynamic resonances of the human vocal tract whose variable frequencies encode meaning in speech. Removing this higher frequency range from human speech is what happens when we “hum”. Attempting to communicate with just humming offers a very limited repertoire of information. You can tell man from woman from child, big guy from little guy,and happy from angry, but that is about it. Flip the situation, and remove this lower frequency range while keeping the higher voice formant frequency range, and you get “whispering”.

There are about 40 unique “phonemes”, or elements of speech which are strung together to create speech.  Each phoneme is actually a musical chord formed of a particular pattern of notes corresponding to the voice formants at a particular moment in time as they change rapidly to create speech. Our hearing is so hardwired for perceiving speech in the voice formant frequency range that we have lost the ability to perceive these phonemes as the musical chords they are. Each moment of a child’s whisper is a musical chord.