A brief overview
The purpose of the responsive back is to transfer vibrations better to the musician. The underlying concept is that the most important judge of guitar sound is not the audience, but the performing musician. It has been noted in the past that the audience has a much harder time judging the quality of the sound of an instrument than the musician playing it. This is due to the proximity of the instrument and also the room acoustics, which can vary significantly from venue to venue. Therefore one can assume that if a musician hears their instrument well and is literally in love with its sound, this joy is reflected in a more passionate and dedicated performance of the player. Which are all attributes that are certainly noticeable (and very crucial) to the audience. But the flexible back also has a reputation of poor projection – please read on to find out how I solve this problem.
How the back contributes to guitar sound
and a description of the two major principles of back construction:
The acoustic function of a guitar back are described as follows: (see Meinel, E.: Lehrbrief 2, Markneukirchen, 2007, p.70): – Creation of a hollow space (with the sides and the top), this prevents an acoustic short between the front side and back side of the top. The lowest air mode (Helmholtz resonance) increases the area of emittance significantly and permits a sonorous, voluminous sound. – Natural oscillations of the back contribute to the overall oscillation characteristics [of the instrument] in a complex manner. – It reflect the sound that the top emits to the inside of the instrument. There are two fundamental concepts of guitar back construction:
1) a stiff and heavy back
The acoustic principle reminds one of that of a timbal (kettledrum). The back and the sides are less able to oscillate. Back vibrations, which are not preventable entirely, are not damped by the body of the player. A relatively low oscillation transfer from the top to the back exists, resulting in a concentration of vibration energy on the top, from where it is emitted with more direction towards the audience […].
2) light, oscillating back
The acoustic principle reminds one of the drum. Stronger coupling with air cavity and top resonances. Intense back resonances fill resonance gaps of the top. (The body oscillates more as a whole, including the sides; the ability to vibrate more as a whole does not necessarily equal an improved emittance!)
Sound transfer – some background information:
The transfer of sound from the top to the back takes several routes. In the following description, the word “coupling” is used, which can be regarded as the same thing as sound transfer. Coupling is basically understood as an interaction of physical systems. In engineering for example, coupling is reduced by vibration insulation. In the top-back-system of a guitar, this coupling should be improved so that the energy is not lost in the vibration of the sides. If a comparison to the mass-spring model is made, one can see the back and top of the guitar as the mass, and the sides as the spring. If the spring (side) is too soft, the energy from the mass (top) to the other mass (back) is not transferred completely. The coupling would be insufficient, and an undesired transfer loss would occur. But this must not be misunderstood; the goal of this construction is not to transfer all vibration energy from the guitar top to the back. This was described in more detail under “mechanical impedance – veneered sides”. The purpose of the veneered side is to transfer only the energy that has managed to “jump” the impedance barrier from the top to the sides – something the luthier tries to avoid in the first place. The hypothesis is that the impulse of the top vibration (if it manages to overcome the impedance barrier) should be transferred with minimum loss of energy to the back of the instrument. In order to minimize that transfer loss, the core material of the veneered side is oriented with the grain perpendicular to the top. In theory, the “weaker” higher frequencies should be reflected by the veneered side back onto the top, whereas the more powerful lower frequencies permeate through the sides without transfer loss into the back. Professor Meinel wrote in an email to me: The reflection at the perimeter is surely depending on the frequency. The impedance is a complex factor in a mathematical sense and therefore dependent on frequency. I also presume that the higher frequencies are reflected more [than the lower ones] (personal communication on the 8th of April, 2008).
The middle course:
The bracing and composition of the back is fashioned in a way so that the back is rigid but able to oscillate, and still bears the characteristics (strong coupling, easy dampening by the player) of the light, oscillating back. In order to better understand what makes the back more able to vibrate, one must distinguish between the two following effects: – Back vibration through coupling of the air cavity – Back vibration through coupling through the sides In lutherie it is the custom (exceptions exist!) to work the perimeter of a plate thinner to enhance its ability to vibrate. In this case, the coupling is primarily achieved through the air cavity. Since a special method of building the sides (see “veneered sides”) facilitates a good energy transfer, it seems appropriate to construct the back in a fashion to have it coupled directly to the sides. For this the perimeter of the back is of normal thickness, and the center is thinner (including appropriate bracing). The energy from the sides is transferred into the perimeter of the back and then transformed into vibration in the center of the back. The use of coupling through the sides for the purpose of enhancing back vibration should have no negative impact on the projection of the sound and at the same time transform a maximum of energy into sound. All this while maintaining the useful properties (ability to modulate sound by varying body contact with the instrument back, lower Helmholtz frequency, etc…) of the light back.