Paul Chambers Guitars - Luthier and Maker of Fine Acoustic and Classical Guitars

The Acoustic Tuning Process

Each Chambers guitar undergoes a complex acoustic tuning process to measure and control the mechanical and acoustic properties that determine the final sound of the instrument. This takes into account the chosen features of a particular customer, the characteristics of the guitar model in question, and the specific properties of the woods used.

Step 1 – Bridge Torque Model

Bridge torque model

I start by considering the customer’s desired scale length, string gauge, and the resulting string tension and action. This, combined with the bridge design and break angle of the string over the saddle, determines the ‘torque model’.

In turn, this determines the forces on the soundboard and thus the required stiffness of the soundboard and bracing system.

Step 2 – Soundboard Stiffness Model

Measuring the elasticity of the soundboard

Next, the top is tuned according to the customer requirements defined in the torque model and choice of guitar model itself. The soundboard is measured in a number of different ways to determine a set of static and dynamic properties of the wood.

These measurements are used, along with the torque model, to mathematically determine the desired stiffness-to-mass ratio and a set of target acoustic resonances.

Deflection testing the soundboard in a test rig

The deflection of the top is measured under controlled conditions to determine its elasticity. The soundboard is then measured with reference to the desired stiffness model, a set of deflections, to see how closely it complies with the desired outcome.

The top plate is thicknessed, with additional thinning around the periphery of the lower bout, and the braces carved until compliance with the desired deflection model is achieved.

Once this process is complete the soundboard will be optimised in terms of its stiffness within the requirements of the torque model (scale length, string tension, and so on).

Step 3 – The Acoustic Resonance & Coupling Model

Measuring the frequency resonance of the top in a test rig

Once mechanical compliance is achieved the next step is to optimise the acoustic resonances of the top plate and back plate, and air resonance within the body, and the coupling between them. This involves measuring the frequency response of the top, back and air, and carving the bracing system of the back until specific frequencies result.

A desirable acoustic outcome is if the first top plate resonant frequency is an octave higher than the air resonance, and also a semitone above or below the back plate resonance. This frequency relationship achieves an acoustic coupling between the top, back and air resonance, which results in a spreading of the combined resonances across a broad range of frequencies.

This produces a balanced sounding guitar across the full range of the fingerboard. The air resonance is determined by the product of the volume of the body and diameter of the soundhole. This is taken into account at the design stage for a particular model.