The Structure of the Classical Guitar
Position of the harmonic bar determines the timbre

Pieces of wood called harmonic bars are placed on the inside of the top plate of the guitar.
One of the purposes of these harmonic bars is to provide the top with sufficient strength to support the tension of the strings. Another purpose is to create a certain timbre. The guitar's timbre is affected greatly by the size, shape, and layout of these harmonic bars.
Let's compare the layout of the harmonic bars in a classical guitar and in an acoustic guitar.

The strings of an acoustic guitar are made of steel and have much more tension, which requires larger harmonic bars arranged in a layout that provides greater strength. By comparison, classical guitars use much smaller harmonic bars arranged in a more precise layout. While these harmonic bars provide strength for classical guitars that use soft nylon strings, they have been arranged to deliver the best sounding timbre