-6db low pass filter and synthesised tone

Hi! I want to synthesis a harmonic sound with fundamental frequency of f0 and a serie of amplitudes (a0, a1, a2, a3 ....a20) for partials. The amplitudes should reduce a - 6db per octave like a low pass filter but I need the amplitueds as number. That means I do not need a real low pass filter for a sound signal rather the calculated amplituds after the signal is filtered. How can I claculate them?