Help with function for under damped harmonic oscillation decay function, reversed along x axis.

Trying to figure out a F(x) that would look like a under damped harmonic oscillation decay, but swapped on the x axis so the amplitude + with time, domain and 1/2 the amplitude for every 2x of freq.The function that I have come up with for now is f(x)=[e^(-γx)ωcos(sqrt{ω^2-γ^2}x-α)]. Where gamma= the damping coefficient, which I would guess was about0.02-0.05 ish. Omega is the initial amplitude, 20? and I think alpha is the phase angle?

The hint I was given was "A first order low pass filter will work if you're trying to maintain equal peak velocity as frequency changes." and "Halve the amplitude for every doubling of frequency." Another person said that matlab can generate wave forms.??? I am just now taking Math 251 next term and I am pretty sure solving this is differential equations.

The idea is to use it as a drive file for a 4 post simulator for 1/4 car suspension simulation. http://hyperphysics.phy-astr.gsu.edu/hbase/images/oscda8.gif