  Behavior of the photoacoustic signal produced by nanoparticles as a function
of their concentration was studied in detail. As the concentration of
nanoparticles is increased in a sample, the peak-to-peak photoacoustic
amplitude increases linearly up to a certain value, after which an asymptotic
saturated behavior is observed. To elucidate the mechanisms responsible for
these observations, we evaluate the effects of nanoparticles concentration, the
optical attenuation and the effects of heat propagation from nano-sources to
their surroundings. We found that the saturation effect of the photoacoustic
signal as a function the concentration of nanoparticles is explained by a
combination of two different mechanisms. As has been suggested previously, but
not modeled correctly, the most important mechanism is attributed to optical
attenuation. The second mechanism is due to an interference destructive process
attributed to the superimposition of the photoacoustic amplitudes generated for
each nanoparticle, this explanation is reinforced through our experimental and
simulations results; based on this, it is found that the linear behavior of the
photoacoustic amplitude could be restricted to optical densities $\le0.5$.
