  We report the experimental realization of acoustic coherent perfect
absorption (CPA) of four symmetric scatterers of very different structures. The
only conditions necessary for these scatterers to exhibit CPA are that both the
reflection and transmission amplitudes of the scatterers are 0.5 under one
incident wave, and there are two collinear and counter-propagating incident
waves with appropriate relative amplitude and phase. Nearly 1000 times in the
modulation of output power has been demonstrated by changing the relative phase
of the incident waves over 180{\deg}. We further demonstrate that these
scatterers are sensitive devices to detect the small differences between two
nearly equal incident waves. A 27 % change in the strength of the scattering
wave has been demonstrated for every degree of phase deviation from the optimum
condition between the incident waves.
