We have studied static and dynamic magnetic properties of a general asymmetric trilayer system using numerical simulations. For ferromagnetic, 90degrees, and antiferromagnetic coupling, the magnetizations of the two magnetic layers exhibit one, two, and three phases with increasing external field, respectively. The total magnetization and ferromagnetic resonance accordingly follow these phases of the magnetization vectors. The resonance condition is related to the interlayer coupling strength in such a way that a larger coupling constant yields a higher value of f sub(res)/H, where f sub(res) is the resonance frequency at the external magnetic field H. Based on the simulation results, it is proposed that measurements of the acoustic mode resonance alone at unsaturated conditions provide a sensitive and accurate technique to extract the antiferromagnetic coupling strength. The technique is demonstrated experimentally with the broadband ferromagnetic resonance measurements of two trilayer films with weak and strong coupling strengths. The technique offers an efficient and sensitive method for antiferromagnetic coupling strength extraction, yielding coupling constant values with a precision of better than 0.03 erg/cm super(2). Also, separation of the bilinear and biquadratic coupling contributions is possible with the technique.