1. A frequency conversion system comprising: a first electromechanical structure resonant at a first frequency; a second electromechanical structure resonant at a second frequency, each of said structures having input and output energy means, the output energy means of said first structure and input energy means of said second structure comprising impact forces; means for holding said impact forces in special relationship froM one another; means to excite said first resonant structure to vibrate at its said frequency; a non-resonant projectile mass mounted intermediate said impact forces for free movement back and forth therebetween, vibration of said first resonant structure effecting oscillatory movement of said mass back and forth between said impact forces, whereby said projectile mass transmits momentum and kinetic energy during its periods of free flight between said forces and absorbs or delivers momentum and kinetic energy during impacts with said forces, thereby causing said second structure to vibrate at its resonant frequency; and utilization means connected to said second structure.