One of many Mason & Hamlin innovations, the Tension Resonator was first patented in 1900, and it's been an exclusive feature of all Mason & Hamlin pianos since that time.  As part of the Crown Retention System, the Tension Resonator helps to preserve the crown of the soundboard for proper resonance. 

Made of inflexible steel, the Tension Resonator’s radial arms are bolted to the piano’s inner rim underneath the soundboard.  By holding the rim in a rigid position, the Tension Resonator maintains the soundboard crown in its original shape, regardless of climatic conditions or the age of the piano.  This assures Mason & Hamlin owners that the original pure tone of their piano will last for the life of their instrument.