This unmarked archway possesses a mystifying acoustic property: when two people stand at diagonal arches and whisper, they can hear each other's voices from across the way. This is the result of the curvature of the domed ceiling and the way sound is carried along its surface. The design of the archway was created by Rafael Guastavino, a Spanish immigrant who perfected the use of Catalan Vaulting technique in America. This design did not require the use of structural support to add strength, the result of which was graceful and elegant design of the Whispering Gallery. One of Guastavino's innovations was to replace the brick with fast drying Portland cement; this decision permitted him to create arches several times larger in size than traditional methods allowed. To this day, no one is certain that the acoustic effects of the Whispering Gallery were deliberately planned by its designer.