TABLE OF CONTENTS Hermann Von Helmholtz (1821-1894) performed some interesting acoustical experiments with resonators. His resonators were a series of metal spheres of graded sizes, each fitted with a neck, appearing somewhat like the round-bottom flask found in the chemistry laboratory. In addition to the neck there was another small opening to which he applied his ear. The resonators of different sizes resonated at different frequencies, and by pointing the neck toward the sound under investigation he could estimate the energy at each frequency by the loudness of the sound of the different resonators.
There were numerous applications of this principle long before the time of Helmholtz. There is evidence that bronze jars were used by the Greeks in their open-air theaters, possibly to provide some artificial reverberation. A thousand years ago Helmholtz-type resonators were embedded in church walls in Sweden and Denmark with the mouths flush with the wall surface, apparently for sound absorption.1 The walls of the modern sanctuary of Tapiola Church in Helsinki, Finland, are dotted with slits in the concrete blocks2 (Fig. 15-1). These are resonator "necks" that open into cavities behind, together forming resonating structures. Energy absorbed from sound in the room causes each resonator to vibrate at its own characteristic frequency. Part of the energy is absorbed, part reradiated. The energy reradiated is sent in every direction, contributing to the diffusion of sound in the room. The resonator principle, old as it is, continually appears in modern, up-to-the-minute applications.
