D. Lubman & Associates, 14301 Middletown Lane, Westminster, CA 92683
Periodic motion of a sound reflector in a tonally excited reverberant sound field results in a multitonal reverberant spectrum consisting of symmetrical ``sidebands'' centered around the forcing frequency. This surprising and useful phenomenon does not violate linear system theory! It was first reported in 1968, late in the history of what some consider to be the ``mature'' field of architectural acoustics. It was discovered in connection with the invention of rotating diffusers (RDs), which were themselves a surprise. (Moving diffusers were shown to be the only class of diffuser that can improve spatial uniformity of reverberant sound.) Interest in RDs was spurred by their ability to improve the precision of tonal sound power determination in reverberation rooms. But neither the reason for multitonal ``sidebands'' nor their role in improving spatial uniformity was understood. Some workers pronounced sidebands to be trivial doppler shifts. Others denied their very existence, dismissing data as mere illusions of reactive sound. At the recent Sabine Centennial, this writer proposed a new ``chopper'' hypothesis to account for RD sidebands. More physically compelling than the doppler hypothesis, it shows strong potential for analytical development, and may facilitate RD design optimization. Chopper theory is reviewed and recent developments are reported.