The thresholds of audibility for 100 and for 1000 cps have been measured on five groups of originally naive listeners by various experimental techniques. All the experiments showed improvement of the threshold with practice. The improvement was greater at 100 cps than at 1000 cps. Pretraining at 1000 cps did not affect the threshold change at 100 cps. The improvement of the threshold with practice was enhanced considerably by reward and feedback.

The question whether the measured threshold of audibility improves with experience has widespread implications. It refers to the nature of the threshold itself, but it also affects the psychophysical experiments involving comparisons of thresholds of audibility. If practice has no effect, then the amount of experience of the listeners can be disregarded and results obtained on groups of listeners with unequal sophistication may be compared with each other. If the opposite is true, experiments using threshold comparisons would require either pretaining of the listeners or careful counterbalancing. In several experiments, inexperienced listeners showed statistically significant improvement in their threshold of audibility with practice. Depending on the experimental conditions, the average improvement amounted to from 4 to 10 db. Experiments performed on several groups of listeners under similar conditions, yielded practically identical results.

This paper reports on the development of a Digital Loudspeaker Array (DLA) solution based on Pb(Zr 0.52 ,Ti 0.48 )O 3 (PZT) thinfilm actuated membranes. These membranes called speaklets are arranged in a matrix and operate in a binary manner by emitting short pulses of sound pressure. Using the principle of additivity of pressures in the air, it is possible to reconstruct audible sounds. For the first time, electromechanical and acoustic characterizations are reported on a 256-MEMS-membranes DLA. Sounds audible as far as several meters from the loudspeaker have been generated using low voltage (8V).