| 摘要 |
<p>Ultrasonic signals are used to transmit sounds from a modulated ultrasonic generator (14) to other locations from which the sound appears to emanate. In particular, an ultrasonic carrier is modulated with an audio signal and demodulated on passage through the atmosphere. The carrier frequencies are substantially higher than those of prior systems, e.g. at least 60 kHz, and the modulation products thus have frequencies which are well above the audible range of humans; as a result, these signals are likely to be harmless to individuals who are within the ultrasonic fields of the system. The signals may be steered to moving locations, and various measures are taken to minimise distortion and maximise efficiency. <IMAGE></p> |
