| 摘要 |
PURPOSE:To secure an automatic adjustment for the sound volume of the acoustic device according to the external noise by utilizing the speaker as the microphone and thus to stabilize the acoustic device to the substantial external noise without any external connection to the car or the like. CONSTITUTION:The following units are provided: speaker 5 conncted to power amplifier 1; differential amplifier 4 which supplies the adverse electromotive force induced by the external noise of speaker 5 via differential amplifiers 2 and 3 and then detects the reverse electromotive force by the output amplifiers 2 and 3; band pass filter 6; and voltage divider 7. Thus the output of divider 7 is applied to variable resistance element 9 via DC converter 8 to ensure the automatic control for the gain of amplifier 1. Then the output of amplifier 1 is controlled automatically according to the external noise delivered from speaker 5. In such way, the sound volume of the acoustic device can be controlled automatically to the external noise. |
