| 摘要 |
PURPOSE:To obtain loudspeaker sound with high articulation at all times independently of the fluctuation of noise level by detecting the noise level around a loudspeaker and changing the reproduction level in response to the noise level in a location such as a station platform where surrounding noise is high and the noise level is fluctuated. CONSTITUTION:A diaphragm 13 of a speaker 8 is vibrated also by the surrouding noise in addition to a loudspeaker signal and an electromotive force is produced in a voice coil 12. The electromotive force includes the loudspeaker signal where its low frequency component is cut off by a high-pass filter 7 and a surrounding noise signal. Thus, a noise signal component is extracted by a low-pass filter 9. This signal is amplified by amplifiers 10a-10d, converted into a DC voltage by rectifier circuits 11a-11d and impressed to AGC circuits 5a-5d. The AGC circuit adjusts automatically a reproduction level of the loudspeaker signal in response to the level of noise signal converted into the DC voltage. Thus, even if the surrounding noise level is fluctuated, the reproduction level having excellent articulation and not masked by the surrounding noise is obtained at all times. |
