| 摘要 |
PROBLEM TO BE SOLVED: To secure sufficient sound pressure corresponding to a frequency change when a sweep sound is output, and simultaneously reduce current consumption of a speaker. SOLUTION: An alarm 10 outputs a sweep sound whose frequency changes in a predetermined range with lapse of time from a speaker 46 as an alarm when detecting a fire. A reference current setting part 54 sets a driving current as a reference current Ir, the driving current flowing when a predetermined amplitude voltage Vr having a predetermined frequency having a high sound pressure in the speaker 46 and having a high impedance is applied. A PWM processing part 56 outputs a PWM pulse string in which a duty ratio is set so that the driving current of the speaker 46 is kept in the vicinity of the reference current Ir, converts the PWM pulse string to a sound signal via a low pass filter 42, and then amplifies the sound signal by an amplifier 44 to output from the speaker 46. COPYRIGHT: (C)2010,JPO&INPIT |
