an audio signal voltage generator that generates an audio signal voltage based on a level of an input audio signal;

a capacitor that is chargeable and dischargeable by the audio signal voltage and develops a monitor voltage;

an indication circuit that is activated depending on the monitor voltage to provide an alert indicating that a howling noise is contained in the input audio signal;

a discharging element that discharges the capacitor according to a first time constant when the audio signal voltage is lower the monitor voltage, thereby lowering the monitor voltage; and

a charging element that charges the capacitor according to a second time constant being set greater than the first time constant when the audio signal voltage is higher than the monitor voltage, thereby raising the monitor voltage so as to activate the indication circuit,

wherein the audio signal voltage generator generates the audio signal voltage, which has a variable voltage level when the input audio signal is free of the howling noise, and which has a monotonous voltage level when the input audio signal contains the howling noise, the monotonous voltage level being less variable than the variable voltage level, such that the variable voltage level of the audio signal voltage enables the discharging element to avoid raising of the monitor voltage, while the monotonous voltage level of the audio signal voltage enables the charging element to enhance raising of the monitor voltage.