On the upcoming Fisher Impulse AQ detector, what does the threshold control do that distinguishes it from the volume control? Is it somehow changing the way the return signal is sampled?
The threshold control allows the operator to choose how loud they want the “operating hum” of the detector to be. It is adjustable from silent up to quite loud. This hum is generated by the detector when it is operating and no target signal is present. When a target signal is detected, a separate audio output is sent. On the AQ it will be high tone in all metal (in multitone it can be high or low and in mute - high conductors will emit no tone).
The volume control adjusts the amount of amplification of this target signal to make it louder or softer.