A weight plate have integrally formed handles is provided. A wide-band acoustic signal is produced by extracting at least one essential attribute from the narrow-band acoustic signal. A reference resistor is selectively connected to the equipotential preamplifier for setting a reference current, wherein the equipotential preamplifier is set to produce a preamplifier output voltage having a magnitude depending on whether the sense current is smaller or greater than the reference current. When a data in the verification server matches the first key, the verification server makes no response and asks a network-service server to provide a network service to the user terminal.