VLF detectors work using two coils in a perfect balance. The outer coil works as a transmitter. It creates a magnetic field using alternating current. This magnetic field is distorted by the presence of any metallic object. The inner coil works as a receiver. It reads the secondary magnetic field made by the metal. This converter then amplifies the magnetic field and converts it to audio tones. The device then uses phase demodulators to differentiate different objects.