This current induces an alternating electromagnetic field around the search coil. This field is uniform and looks similar to the field which iron filings pick out around a bar magnet. However, the field is easily distorted by a metallic object, for example a coin or a gold nugget, or an object which is magnetised, such as a nail or naturally magnetic mineral like magnetite. The opposing electromagnetic fields are produced by eddy currents within these objects. When the uniform electromagnetic field of the search coil passes over a metallic object with an eddy current, the detector's field is weakened and distorted.
All metal detectors have an audible note produced by the mixing of two oscillators.
One oscillator produces a 'reference' frequency which is very similar to that produced by another oscillator circuit within the search coil. The search coil frequency is dependent on the distortion of the electromagnetic field it produces, i.e. the materials it passes over buried in the ground. When a magnetic material enters the field of the detector and concentrates it, a lower frequency is produced in the search coil oscillator, resulting in an opposite audible beat.