The output of the microphone is an alternating electrical current, whose frequency is the same as that of the sound wave that caused the diaphragm to vibrate. The amplitude of the electrical signal generated depends on the mechanical characteristics of the transducer, but is proportional to the velocity of the coil. The ribbon microphone consists of a long thin strip of conductive metal foil, pleated to give it rigidity and ‘spring’, lightly tensioned between two end clamps. The moving-coil microphone is widely used in the sound reinforcement industry, its robustness making it particularly suitable for handheld vocal use. In normal operation, the driving force on a pressure-gradient microphone is related almost totally to the phase difference of sound wave between front and rear of the diaphragm (caused by extra distance traveled by the wave). Omni microphones are usually the most immune to handling and wind noise of all the polar patterns, since they are only sensitive to absolute sound pressure.