A safety helmet includes a cap equipped with at least one visor, which is hinged to the sides of the front opening of the cap itself and can be rotated between a position of interception of the front opening and a raised position with respect to the opening. The surface variations can be provided in the form of flow boundary disrupting protrusions disposed within the downstream portion of the vane island passages to prevent flow separation. Each of the channel driving circuits includes a first input terminal, a second input terminal, a third input terminal, an output terminal, a series circuit, and a parallel circuit. Based on this comparison, a tailor-made equalizer is designed to compensate for the differences between the sounds as picked up by the first microphone and the reference sounds of the musical instrument. When the photodiode portion is in its first state, the reset portion normalizes a voltage across each of the resonant tunneling diodes.