The frequency has the SI unit Hz (Hertz) and denotes the number of periodic repetitions per time unit. 1 Hz = 1/s . The standard concert pitch a 1 in music is 440 Hz. The audible range for humans is 20 – 20,000 Hz, and perceptible light is between 400 THz and 750 THz. The power grid in Europe is operated at 50 Hz. To ensure grid stability, the range of 50.2 – 49.5 Hz must be maintained. When electricity production exceeds demand, the frequency increases; when too little electricity is produced, the frequency decreases. Deutsche Bahn uses 16.7 Hz for its traction power supply.