Power spectral density is commonly expressed in watts per hertz (W/Hz). The SI unit of frequency is the hertz (Hz). It is equivalent to 10 watts per square metre per hertz. Europcar, Hertz, Sixt and Thrifty all operate at Brussels Airport. Frequencies are measured in units of hertz. The microbarometers used are designed to monitor frequencies below approximately 20 hertz. Each musical note corresponds to a particular frequency which can be measured in hertz. Companies like Europcar, Avis and Hertz have offices near the airport. It is tuned to low frequency signals (below one up to tens of Hertz). The number designates the frequency in hertz. Frequency of PD tremor is between 4 and 6 hertz (cycles per second). The device operated on a frequency of 25 to 300 hertz. Alamo, Avis, Budget, Dollar, Enterprise, Hertz and National all are on-site. Therefore, it is quoted in hertz (frequency) and not ppm (chemical shift). The channel capacity is proportional to the analog bandwidth in hertz.