In order to accurately measure brainwaves, the Sleep Shepherd is finely tuned to its environment. Ambient electrical noise must be filtered to ensure proper signal strength. Hats to be used in the U.S. (along with the rest of North America and certain other countries) contain a 60 Hz notch filter to account for the 60Hz frequency of AC power transmission. However, many European, African, and Asian countries use a 50Hz frequency for AC power – which can disrupt the sensitive measurements made by the Sleep Shepherd. It is important to confirm the mains frequency of the country in which that hat will be used. The chart below lists the mains frequency by country.