It is very important to make sure that the audio guide system that you purchase has a transmitter with an FCC ID or IC ID (for Canada) clearly identified. Most well-known brands will have it, but some overseas vendors sell devices which are illegal to use in the USA and Canada due to the lack of official registration with the proper organizations that control this activity. Another important factor is to make sure that the brand has local US support, preferably live over the phone, and US-based warranty protection.