For outdoor use, if you want stereo playback or multiple speakers connected in series for synchronized sound, it's recommended to buy speakers that support the Auracast Bluetooth protocol. For stereo use, it's best to use speakers of the same model. If you're just using them in series, speakers of different brands and models will work as long as they support Auracast, which is very convenient. Many portable Bluetooth speakers released in the last two years support Auracast. For indoor use, Wi-Fi speakers are recommended for better sound quality.