You don't need to buy an additional jack cable as the speakers already have a 3.5 mm jack socket for connection via cable. You can connect them either via Bluetooth or with an existing 3.5 mm jack cable. If you don't have a suitable jack cable, you would have to buy one, but it is not essential if you want to use Bluetooth. The speakers have a 3.5 mm input and an output, so a jack cable can be used for wired connections.