NHT Xd system Setup Manual page 13

Connect the signal cable
between the XdW and XdA.
Single XdW Dual XdW
The factory setting of the Ext. Trigger switch is in the Audio position. This turns
the XdA on automatically when it 'hears' an input signal. For more information on
the Ext. Trigger modes, check the web site: www.nhtxd.com/xdsetup