Watch the Drop 3, 4 Voicings: online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
The drop 3, 4 voicings of the Add 9 chord present a very open sound. Their point of origin will always be the same inversion of the close voiced forms. In this lesson you will see how the root position close voiced Add 9 chord changes texture by simply lowering the root (4th voice) and 9th (3rd voice) down one octave. This will create some big stretches, but the results are worth the effort. These open voicings will require finger picking or hybrid picking because of the unused string between voices 2 and 3. Be sure to guard against intonation problems by not distorting the string placement either along the string axis or fret axis.