Chapter 7, Verses 23 and 24 - Sun Tzu

The Book of Army Management says: On the field of battle, the spoken word does not carry far enough: hence the institution of gongs and drums. Nor can ordinary objects be seen clearly enough: hence the institution of banners and flags. Gongs and drums, banners and flags, are means whereby the ears and eyes of the host may be focused on one particular point.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.