I never fully understood the concept of voicing:

• if 'b' and 'p' are really the same except for the voiceness, how can we still tell the difference in whisper?
• how can we produce 'b' unvoiced as well as unvoiced vowels, etc.
• i don't feel my lips are moving the same way when producing 'b' and 'p'

The answer: they are not exactly the same. The textbook is lying to you. See the post for more.