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Remember the story of Eliza Doolittle in My Fair Lady? Henry Higgins realized he couldn't pass a common street vendor off as a lady until he taught her how to speak properly.
Because the voice is so deeply entwined with emotions and the expression of personality, any lack of self-confidence or fear will be communicated with your words. Hopefully all the previous lessons about sitting, walking, and moving like a lady have contributed to pride in yourself, and that will automatically translate to a more assured and pleasing speaking voice.
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Vocal habits to avoid:
Shrill or whining tone
Loud dominating tone
Too soft timid tone
Monotonous pitch
Speech habits to avoid:
Mumbling
Talking too fast and slurring words together
Lack of enunciation
Slang phrases
Filler sounds - uh, um, you know, like
Use of off color language or swear words
Make a list of the poor vocal and speech habits you need to work on to correct. Then start listening to yourself as you talk during the day. Become more aware of what you are saying and how you are saying it. It also helps to listen to others around you and privately analyze their vocal patterns. You'll soon learn to "hear" and start to naturally correct your poor speech habits.
When listening to the recording of your self, you may be surprised to find your speech filled with slang expressions, swear words or annoying filler sounds that you weren't aware you were using. By becoming aware of them, you are halfway toward eliminating them. If necessary, make a game of it and start "fining" yourself a dime or a quarter each time you use one. Enlist the help of family or friends to help you notice your use of these speech habits. With persistence, you'll soon find they're gone.
The best way to work on improving your voice and speech is to read aloud. Choose a quiet time and a private space. Choose reading material that requires discriminative thinking and the full use of your imagination. Try classic poetry or Shakespeare. Read Jane Eyre again, this time out loud. Buy a copy of Bartlett's Familiar Quotations and practice one in front of the mirror every morning before heading out the door. Become aware of your voice and speech patterns, and strive to sound like a lady at all times!