Speaking clearly. Improving voice and articulation: Фонетический практикум. Фомиченко Л.Г. - 8 стр.

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you won’t have to unsay it. You never have to take a dose of your own
medicine if you know when to keep your mouth shut.
      If you didn’t succeed, the following exercises will help you gain
control over your flow of breath.
      Take a deep breath and release it slowly, making the sound /s/.
Keep it even and regular, free of jerkiness and bumpiness. Try it with
the sound /f/.
      An interesting experiment: Hold a small, lighted candle about six
to eight inches in front of your mouth. Sustain /s/ and then try /f/.
Keep your exhalation regular and constant. The flame shouldn’t
flicker and certainly shouldn’t go out.

      UNIT II
                ARTICULATION EXERCISES
      If you have a tight, constricted throat with rigid walls your voice
may be strident, jarring, and rasping. Openness of throat and relaxation
of the walls and surfaces will promote a mellow, velvety, and molasses
— rich quality.
I. Exercises for the Opening of the Mouth
      1. a) Keep the mouth closed with the li ps pressed together.
         b) Drop the lower jaw as low as possible. The mouth should
            be wide open.
         c) Come back to the a-position.
      2. a) Keep the mouth closed with the li ps pressed together.
         b) Open the mouth as wide as one third of the opening.
         c) Open the mouth one third wider. The separation of the
            jaws is considerable.
         d) Shut the mouth.
      3. a) Open the mouth as wide as possible.
         b) Close the mouth gradually in three equal times.
II. Exercises for the Lips
      1. a) Keep your li ps pressed together.
         b) Now open the mouth.The li ps should be in their neutral
            position.