Voluntary control over sound production is achieved in the Perceptual-Motor stage after 6 months.
In terms of the vocal mechanism, from about 6 months on, the child is in a vocal stage, we will describe later as exhibiting lalling and jargon. The sounds that the child is making are no longer simply the result of hit-or-miss attempts (motor-perceptual) but are purposefully orchestrated. That means an auditory image from the Mixer is dictating what the motor pattern (sound) will be--a perceptual-motor process.
Please switch to the graphic version to see an illustraion of an evaluation of a A VACUUM TUBE which IS typically NARROW, LIGHT AND SMOOTH-AND FUN.