Control the tonal quality of Amazon Polly’s voice output with the new vocal-tract-length Speech Synthesis Markup Language (SSML) tag. Adjusting the “vocal tract length” will modify the tone quality, called timbre, to make a voice sound characteristically different without adjusting rate and pitch. This tag can be used with any of the voices in the Amazon Polly Text-to-Speech portfolio to produce many variations for a single voice.