A fundamental distinction between speech sounds depends on whether the excitation is a noise source or a quasiperiodic one. The noisy or voiceless sounds are presumably produced with an opened and quiescent larynx, while for voiced sounds the larynx is closed down and in rapid oscillation. Direct evidence of this has come from motion pictures taken through the open mouth, a method obviously limited to particular sounds. Running speech can be studied by a transillumination technique. A fiber‐optics bundle or a miniature incandescent bulb introduced into the laryngeal vestibule through the nose illuminates the glottis, while a photocell placed below the thyroid cartilage registers the variable light transmitted through the glottis and the tissues of the neck. The “glottograms” so obtained are compared with simultaneously recorded acoustic waveforms and intraoral air‐pressure traces to determine how the voiced‐voiceless distinction correlates with the closed‐versus‐open states of the larynx.