Vocal effort is a quantity varied by speakers when adjusting to an increase or decrease in the communication distance. The communication distance is the distance between the speaker and the listener. Vocal effort is a subjective physiological quantity, and is mainly dependent on subglottal pressure, vocal fold tension and jaw opening. Vocal effort is different from sound pressure. To measure vocal effort, listeners are asked to rate the distance between speaker and addressee.
^Hartmut Traunmüller & Anders Eriksson (2000). "Acoustic effects of variation in vocal effort by men, women, and children". Journal of the Acoustical Society of America. 107 (6): 3438–3451. doi:10.1121/1.429414. PMID10875388.