Vongpaisal, T., and K. Pichora-Fuller. “Hearing One or Two Voices F0 and Vowel Segregation in Younger and Older Adults”. Canadian Acoustics, vol. 32, no. 3, Sept. 2004, pp. 184-5, https://jcaa.caa-aca.ca/index.php/jcaa/article/view/1689.