Ryan, J., E. Shaw, A. Brammer, and G. Zhang. “ENCLOSURE FOR LOW-FREQUENCY ASSESSMENT OF ACTIVE NOISE REDUCING CIRCUMAURAL HEADSETS AND HEARING PROTECTORS”. Canadian Acoustics, vol. 21, no. 4, Dec. 1993, pp. 19-20, https://jcaa.caa-aca.ca/index.php/jcaa/article/view/820.