Choosing an assessment room
Acoustic measurements must be made in a quiet room. It has been shown that to reliably measure voice qualities using the Boersma algorithm requires a signal-to-noise ratio (SNR) of at least 26 dB. However, to obtain shimmer results accurate to 1%, the SNR should be at least 41 dB. In order to validate your assessment environment, you should perform a noise calibration (select Analysis > Calibrate Noise... from the Analysis menu in Sonneta).
Noise comes from many sources. There are continuous sources such fans and the hum of fluorescent lighting, and changing sources such as people talking in the hallway and city noises coming in from the window. The loudest of all of these sources will dominate the noise level for your environment.
Deliyski, D. D., Shaw, H. S., & Evans, M. K. (2005). Adverse effects of environmental noise on acoustic voice quality measurements. Journal of Voice, 19(1), 15-28.