I'd agree that the airflow is the problem. The increased risk is because of the airflow over the audience coming into the sound room. There have been spreading problems due to airflows like this - see My linkhttps://www.erinbromage.com/post/the-risks-know-them-avoid-them Ideally you'd need some positive pressure in the sound room to keep the flow the other way. Fans from the outside, etc are all possible but, to be honest, much more complex in our situation than the window approach.(... and before anyone suggests it, I've tried farting a lot but keeping it up for three hours is pushing it, even for me ) That blog post is not representing where UK population health science is (see Brian's HSE advice). If your organisational risk assessment suggests that the airflow through the sound booth is of high risk to mandate a barrier, then the barreir itself merely changes the airflow and that risk is transferred to others - FOH stewards etc? If the risk (once assessed) is so high then it would mandate the wearing of a minimum of a Type 3 fluid resist mask for staff surely.