It is theoretically possible to take measurements at the operatives ear positions whilst operating the equipment etc, calculate exposure times for the different tasks and come up with an estimated daily noise dose but it is likely to be an innacurate way of doing this. Your idea of dosimetry is a much more reliable approach and will allow you to reach a more reasoned conclusion on whether your employees are exposed to high noise and how much. Once you have that information you can then make a better judgement on what to do to reduce their exposure and whether your existing arrangements are adequate. Health surveillance is a requirement if there is a significant risk to your employees health and is good practice anyway to identify NIHL and to check whether your noise controls are actually working.