Here's a really good technical (but very readable) article on noise: it's impacts on health; the measurement of different aspects of noise; enforcement of compliance with noise controls; and a whole lot more.
A whole lot of it is relevant to motocross noise, even though the article is focussed on traffic noise, and there are many things that will give you ideas to follow up in relation to motocross noise issues.
Just as a small taste of a much longer document:
"Unwanted noise has wide-ranging effects on human health to a far greater extent than merely to cause damage to hearing. Noise results in cardiovascular issues, sleep disturbance, headaches, nausea, impaired task functioning, depression and is implicated in many other health problems as well as inducing anti-social behaviours. (references 3,4,12,13,17,23) These unwanted health problems occur with noise levels which are a lot less loud than that which would cause hearing damage. Effects are also cumulative. The costs to health-care systems is enormous and researchers are finding more and more stress related problems that can be attributed to noise.
Governments, city planners and local councils are, in general, aware of the noise problem, and try to provide urban planning guidelines and rules to mitigate noise, but prefer to take 'baby-steps' and are over-cautious in order to avoid the creation of higher compliance costs. In most cases, authorities do not apply sufficient urgency or big enough solutions to the problem. There are already a large number of studies and comprehensive research documents published on the subject. No more are needed. Instead, some definitive progress is called for, which will certainly have a cost. However, studies have already shown that the costs of healthcare triggered by unwanted noise is more than a hundred times than what is currently spent on reducing or mitigating noise from vehicular traffic."
The article is from the website of a New Zealand company (Axino-tech Consulting and Services Ltd) and appears to be offered as a contribution to wider understanding of noise issues. There should be more companies sharing their expertise in such an altruistic way.