Minnesota's boating noise laws
http://www.boatus.org/onlinecourse/statelaws/Minnesota.html"An effective muffling system in good working order is required for the exhaust of each boat engine. This system must be effectively installed to prevent excessive noise. Loud boat noise not only bothers other operators and users of the waterways, it can also disrupt peace ashore because of sound's ability to travel great distances over water.
Excessive noise can also prevent you from hearing other boats' sound signals and warnings, and approaching law or rescue personnel. Specifically, it is unlawful to modify, equip, sell or operate a motorboat that exceeds the levels indicated below.
Specifically, no boat shall be operated if it exceeds the following noise levels:
•Boats built before January 1, 1982 must not exceed 84 decibels (dBA) during a pass-by test or 86 dBA measured during a stationary idle test.
•Boats built on after January 1, 1982 must not exceed 82 decibels (dBA) during a pass-by test or 84 dBA measured during a stationary idle test.
A Pass-By Test is usually taken at 50’. An equivalent reading may also be taken at other distances. A Stationary Idle Test is taken at least 4’ above the water and at least 4’ behind the transom of the boat."
Make sure you see the actual DB meter reading and make sure he is standing the 4 feet back. Get a copy of MN's staute on this and keep it with you on the boat for next time. That is if you are within the DB.