BandB-
I didn't have a chance to log sound levels versus frequency, which would be more useful information. Perhaps I could calculate dbA with that info?
In any case, I like all three of Keysdisease's suggestions. The engine is a new Mercury Bigfoot 60hp and there is no sound insulation in the cowl cover. I'll search to see if I can find some mass loaded insulation in Qatar. If not then I can bring some back from vacation this summer in America.
I like the idea of installing it both inside the cover and also on a canvas engine cover, so in effect there would be two layers. A lot simpler to build those than to create the hard fiberglass box.
This page discusses converting, although I've never done it, just used a dBA test unit. There is one chart in the middle of the page that shows some frequency conversions.
Decibel A, B and C