I bought a Wensn 30-130 Decibel sound meter off fleabay. I use it to record sound levels in front of my face in flight.

 I first tested my RV-8 after installing a turn down tip, but was also experiencing  severe exhaust leaks from my turbo: 19.7 DBa 

After fixing the leak, I recorded 16.8 DBa

Then I squeezed the turn down tip to a flattened rectangle opening, and replaced the lost area with 1/4" holes, which I bent to face backwards with a Philips screwdriver. a Piccolo Pipe, as it's known. Kind of a cheese grater for sound waves. Next reading: 14.6 DBa

This is still to loud for my worn out ears, so I bought an Aeroturbine 2525 muffler, as described on this forum. I've been collecting 304 stainless pipe bend, ball joints clamps and hangers. I hope to get something cobbled together this weekend.

I, too, am looking forward to your report on how well it works.

Suggest checking what's going on with your dB readings, though. 16 dB is quieter than a bedroom at night, & likely inaudible to those of us who have been flying for a decade or two. Realistic levels expected in the cockpit of a homebuilt will be between 85 dB (pretty good, but still loud) & 120 dB (loud rock concert levels and painful to most of us).

I'd also suggest using the C scale instead of the A scale. The A scale is very commonly used, but it presents a very distorted view of the actual sound level. It radically filters the low end of the spectrum, and to a lesser extent, the high frequencies, as well. It will mask the level of your actual noise exposure, especially at the low end of the noise spectrum.

Charlie