Resonated tips will quite down the exhaust tone just a bit, kinda like a mini muffler at the end of your exhaust tips.
Example... Flow Master 50 series shouldn't require the use of any resonated tips. 50 Series muff are quite.
40 Series muffs may require the use of resonated tips due to the loudness of the 40 series.
I had regular straight tips when I had my 40's installed. The drone was so bad at highway speeds my brain would vibrate, I had really thought I made a huge mistake on my exhaust. I've always used F/M 40 series, but that was always on V8's. This was the first non-V8 that I installed a set of F/M 40's on. Not really wanting to spend the money to buy a new set of F/M 50's, I decided to try a set of resonated tips. They made all the difference in the world. They quited down the tone just enough to eliminate the cabin drone and my brain doesn't vibrate when I go on road trips.
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Bobby
Died 5/1/08, resurrected 5/15/08
ASCD SD hood, 17" Bullitts, Overkill PCM, 3.4" Pulley, NGK TR55IX, K&N 9" cone filter, JimmyC D/P, U-bend removed, Flowmaster 40 series, Hooker big bore tips with resonators, S/B brake lines, drilled/slot rotors, GMPP handling kit, KYB struts, F&R strut supports, HID's, LED's, 2.5" Air Dam, red GTP overlays on custom badges, white rear insert.