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im pretty sure the glasspacks which i have by the way are a form of resonation. straight pipes are not. removing your rear resonators and replacing with some bb glasspacks really loosens up the sound but you give up a small bit of low end. i hope this helps.
 

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The way I understand it, all mufflers have some degree of resonator behavior in them, by virtue of being a constrained air mass in the body of the muffler with an entrance and exit of distinct diameter. Naturally, the muffler handles more tasks than just being a resonator, but those are the chief aspects that give it a mild resonator behavior. The muffler typically has more elaborate construction inside to achieve its primary tasks (internal baffles, channels, flow-paths, etc).

An actual "resonator" is mostly just a hollow shell with an entrance and exit (or maybe just a perforated, flow-through design) so that it can achieve a singular task of introducing a Hemholtz resonator effect in series or in parallel with the exhaust. Designs may vary across brands, but they generally are not built to fulfill the full application that a muffler does (muffling and high-temp durability of fresh exhaust). The case material is typically a thicker gauge material so as to ensure a stable tuning effect (think of it like a loudspeaker, where you want sturdy, non-flexing walls in the cabinet).

So what does the resonator effect do? It introduces an acoustic notch at a distinct frequency. The tuning of the resonator is important such that the notch frequency coincides right where the drone frequency of the exhaust system is. It cancels out that drone, so that you still get most of the sound of the exhaust system at other frequencies, w/o the drone frequency hammering you over the head when you cross a certain rpm.

Perhaps a chief functional difference between using a glasspack vs. using a resonator, is that the glasspack can serve as a mild resonator with some additional damping in effect by virtue of the glasspack media (hence, making the operation of the notch frequency a bit less pronounced, allowing a more "assertive" exhaust sound with some amount of drone getting through). The resonator is just a resonator with little or no damping. As the vids posted earlier demonstrate, the glasspacks also have their own distinctive sound (whether by the flexing of the glass media or just being a flow-through design that allows higher sound harmonics from the engine pass through, I dunno), with the rhythmic, high-pitched cracking sound.
 
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