resonator is part of some muffler, as far as I know, and it's the baffled type (OEM).

SILENCER is another part to quite your muffler.
Apex'i Silencer



Straight-thru Design:


"Straight through design" mufflers are the most common import mufflers today. straight thru muffler are "glass-pack" (a fiberglass sheet wrapped around perforated tubing).
This will cause a relatively low suppression life. Carbon will attach to the only exposed side of the fiberglass mat and create a sound reflective surface instead of a sound absorbing surface. Also, most of them do not use any stainless steel wool to protect the fiberglass from burning off, which makes the exhaust louder. These types of muffler will result in a short muffler life.
Baffled/Chambered Type (Most OEM)

Most of the OEM muffler and Aftermarket mufflers for normally aspirated vehicles (non-turbo) use baffle type systems. This construction uses baffle plates to bounce the exhaust noise around inside the muffler to suppress the sound. This is a very effective way of suppressing the sound but at a high engine speed, the baffle plates becomes restrictor. The restrictor will cause high exhaust pressure and it will be difficult to gain peak power. Also, some of the baffle plates are not very resilent so it can start to rattle and produce an unpleasant exhaust sound.
Inner Silencer Type (i.e. some Apex'i muffler)


Some aftermarket mufflers today have insufficient sound suppression so they include an inner silencer. Most of these silencer have inside diameter of 35mm ~ 45mm (1.6 ~ 1.75in). This becomes a restrictor and it can cause a 5 ~ 7% power loss. Almost same as stock! It really defeats the purpose of installing a performance exhaust system. Also by having the additional silencer will increase the cost of the system. This is the reason why we do not offer these type of mufflers.
Now, look at GReddy exhaust, a very nice piece similar to Prodrive design.


I hope this enlighten some of you...