I asked doctor google -
 
Resonators are straight through exhaust devices. They use a single perforated pipe which is surrounded by sound-deadening material. Resonators are designed to cancel out the sound and pressure waves before they leave the resonator body and enter the tail pipe. This device works just like the resonator chamber in the muffler -- the dimensions are calculated so that the waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust.
 
Matt

It all comes down to the quality, size and design of muffler...
I have a 2.5" side exit exhaust on the Autocross car that used to be very quiet and was well within legal limits, but it had a full Falcon sized muffler sitting under the drivers seat.  Said muffler had a hernia, and was replaced with a 14" long generic brand one recently which has increased the sound considerably, but it is a competition car so who cares too much.  Still it is CH plated and would be OK for the street.  I have seen 2" full length exhausts on a standard 2Lt Escort with no mufflers that was relatively quiet till given the boot.  But my 2.25" race car ehaust with full muffler, but no resonator is a little lound, but only when given throttle....
 
The autocross car (with the side exhaust) has been through about 5 or 6 mufflers in the 12 odd years I have been punting it around...  A single resonator was quite raspy, and spat flames out when doing motorkhana handbrake turns.... Full late model Falcon mufflers make it very quiet (even at full revs), but may be alittle restrictive (hence thre 2.5" system!), and the most recent 14" long muffler as a virtual straight through is quite raspy and a little noisey it it was a road car...