Cat-back Exhaust System
An exhaust system is usually tubing used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:
Cylinder head and exhaust manifold
A turbocharger to increase engine power.
A catalytic converter to reduce air pollution.
A muffler (North America) / silencer (Europe), to reduce noise.
Cat-back (also cat back and catback) refers to the portion of the exhaust system from the outlet of the catalytic converter to the final vent to open air. This generally includes the pipe from the converter to the muffler, the muffler, and the final length of pipe to open air.
HKS Cat Back
JDM Cat Back for Nissan skyline R33
Cat-back exhaust systems generally use larger diameter pipe than the stock system. Good systems will have mandrel-bent turns that allow the exhaust gas to exit with as little back pressure as possible. The mufflers included in these kits are often glasspacks, again to reduce back pressure. If the system is engineered more for show than functionality, it may be tuned to enhance the lower sounds that are lacking from high-RPM low-displacement engines.
A muffler (or silencer or back box in British English) is a device for reducing the amount of noise emitted by the exhaust on most vehicles. On internal combustion engines, the engine exhaust blows out through the muffler. The internal combustion engine muffler or silencer was originally invented by Milton O. Reeves
But you might have seen some cars with loud muffler sound its so common because its no longer used to decrease the sound but let the exhaust gases go without any problem.
HKS universal Muffler