So tell me what engine you have and if you have the stock exhaust header or an aftermarket one - short or long -- tuned equal length? Is it a 2 1/2 outlet to muffler pipe or 3 inch out? Does it go straight out the back like
@Rttoys? or does it exit to the drivers side like some of the turbo cars? Do you want a single exit muffler pipe or a center twin, or outside twin exits? How much quieter do you want it? Do you want high pitch (rice rocket sound) or a low rumble old school hotrod sound?
Intakes also cause noise situations. The Dual side draft Weber carbs on my Birkin make more noise sucking air in than the exhaust. Listen to some videos of European hill climb cars and those bike carbureted, individual throttle body (roller and slider actuated ones too) cars make some screaming noises (they also run at high RPM).. People take notice when the Birkin is running. It also produces a different total sound.
Mufflers are just like engines and can be tuned to a certain sound, but it might take a few different muffler builds or combinations to get you there. And the way you drive also is a factor. Easy on the throttle and it will be quieter than always flooring it.
You don't want to impede air flow and create too much back pressure. You don't want to big of piping either.
There are many different configurations you can choose -- auger type, perforated cone type (
@Desert Sasqwatch), Perforated pipe (straight through or a few internal to the muffler body (( those usually chambered as well )) with or without deadening wraps, baffled type like Flowmaster. There are ones with half circles (Spintech) and there used to be a Wave muffler (internal washboard baffles). The old dimpled side pipes of years gone by. There are also disc type inserts like the cone insert. You also have to consider the engine RPM range you normally drive for exhaust tuning (you can always have a quiet street muffler with an open flow vacuum, electric or manual operation. (old school cutout type). Sine, Sawtooth, Square wave sounds create differences and Frequency/Hz ranges. Whew!!
Take for instance a 3.4 GM V6 (which is basically a 350 V8 minus two cylinders) and it sounds close to a 350 V8 if you muffler it right. The 3.8 GM V6 has a totally different sound and people have a disturbingly hard time getting it to sound good.
A few Goblin builders have created some visual works of art for exhaust and like you as a builder it was their individual preference.
Bottom line: If you find a solution that works for you, then make it happen. You are the one that is driving the Goblin, it is yours, and you apparently want to honor your neighbors sensitivities as well as your own.