AFAIK the measured decibel level is only part of how the passengers perceive exhaust noise.
High frequency noise is supposedly much more irritating, causing headaches, tiredness, loss of concentration, etc. while low frequency noise at the same pressure level is more tolerable.
From that, I would guess that a manufacturer would go to some trouble to minimize high frequency noise.
In general simple decibel reduction can be done by shapes and baffles (there are literally thousands of patents), but the simplest way to reduce sound without power loss is very large muffler volume, especially cross-sectional area.
