Nissan engineers reduced noise from the inter-cooler. They did not measure noise to reduce. They measured the function. The function of inter-cooler is to cool exhaust gases, also known as heat exchanging. It is done by letting hot exhaust gas go through many tubes with fins.
Input is input gas flow and output is the gas-flow velocity in tube.
For the study, engineers measured the relationship between input gas flow and gas-flow velocity. The ideal relationship between input and output is called ideal function and it is usually based on physics. They found design parameters that can reduce the variability of the relationship between input and output based on the ideal function. The heat exchanging efficiency was improved by 20 percent. One of the design parameters that contributed to the improvement was intake angle.