Here is a blurb i kept in my bookmarks when I wondered why the E0series V8's had the donut tube on the air intake
"Resonant RPM: The airbox and inlet tube will resonate at a certain RPM. This will allow it to pass air better at this RPM and increase power there. In an airbox system the airbox itself acts as an air spring. The mass of air in the inlet pipes act on this air spring. The system resonates and passes air easier when the inlet pulses are timed just right (similar to pushing somebody on a swing. It is easy if you time the pushes just right).
Resonant RPM =19,100 *square root ((116.5 *Inlet Cross-Sectional Area))/(Air Box Volume)(Inlet Pipe Length) this is for a 4 cylinder 4 cycle engine."
If you are mad keen on understanding the science go here
http://motorcycleinfo.calsci.com/Airboxes.html