Helmholtz resonators (also known as anti-drone tubes) are an effective way to reduce undesirable frequencies at a targeted point in the RPM range when engineered and utilized correctly. Through calculations on factors such as number of cylinders, target RPM, and other factors you can determine the appropriate volume necessary to create the Helmholtz resonator. These “anti-drone tubes” can effectively reduce drone at a certain frequency without reducing desirable tone or volume like a muffler. The only downside to them is the added weight and reduction in space in which to craft the exhaust system itself, but the pros far outweigh the cons as you’ll be able to happily cruise on the highway yet still enjoy an aggressive exhaust note when you drive more aggressively. Have a look at our blog post here where we crafted the best possible 991.2 Carrera non-PSE exhaust system utilizing them!