Helmholtz Resonance-
the concept of this resonator is quite complicated when trying to understand how sound and frequency works, but the steps involved are fairly simple. here's how:
1. you will need two variables: the drone RPM, and # of cylinders your engine has.
2. divide the RPM by 60 to get the RPS, or revolutions per second.
3. divide the # of cylinders by 2 to get # fired per revolution.
4. multiply RPS by # fired per rev to get the drone frequency in Hz.
5. divide the speed of sound by the Hz. the speed of sound is a constant 1,260 ft/s. the number is funky because sound travels faster in warm air, and the air coming from an exhaust is very hot.
6. take the result from #5 and divide by 4.
7. take the result from #6 and multiply by 12 to get the resonator length in inches needed to cancel the drone at that RPM.
here's an example:
1. my car drones the worst at 3,300 RPM and has 4 cylinders.
2. 3,300 / 60 = 55.0 RPS
3. 4 / 2 = 2
4. 55.0 * 2 = 110 Hz
5. 1,260 / 110 = 11.45
6. 11.45 / 4 = 2.86
7. 2.86 * 12 = 34.36 inches
from the result shown, my car needs a resonator of approximately 34.4 inches to add a damper that is tuned for my drone.
Hope this helps.
Interesting.
In step #5 above
a questionable assumption is made
to ignore how the speed of sound in a gas varies with
temperature,
pressure,
and what the gas is a mixture of ( various percents of nitrogen, carbon dioxide, water vapor, oxygen)
Maybe 1,260 feet per second is “close enough.”
In past Moparts posts on “Drone” we have discussed “stubs” for reduction
instead of resonators.
Another post above points out that whether the air/fuel mixture is rich or lean affects Drone.
This is also saying: rich or lean affects the speed of sound of exhaust gas.
Drone will change if you climb a steep hill at constant speed. (pressure of exhaust gas goes up)
Drone will change if you vice grip a large socket into your exhaust pipe tip. (pressure again)
