fast, here's a pretty good read on how it works: enoisecontrol
The article's written for deisel engine generators, but if you start at page 35, it talks about tailpipe design, tailpipe resonance and finally side branch resonator design. If you're into math, everything you need is there. I used this along with the autolounge calculator and it worked perfect. You'll need to measure or assume an exhaust gas temperature to get the correct speed of sound to plug into the formula...not super critical, but it does make a difference.

Basically, the length of the side branch resonator is 1/4 the wave length of the frequency you want to cancel. The wave traveling down the side branch resonator reflects off the closed end and is reflected back toward the tailpipe. When it arrives back at the tailpipe, it will be 180 degrees out of phase with the source wave.

 

DAMN.. not all of us(me anyway) are capable of the physics calculations and or the advanced math... maybe no advanced to you but come on.

 

32" seems to work well for me on both my xcabs, gas and diesel. And just did another yesterday on a xcab and worked well on it too.

 

ive used 22 magnaflows, a single 3 in/out on my old 08 and a single 3.5 in/out on my older 01, both single cabs with long tubes and no cats,

no drone whatsover..... simple

 

It's really pretty simple. First, you have to determine the frequency of the sound wave in hertz where you have resonance or drone. If it happens at 1800 rpm and you have 8 cylinders, 1800 x 8 = 14400. But since each cylinder only fires every other other rev, divide 14400 by 2 = 7200. This is pulses per minute, so divide 7200 by 60 seconds to get hertz (hz). 7200/60 = 120 hz. Now you have to find the wave length of the 120 hz sound wave at whatever your exhaust temperature is. If you assume exhaust temp is 200° F, sound velocity is 1275 feet/second and wave length is 127.5". The ideal resonator length would be 1/4 of the wave length, or 31 7/8". The reason behind the 1/4 length is, the sound wave bounces off the end of the resonator and when it returns to the tailpipe, it has traveled a distance 1/2 of the wavelength and is 90° out of phase with the sound wave in the tailpipe (sound waves are sine waves), thus canceling the sound. There are calculators all over the internet to help you find wave lengths and speed of sound at temperature. Bottom line is a resonator pipe somewhere between 30" and 36" will kill drone pretty much anywhere between 1500-2000 rpm.

 

I have been reading these posts and according to this analysis i made my DroneZONE is in the 287HZ range, it looks like.

Attached is the Frequency Analysis from Audacity.
I calculated 287hz or 275hz as the frequency desired to cancel.

speed of sound is 1072
((1072/287) / 4) * 12 = 11.2inches
((1072/275) / 4) * 12 = 11.69inches

What length should i go with? Is this correct?

Stock Dual Exhaust setup with the stock tail resonators removed and the mufflers replaced by 40 series Flowmasters.

2 inch to 2 1/4 inch range pipe.

4 Cats intact.

Any help would be much appreciated.

Frank

 

This should remain at the top of the 8.1 section as many of are fighting with this drone stuff. Great info in here.