So after showing a Jeep Rubicon how to rally over snow banks, I found my factory exhaust snapped and hanging.....the only solution was, of course, a full exhaust upgrade. After which the car sounds great...but on the first road trip the 80dB+ drone at 100hz was killing my wife and I. She puts up with all my loud cars....but this one is her’s and it’s the car we use for longer trips now.

So being a bit of an Audio nerd I knew a Helmholtz resonator could fix it While searching for a way to calculate that I found something even simpler. A 1/4 wave tube, AKA J-pipe, Side branch resonator. The basic idea is the pipe is 1/4 of the wavelength of the frequency you are trying to eliminate. In my case this was 100hz. So my side branch resonator is 34" long. The results are downright amazing. No change in exhaust volume outside, but between 2500 - 3300rpms there’s no ear-rattling drone in the car. In fact, on the freeway, while cruising it sounds stock.


Find the drone frequency use an RTA app or use this:

RPM x engine pulses per RPM x 1min/60sec, a 4 cylinder engine a Subaru boxer has 2 pulses per RPM


In my case it was: 30000 rev/min x 2 pulses/rev x 1/60 = 100Hz

See that bump at 102Hz that’s what we are trying to fix. Note this is not a calibrated mic it’s just the one on my phone I didn’t feel like bring my laptop and full Room analysis setup so I just used a cheap phone app. Basically to confirm my rpm based calculations.


Next: wavelength = Speed of sound/ frequency

Wavelength: 1125.33 ft per sec / 100 Hz = 11.25 ft

Resonator length: 11.25 ft / 4 = 2.81ft of 33.75"

Note the speed of sound changes with temperature and pressure.....but the side branch resonator won’t have any exhaust gasses in it so it won’t get much hotter than outside temp on average. Since everything is relative, the length should work as temps change.


So far the results are amazing, I was able to have a phone conversation with my Bluetooth. I’ll try to post an RTA graph of the after I thought I save a screenshot but I didn’t....the hump at 100hz is completely gone. Next half of this road trip is tomorrow and Sunday I’ll try to get screenshots of results then.

Outback all cleaned up.