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My 2013 Civic SI Sedan's exhaust had a really bad drone in the range of 2000-2500 RPM, and upon some research I came across what is known as a quarter wave resonator, or commonly referred to as a J Pipe. No where online could I find a through explanation of the engineering behind why this modification works, so I decided to make a YouTube video explaining everything. Here is a supplemental write-up.


What is a J Pipe/Quarter Wave Resonator?

A J Pipe is welded onto your exhaust, typically in the rear of the car near the muffler, and branches out in the shape of a J. The pipe is closed at the end after it branches out of your exhaust.

This pipe is calculated to be 1/4 wavelength of the frequency of your exhaust drone. Hence the name "Quarter Wave Resonator"

To understand the way this works to eliminate drone, we have to understand the concepts of wavelength, frequency, and phase shift.
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The wavelength of a sound wave is expressed as the distance of one full cycle of the wave. In this picture above, it is expressed as the distance between the two peaks.
Now, a wavelength can also be expressed in degrees; whereas a full wavelength = 360 degrees.
Therefore, a 1/4 wavelength = 90 degrees.

So how does making the pipe a 1/4 wavelength cancel you exhaust drone?
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When the exhaust gases from your main chamber enter this J Pipe, they travel a distance of 1/4 wavelength into the pipe, and 1/4 wavelength back into the main chamber. For a total distance traveled of 1/2 wavelength, or 180 degrees.

So what you are essentially doing, is creating a 180 degree phase shift between the sound wave in your main exhaust chamber, and the sound wave coming from the J Pipe, re-entering back into the main exhaust chamber.
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The reason this has the effect it does is expressed in the figure above. You can see the red wave (representing your j pipe) is 180 degrees out of phase of the blue wave (representing your main exhaust chamber).
Notice how the amplitude of these two waves is completely opposite one another. That means if you take the average of these two waves at any point, the amplitude is zero.
So you are essentially 'cancelling out' your drone by creating a 180 degree phase shift in the sound wave that you want to eliminate.


Now, you may be asking... How do I know what frequency my exhaust drone is at?

There are two ways you can find this.
  1. The first, and best way is by downloading the mobile app called 'Spectroid' which will actually measure the frequency spectrum of your exhaust. Mine happened to be at 70Hz, as shown below.
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2. The second method is calculating this frequency based off the cylinder configuration of your vehicle. In a four cylinder engine, there are 2 pulses per revolution.
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By choosing your specific RPM, you can calculate the frequency of your exhaust drone, as shown above.
Now, I will say this method isn't as accurate as (1) because every 4 cyl engine sounds different. This is a generalization made, and may not be the same for all engines. Take a 4 cyl Subaru boxer engine vs. a 4 cyl Honda engine. They sound completely different. Therefore, I would highly recommend following option (1).

So once you know what frequency you have to eliminate, you need to find the operating temperature of your vehicle. You can do this be measuring the temperature of the gases at the exit of the muffler
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I used an infrared thermometer directly at the muffler exit to measure my operating temperature. Do this after the car is warmed up of course.

Once you have your temperature, calculate the speed of sound as a result.
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where Tc is temperature Celsius.
The speed of sound for me, at 350 degrees F, is 437 m/s

Once you know the speed of sound and the frequency, you can calculate wavelength.
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REMEMBER TO DIVIDE THIS NUMBER BY 4 TO GET 1/4 WAVELENGTH.

And that's it! Take your measurement to a shop to have them install a J Pipe at your specified length. Mine turned out to be about 60" long.
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So, I didn't want you guys to have to make all these calculations on your own, so I made this handy tool to aid you in your calculation.

I hope with this information, you can begin to understand the process, and background behind how this works. For a comprehensive explanation, please see my YouTube video linked at the top of this thread.

I'd be happy to answer any question you have below! Thanks everyone!
 
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