How to build an exhaust resonator tube to eliminate drone
The concept behind drone is pretty simple; nearly everything has a resonant frequency...exhaust systems, electronic RLC circuits, even gravitation has resonance. At peak resonance, the amplitude of the wave at the resonant frequency shoots up to many times higher than normal amplitude. With sound resonance, it pops up when a certain frequency emitted from the engine resonates with the exhaust system and muffler to create that nasty in-cab drone. Unfortunately this resonance often has its peak right in the area where you want to cruise, between 55-75mph.
The solution is a 1/4 wave resonator tube. Simply put, it is a small piece of exhaust pipe with a flat capped end welded nearly perpendicular to the exhaust flow post-muffler that serves to take some of the resonant amplitude and shoot it back into the exhaust pipe out of phase with the resonant frequency to bring down the amplitude. If you've ever taken a physics class, this is in the 'standing wave dynamics' chapter. 
Mufflers are designed to bring down the amplitude or volume of the exhaust note at all RPMs, but they typically can't do anything about resonance. Corsa mufflers have a built-in resonator that works decent, but only with the entire exhaust kit. Sometimes you'll run across a setup that drones at an RPM the Corsa muffler and exhaust wasn't designed to combat, so theres a chance the internal resonator will do little to no good.
Things you need to know to build the resonator:
-RPM that drone occurs at
-Number of cylinders (engines with simultaneous firing cylinders like the SRT-10 truck will divide this number by two)
You will also need to know the speed of sound. It varies with temperature, but is generally accepted to be 343m/s.
Say your V8 truck resonates at 2000 rpms, right at cruising speed. You'll need to find the frequency of the drone. Units here are pulses per second, or Hertz.
f = RPM * pulses/rev * (1/60)
f = 2000 rev/min * 4 pulses/rev * (1/60)
f = 133.34 pulses/second = 133.34Hz
At 2000 rpms, your V8 fires 4 times per revolution. RPM is in minutes, so divide by 60 to reduce to seconds. Now you need to find the length of the wave at the frequency you just calculated. Wavelength is denoted by lambda (λ), units are meters.
λ = v/f = speed of sound / frequency
λ = (343m/s)/(133.34Hz)
λ = 2.572 meters
This gives you the length of a full sound wave. The idea here is to reintroduce a sound wave into the exhaust that is 180° out of phase with your drone frequency. To do this, you build your resonator tube at exactly one-fourth the length of the resonant wave. By the time the sound wave enters the resonator tube, bounces off the end and re-enters the exhaust stream, the amplitude is exactly opposite of the drone frequency and will lower or eliminate the volume of the drone.
Dividing the wavelength by 4 gives you 0.643m, or about 2.1 feet. Its long and kind of awkward, but someone who has had to deal with a droning truck on a road trip will likely sacrifice the space to make it fit 
The diameter of the tube you'd need is debatable, as I haven't tried different sizes of tubing. When I built mine, I took a shot in the dark and went with 2" pipe on my 3" exhaust. It gives about half the surface area at the end of the tube to bounce off of. I assume a larger resonator tube would allow more sound to bounce back and cancel more of the drone noise. However, space is somewhat limited when you are sending an exhaust pipe off of your existing pipe at some odd angle. You can bend the resonator tube to some extent, but it needs to be mandrel bent, not more than one bend or more than ~30°. If the bend is too sharp, the sound will bounce back too early and the resonator will work poorly or won't work at all.
The reason for this is simply the fact that sound waves do not necessarily 'flow' with whatever medium they are traveling in. Sound is a pressure wave, and any obstacle (bends) will return the wave prematurely. To minimize 'obstacles' in the resonator tube, the straighter it is, the better it will work. The intent is not to randomly scatter the sound wave as a radius will tend to do, but to send it back to the exhaust precisely when it is needed.
So, you just need to mock up your resonator tube in a place that it will fit, cut a hole into the exhaust pipe and weld the tube to it and enjoy a drone-free ride.
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Daze wrote:
