Project Grey Mustang 5.0: Part 6 – Drone Strike

Project Grey Mustang 5.0: Part 6 – Drone Strike with Corsa Performance

by Vince Illi

We’re gearheads.  Gearheads like sweet-sounding cars.  And because I’m a Mustang owner, I like my cars with a V8 rumble that is loud enough to frighten small children.  

OEMs employ acoustic engineers to design their exhaust systems on their vehicles.  They spend many hours with spectrum analyzers and slide rules designing an exhaust system that is the best compromise of longevity, performance, efficiency, and sound quality.

And then dumbass gearheads like me rip them all out in favor of a mandrel-bent, three-inch-diameter system with glasspacks where the mufflers used to be.

Read more about Project Mustang 5.0 here!

The resulting… experience makes it sound as though Cthulhu himself lives under the hood of my car and is about to awaken and take vengeance on all us mortals as the 5-liter V8 screams towards redline.  The problem is that I don’t live my life a quarter mile or autocross course at a time.  I also like to simply drive Project Mustang.  And it was LOUD.  At freeway speeds, centered around 2,000 RPM or so, there was a ridiculously loud humming—known as drone.  I never bothered to use a decibel meter to measure it, but suffice it to say that it was difficult hearing my radio on the Interstate, and forget about having a conversation with the beautiful lady in the passenger seat.

 

This cutaway shows an OEM muffler.  The circuitous route, along with all the baffles and chambers, tones down the exhaust and removes droning, but at the expense of higher backpressure and less awesome noise.

Those acoustic engineers I referred to earlier spend a long time developing the exhaust system (and intake system) to prevent that drone, which is caused by acoustic resonance.  Now, acoustic resonance is a very complex phenomena when applied to exhaust systems and is beyond the scope of this article (and the scope of my knowledge), but it boils down to this principle: when a certain frequency of sound (exhaust pulses) passes through the exhaust system of an automobile, it causes resonance.

Resonance can be thought of in terms of pushing a child on a swing.  If the frequency (number of events per second or minute) of your pushing on the swing doesn’t coincide with when the child is at the most rearward point of the swing’s arc, the child won’t swing much further, and you might get a kid’s shoe in the eye.  But, if the frequency coincides with when the child is at the most rearward position of the swing’s arc (the “natural frequency” of the swing or pendulum), the child will swing farther and farther.

 

This video lets you hear the 5.0’s stock exhaust that Ford’s acoustic engineers spent many hours developing, only to be ripped out by this dingbat engineer/journalist.     It has a nice tone to it, has minimal cabin noise, and no drone.  And don’t let the video fool you; it’s also too quiet.

Exhaust resonance is caused by a similar phenomenon.  The exhaust system and engine have a natural frequency.  When the exhaust pulses leaving the engine coincide with that natural frequency, it causes resonance, which causes the entire system to vibrate at its natural frequency, causing drone, and rattling your dental fillings.

The most obvious solution is to simply bolt the OEM system back onto the car and call it a day.  But I’m still a gearhead, and I still want a nice-sounding exhaust that’s loud when I stomp on the right pedal without causing my ears to bleed on the Interstate.  And the OEM system is too quiet, which is why I swapped it in the first place.

Fortunately, Corsa Performance had a solution.  Whereas most aftermarket exhaust companies simply bend some pipes together, weld them to some mufflers, and sell it as an exhaust system, Corsa takes the time to actually engineer and develop exhaust systems specific to each vehicle.  Corsa has been developing exhaust systems for over 25 years, starting with mufflers for high-performance marine applications and breaking into the automotive aftermarket with an exhaust system for the Corvette in 1998.  Corsa brought a NASA scientist onboard who had experience in acoustics and designed what they call Reflective Sound Cancellation technology, or RSC.  RSC allows the use of a straight-through muffler (which is what I had previously) with a drone-free driving experience (which is what I most definitely did not have previously).  At its core, RSC relies on destructive interference and acoustic waveguides.

Destructive interference is best illustrated by noise-cancelling headphones, like the ones made by Bose and Beats by Dr. Dre.  Sound travels in waves.  If you want to cover up or “mask” a sound, you add the same sound wave, but 180° out of phase (think flipped upside down) of the original.  The two waves cancel each other out, ideally resulting in no sound at all.

 

The image on the left shows destructive interference.  The two sound waves are “opposite” of each other, so when added together, cancel out. Source: Wikipedia

An acoustic waveguide is a tube or channel used to propagate sound waves.  An example is a doctor’s stethoscope, which is a waveguide that transmits the sound of your breathing or heartbeat up to a doctor’s ears.  Sound waves travel through the waveguide and leave the other end.  Or, if the other end of the waveguide is sealed, the sound waves will be reflected back toward the entrance.

 

This cutaway of a Corsa muffler shows the chambers on either side that act as waveguides. Source: Corsa

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