I’m sitting in the Shelby GT500 prototype with Tom Teknos, who leads the noise, vibration and harshness team for Ford Performance vehicles. The windows are down, the 5.2-liter V-8 rumbling softly in the background. He unlatches the door.

A loud drone fills the cabin.

He’s only moved the door a fraction of an inch, but there’s a distinct change in the sound level.

Cracking the door, Teknos explains, disables the Shelby’s active noise-cancellation system. What I’m hearing is the unpleasant hum that Ford’s painstakingly filtered out.

Using an array of microphones, the noise-cancellation system listens for specific unwanted tones and uses the Mustang’s stereo system to produce opposing sound waves to cancel out the drone. Much like a good pair of noise-canceling headphones, they can cut sound out without requiring more insulation from the outside world.

It’s one of the newest tools in Teknos’ arsenal, fighting to make sure GT500 owners hear that signature Mustang V-8 burble without encountering obnoxious, unpleasant, or unstable sounds—a tool he didn’t have when he started, 20 plus years ago, at the Blue Oval. Over the years, he’s helped to design the sound and vibration characteristics of a lot of Mustangs, the 2005 Ford GT, the mighty Raptor, and many more Ford products.

It’s a job that requires hours on the simulator, thousands of real-world miles, and a team of sound engineers that can make music out of the mishmash of explosions in an engine bay.

And the dark art of engine harmonics is why a careful ear can distinguish V-8s from Ford, Mercedes-AMG and Ferrari. They’re all going for different things: Ford wants a burble, AMG wants a growl, and Ferrari wants a scream. Even if you start with the same basic engine layout, you have to make sure your powerplant sounds unique.

It starts with the orientation and firing order of the engine itself. An inline-six will have smoother harmonics than the equivalent V-6, while a cross-plane crank V-8 will be much gruffer than its flat-plane crank analogue. Nowhere is that more apparent than in the Ford Mustang Shelby lineup, where the GT350 uses a flat-plane crank and the GT500 goes cross-plane.

The harmonics differ between the two setups. "Primary harmonics" are the sounds generated by the cylinders firing; "secondary harmonics" are non-combustion sounds, like rushing intake air or spinning metallic parts. A flat-plane engine fires one bank of cylinders after the other, a balanced order that produces strong primary harmonics with limited secondary-harmonic back chatter. A cross-plane engine, however, fires one bank, then the other, then the first bank twice, and repeats the pattern in mirror image: Left-Right-Left-Left-Right-Left-Right-Right. It's the pattern that gives American muscle cars their trademark burble.

It's a big difference, but it doesn't come simply. NVH teams are unlikely to convince the accountants that a new engine design with a unique firing order is necessary to meet the target sound. But they can change a lot of the plumbing between the engine and the exhaust tips to get the right audio profile.

The GT350, for instance, uses the same basic flat-plane-crank engine design as the Ferrari 458. In fact, Teknos and his team benchmarked the 458 during the exhaust design process. When they finished the first mockup, they loaded the engine specs into a computer simulator without messing too much with the exhaust. They had done it: the GT350 almost perfectly mimicked the best sounds Maranello had to offer.

Except, this was a Mustang. While it was impressive that they nailed the exhaust note of an Italian exotic, they realized that wasn’t what Mustang buyers wanted. They needed to bring back that signature burble.

So they did something that would shock and appall anyone tuning their WRX: they got rid of the equal-length headers. With an equal-length setup, the primary harmonics arrive at the same time. They’re all stacked, arriving together and combining into one wail.

For the Mustang, they wanted to space out those exhaust pulses to mimic the chattering sound of a cross-plane-crank V-8—one that happens to offer the free-revving, screaming insanity that only a flat-plane design can deliver. The result is the raucous sound of the GT350, one of Teknos’ proudest accomplishments.

The soon-to-arrive GT500 had the opposite problem. A supercharged, 5.2-liter cross-plane V-8 generates a lot of secondary harmonics. NVH teams control the noise that comes off an engines in four ways: exhaust, intake, air and body structure. While sound deadening can kill a lot of the secondary harmonics of the last three categories, a cross-plane V-8 wants to send a lot of sound through the exhaust.

One of the ways to counteract this is by allowing "crosstalk" between the two cylinder banks, via an X-shaped or H-shaped segment of the exhaust system. Primary harmonics arrive at this junction simultaneously, allowing their energy to combine and continue down the exhaust system. Secondary harmonics, however, tend to cancel each other out, due to their lower energy sound waves and their staggered arrival at the crossover.

Should unwanted noise continue past the intersection, NVH teams can also add exhaust resonators. Simply put, a resonator is a perforated tube within a larger tube, designed to allow certain sound frequencies to pass through while canceling out other, unwanted sound waves. The perforations also generate friction, which scrubs energy and helps reduce unpleasant high-frequency sounds.

Having traversed the headers, the exhaust junction, and the resonator, sound waves now arrive at the muffler. They’ve been aligned to create the desired timing, scrubbed of high-frequency tones, and cleaned of unnecessary mutterings. The sound itself is good, it’s just loud—too loud, it turns out, for street-legal regulations.

Enter the muffler. The exhaust system component you’re probably most familiar with, it mainly functions to bleed some energy off the sound waves to produce a much more docile exhaust note. In performance cars, though, we still want to be able to crank the volume up to 11. That’s why cars like the GT500 have active exhaust systems.

When the exhaust baffle is fully open, spent gasses bypass the muffler entirely. As mentioned, that kind of roar isn’t something Johnny Law will permit on public roads. That’s why you’ll have to put the GT500 in Track Mode to get the full thunder. After all, no buyer would dare use Track Mode on a public highway, when it’s so explicitly labeled "for track use only." Right?

If you seek to stay on the right side of the law, though, the Mustang's "Quiet Mode" seals off the loud route and sends exhaust into an expansion chamber surrounding the pipe. The waves will bounce around the chamber, with only a small perforated tube leading to the exit. Between the expansion—which cools and slows the gasses—and the perforations, enough energy is scrubbed off to make the Shelby usable around town.

While two-mode exhausts are nothing new, this latest generation of active exhaust control allows for more variability. If Quiet Mode is too muted and Track Mode attracts unwanted police attention, the Shelby's "Normal" and "Sport" modes partially open the valve to preset amounts. That way, some airflow escapes undeterred while the rest takes the long way through the expansion chamber. Listen for yourself:

Despite all these tricks, you can't always tune every unwanted sound out of an engine. Be it complex secondary harmonics or just a weird groan, every engine has its quirks. That's why, as we listen to a Shelby GT350 crossing 6000 RPM, Teknos smiles.

The 2018 model, he explains, had an annoying droning around 6000 RPM. At his recommendation, 2019 versions got the active noise-canceling system he showed off in the GT500. Now, you can cross 6000 RPM without hearing any change in character.

That the drone ever existed is probably something most drivers wouldn't have noticed. In fact, most aficionados probably don't realize just how much work and design goes into achieving a car's particular sound.

Even after listening to Teknos, my first thought upon hearing a downshift wasn’t about the intricacies of cross-plane vs. flat plane, or whether the powerplant sounded more Maranello or Dearborn. Instead, the Shelby pinned me to my seat and all I could think was, "holy hell, what a masterpiece."

Job done.

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