Engine Exhaust Sound and Its Suppression

There are two basic passive means of quieting the pressure pulses of waste gas being released during the exhaust phase of piston internal combustion engine operation. One is self-cancellation of sound, and the second is dissipative—the equivalent of screaming into a pillow. As sound travels through myriad tiny passages, sound pressure is changed into heat as it forces its way through them.

First, and more effective, is a box filled with quarter-wave tubes, each tuned to a different range of frequencies. A quarter-wave tube is open at one end, closed at the other, and of a length equal to 1/4 of the sound wavelength to be quieted. A sound wave is a pressure oscillation—positive-negative-positive, etc.—that propagates through the air. As the positive-pressure peak enters the tube and is reflected unchanged in sign (i.e., still positive), that part of the wave that is half a wavelength behind the pressure peak arrives at the open end—and it is the negative peak of the wave. The reflected positive wave and the just-arriving negative wave cancel each other.

This is how vehicle manufacturers can tailor the sound character of engine exhaust to suit the market by letting through the frequencies that add up to a specific “sound message.” Want to kill irritating high frequencies, but let through expressive thunder with just a hint of raspberry? No problem: Tune your quarter-wave tubes to cancel the unwanted frequencies so that what comes through is your “designer sound.”

The familiar dissipative sound reduction is to line an exhaust pipe with perforated metal, and to fill the space between it and the outer shell with tightly packed fiberglass roving. As many of us know through experience, exhaust energy soon breaks the fibers and blows the bits out the open end, causing sound to increase over time. Uh-oh, here comes the guy with the sound meter! We disassemble the muffler(s) and tightly repack with fiberglass. According to the old AMA, “Less sound equals more ground,” but according to NASCAR, NHRA, and certain kinds of popular music “Pinning the decibel meter sells tickets.”

I’ve noted elsewhere that social problems are difficult for engineering. The guys and gals from Styling wave focus-group results saying, “Don’t even touch those Sportster mufflers! They are pillars of that bike’s character.”

The engineers earn zero points by saying, “The bigger the cylinder, the bigger the muffler has to be.”

“No way. Get it done within those hallowed 1957 dimensions.” Anybody remember what happened to Triumph sales in 1971 when it killed its image with a volley of thoughtless changes?

To meet sound within such small volume, Sportster mufflers had to be made very restrictive. So much so that a humorist said, “After you switch off, you hear a long sigh from the mufflers as the last of the exhaust seeps out.”

Erik Buell wasn’t having any of that. He saw a big space under the engine and that’s where he put his muffling apparatus—plenty of volume for even the biggest cylinders—without putting a cork into performance. Manufacturers around the world have adopted his idea.

If you have plenty of electric power and don’t mind extra expense, another way to kill unwanted sound is to electronically generate an anti-sound to add to it, produced by a specialized loudspeaker in the exhaust.

The nature of an engine’s exhaust process is responsible for its sound. The sudden opening of a two-stroke’s cylinder-wall exhaust port, or the equally sudden opening of the exhaust of a peripheral-valve four-stroke Wankel engine, generates a very steep wave front containing a lot of energetic high frequencies.

Here we have to invoke the mysteries of math and Fourier’s Theorem. Old JB showed that any wave form, of whatever shape, could be represented as the sum of many pure-and-graceful sine waves of various amplitudes, added together. Harley-Davidson’s 1980s program “We’re killing the noise to keep the music” employed Fourier analysis to identify sources of high-frequency noise (click-click of cam-gear backlash, panel vibration, pistons changing sides in their cylinders) so they could suppress them. Then they could legally let through more of the deep-and-mysteriously-powerful motorboat sounds.

For years Wall Street theorized that there could be undiscovered regularities (i.e., waveforms) in the market’s mysterious ups-and-downs. Might they have predictive value? Whosoever understood this mystery would become rich beyond imagination. Hot-shot math graduates were hired to look for them, and Fourier analysis was a primary tool.

If you compare the exhaust opening rates of two-strokes with those of four-strokes, you discover that the two-stroke’s piston is moving at its peak speed as it begins to uncover the exhaust area. A four-stroke’s exhaust process is glacially slower, for its exhaust valves are motionless as the process begins. The two-stroke’s exhaust wave front is very steep; that of the four-stroke is gradual. The two-stroke’s square-wave sound front hits us with high-frequency, high-energy components.

I was at Fuji Speedway in Japan one day in 1972, when a Wankel-engined race car went out for a few laps. Loud beyond description.

On the other hand, high sound amplitude at lower four-stroke frequencies also makes us hold our heads. Stand close to Vance & Hines’ Suzuki GS revival in Pro Stock Motorcycle as its throttle is blipped: The air becomes the hammer that strikes from all directions.

When MotoGP brought back four-strokes to the premier class, many hoped for “megaphone music” like that of classic multicylinder race engines of the 1960s. They were designed with equal firing intervals that were easy on powertrains. MotoGP engines are like PA announcements at the airport—loud but incoherent; their unequal firing intervals are designed to maximize tire grip.

Finally, a pinch of realism: An engine’s power strokes are completely silent.