The Art of Noise:
Designing Sound Characteristics
Into a Motorcycle Engine

Early on a cloudless July morning in America's Great Plains, a farmer, clutching his warm cup of coffee, walks toward one of his tractors as he begins his work day. Before he reaches his goal, he hears the crisp exhaust note of a motorcycle, and out of the corner of his eye he notices a single headlight cresting the hill of the U.S. Highway that runs adjacent to his field. Long before the motorcycle gets close enough for him to read the name on the gas tank, the farmer knows that he is sharing this early morning moment with a rider aboard a Harley-Davidson, simply because of what he is hearing.

The Harley-Davidson Motor Company has re-emerged as the leader in the heavyweight motorcycle industry because of countless stories of concise product recognition similar to this one. While improving the quality of their product line, the Motor Company has remained faithful to the styling cues that it was built upon, and in doing so has created an identity by which the rest of the cruiser genre is judged. Just what are some of these styling cues? Many of them are visual, like "Springer" front suspensions, peanut-shaped fuel tanks, and finned V-twin engines. And some are audible, for as Jon Thompson [1994] explains, "An essential element of the H-D equation is the charismatic feel and sound of the Harley engine--a sound one Honda spokesman characterized as, no kidding, 'potato-potato-potato!' "

While this may not sound like the most flattering of descriptions, it is one that Harley has embraced completely, and takes very seriously. It is a cornerstone of their marketing strategy, for even their product guide exclaims:

Potato, Potato, Potato. . . A Harley-Davidson at Idle. Has the wind ever carried a more satisfying sound?...The sound is unmistakable. The signature of a Harley-Davidson motorcycle... you instinctively know: this is the way a motorcycle ought to sound... This did not happen by accident... when we find something more pleasing than anything else, we stick with it. Like the exhaust note of a Harley-Davidson 45° V-twin, the sweetest sound ever carried on the wind. [Thompson, 1994]

Clearly, Harley believes that their V-twin exhaust rumble is instantly recognizable and discernible to the trained ear, and believes that it is a vitally important component of their strong product identity. Because of this, their design engineers are asked to create products that are technologically competitive and yet wonderfully nostalgic -- essentially, they are asked to design with an eye toward the future and an eye toward the looks and sounds of motorcycles long past.

Just how is an exhaust note designed to "sound like a Harley?" What parameters govern such a design requirement? Answering these questions is an everyday experience for some of the engineers at the Harley-Davidson Motor Company, and their effectiveness in garnering solutions to them helps ensure their product's continued success in the marketplace.



Design Parameters

The exhaust rumble that is created by a running motorcycle engine is a very complex sound, and it is one whose origin is equally complex. Beginning before the combustion process and ending with the muffling of the exhaust, the tone of the sound that is ultimately heard by the customer is one that has been influenced by a great number of parameters. Many decisions are made with a special cognizance of their effect on this tone, and when they are coupled together the result is, according to Harley, "the way a motorcycle ought to sound." While all of these design decisions contribute to this characteristic exhaust sound, only a small number of them are regarded as absolutely essential to it. Thus, a closer investigation of these critical ones is truly necessary in order to obtain a more complete understanding of the origin of this styling cue, this "Harley sound."

Carburation and Ignition.