No. 2984: WHEN ENGINES ARE TOO QUIET

by Andrew Boyd

Click here for audio of Episode 2984

Today, itís time to make some noise. The University of Houstonís College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Itís happened more than once, and itís always unsettling. Driving through a parking lot, a pedestrian mindlessly steps in front of my car causing me to slam on the brakes. And it isnít a case of distracted walking. No cell phones or texting involved. What, I ask, are these people thinking?

The reason became all too clear when the tables were turned and I found myself stepping in front of a car. The culprit? The carís engine. Electric cars, or hybrids running on batteries, donít make much noise. People were stepping in front of my Prius because they couldnít hear it.

prius
Toyota Prius Plug-in Hybrid. Photo Credit: Wikimedia Commons

In December of 2010 this quiet problem led Congress to take action. The Pedestrian Safety Act was passed with overwhelming support, including a unanimous vote in the Senate. The three page act was quite specific, aimed at hybrid and electric vehicles traveling below whatís known as the crossover speed. Above the crossover speed, tire and wind noise are prominent. Below this speed, engine noise takes over -- except in the case of very quiet engines. The congressional act called upon the Secretary of Transportation to establish and enact minimum noise standards. Below the crossover speed, cars must be loud enough for pedestrians to hear and differentiate acceleration, deceleration, and constant speed. But, and hereís the interesting part, the sound itself doesnít have to mimic the sound of a conventional engine. Thatís offering a surprising new challenge for engineers: what to make cars sound like.

Hybrids and electrics arenít the only cars suffering quietude. Oddly enough, so are sports cars. Todayís high end sports cars are so refined, with purring engines and sound insulated passenger compartments, they lack an ingredient many people badly want — the sound of an engine revving through one gear after the next.

One solution is strikingly simple: make a recording of engine sounds and run them through the carís speaker system. The methodís actually proven quite effective, providing discerning drivers with the aural experience they crave. Further, it doesnít increase noise for passersby, and the cars can be designed with a noise shutoff switch — in case youíre in the mood for a quiet ride.

Mustang
Ford Mustang Boss 302 coupe. Photo Credit: Wikimedia Commons

Of course, the problem with this solution is that it seems like cheating, and many sports car enthusiasts feel just that way. So some manufacturers have created sophisticated mechanical means by which to capture engine noise, acoustically amplify it, and transmit it without speakers into the passenger compartment. Cheating problem solved.

Porsche
Porsche Panamera 2011. Photo Credit: Wikimedia Commons

As you walk the city or through a parking lot, keep an ear out for strange new sounds coming from the latest generations of hybrid and electric cars. Theyíre sure to provide an ever changing symphony.

Iím Andy Boyd at the University of Houston, where weíre interested in the way inventive minds work.

(Theme music)

K. Colwell. Faking It: Engine Sound Enhancement Explained. Car and Driver, April, 2012. See also: http://www.caranddriver.com/features/faking-it-engine-sound-enhancement-explained-tech-dept. Accessed January 7, 2015.

Federal Motor Vehicle Safety Standards; Minimum Sound Requirements for Hybrid and Electric Vehicles. From the Federal Register website: https://www.federalregister.gov/articles/2013/01/14/2013-00359/federal-motor-vehicle-safety-standards-minimum-sound-requirements-for-hybrid-and-electric-vehicles. Accessed January 7, 2015.

The Pedestrian Safety Enhancement Act of 2010. From the govtrack website: https://www.govtrack.us/congress/bills/111/s841. Accessed January 7, 2015.

This episode first aired on January 15, 2015.