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No. 2966:

Today, we pipe down. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them. 

If you've ever been awakened by a motorcycle in the middle of the night, you may have asked yourself, why are motorcycles so noisy and cars so quiet?

man riding a red motorcycle
flickr image

To understand the reason, we need to take a closer look at a sophisticated component of the modern motor vehicle: the muffler.

A major source of engine noise is the release of exhaust from an engine's combustion chamber. It's a lot like popping a balloon. High pressure exhaust is allowed to quickly escape into the lower pressure exhaust pipe. The exhaust and sound wave travel down the pipe, much like air and sound move through a trombone. But the goal of a motor vehicle's exhaust system is to muffle the sound, not amplify it.

car parts
Photo from Wikipedia

black and white graphic

Graphic from Wikipedia

Mufflers use sound absorbing materials, but the materials can only do so much. A more significant function of the muffler is to reflect the sound waves in such a way that they cancel each other out. In this sense, the muffler is like a well-designed recital hall. It's built to provide the best possible acoustics, in this case, for one particular soloist: the car's engine. 

A side effect of mufflers is that they slow the flow of exhaust making it harder for the engine to vent gas. The net result is a small loss in engine performance. So performance enthusiasts often prefer glass packs — mufflers that don't restrict the flow of exhaust. Glass packs achieve this by relying entirely on sound absorbing materials, not acoustic chambers.

motorcycle exhast
public domain image

And that's the reason given by many cyclists for replacing factory installed mufflers with noisy substitutes — performance. Motorcycles come off the factory line no noisier than cars, thanks to noise limit regulations established in 1980 by the EPA. Detailed laws, however, were left to the states. Only fourteen states have no explicit guidelines for motorcycle noise. Others, like California, have carefully outlined laws. Seven states forbid modifying the muffler to make it noisier than the original.

So why all those noisy motorcycles roaming the streets? Social norms and lack of law enforcement. With today's engineering, the effect of quiet mufflers on engine performance is negligible. For many bikers, glass packs simply provide that special sound — the sound of biking. And in most parts of the country, noise regulations aren't enforced. With so many other concerns, and with passive public acceptance of the noise, why open a can of worms?

In time, we'll likely hear fewer ear throbbing motorcycles. Manufacturers, including Harley-Davidson, have called upon bikers to pipe down. Perhaps it's time to set aside raw power and embrace a new measure of performance: the orchestrated performance of an acoustically refined muffler.

I'm Andy Boyd at the University of Houston, where we're interested in the way inventive minds work.

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Notes and references:

Most muffler systems are passive — they don't adjust themselves during operation. However, newer, more expensive, active systems that share similarities with noise reducing headphones are being developed. 

M. Gibbs. Deafened by Hogs: Motorcycle Noise and the Environment. Forbes, October 31, 2012. See also: Accessed August 19, 2014.

Loud Pipes' Cost: Harley-Davidson Tries to Quiet Motorcycle Noise. From the Motorcycle Cruiser website: Accessed August 19, 2014.

Motorcycles. From the Noise Off website: Accessed August 19, 2014.

K. Nice. How Mufflers Work. From the How Stuff Works website: Accessed August 19, 2014. 

This episode was first aired on August 21, 2014.