Interior noise reduction: attacking sound at the source

Automotive Industries,  July, 2003  by Brent Haight

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Customers are beginning to make greater demands for quieter automobiles, and because of this, automakers have begun to attack sound inside the passenger compartment with greater ferocity. Thanks to advanced engineering capabilities and improvements in materials technologies, what used to be thought of as an uncontrollable source of unwanted sound is now fair game in the quest to reduce interior noise. From front and side windshields, to tire wells and the engine compartment itself, nothing has been left unexplored when it comes to finding new ways to ensure a quieter driving experience.

"There is an increased sensitivity toward noise, from a personal standpoint and a societal standpoint," says Daniel Maguire, technical director--active systems, at Cooper Standard. "You see increasing links between noise and vibration and fatigue, noise and hearing loss. There is a technical interest coming about, and you also have personal interest. In the very high-end vehicles especially, it's clear that the companies are devoting significant effort in the development of the vehicle to minimize the noise and vibration harshness that exists in these vehicles. And that is something that is of late. When you look at a high-end vehicle and consider the amount of attention and money that is spent on the vehicle's ride quality and NVH characteristics, it is significant and very well representative of the importance that you see."

Cooper Standard's product line includes passive and active vibration control components that are intended to reduce both vibration and the noises created by the vibration within the vehicle.

"Our entire product line goes to vibration control, but then noise control as well," says Maguire. "The reasoning is that if I'm sitting in a car and the engine is running, because it is a piece of rotating machinery, it is creating some broadband noises, but more significantly it is creating a lot of tonal noises at varying frequencies. I'm sitting in my vehicle and I'm hearing noise and that noise is propagating through the firewall and up to me. The vibrations that are caused from the engine are propagating their way through the mechanical system. The vehicle is essentially a collection of devices that are waiting to be excited by particular frequencies of energy and they are going to make noise."

Maguire compares it to a pebble in a pond. "You have engine noise and muffler noise, but then also noise that is created by vibration of the engine and muffler. It's that composite that creates the whole spectrum of noise that drivers and passengers are subjected to."

Cooper Standard's goal, according to Maguire, is to isolate the engine from the rest of the vehicle, in part because of direct connections to the occupants of the vehicle, but also because "everything in the vehicle is a musical instrument waiting to be excited. The less energy that is making its way through the vehicle, the less excited the structures."

Cooper Standard isn't the only company that is targeting the engine as the main source of noise and vibration. Headquartered in Europe, Carcoustics Inc. offers blow molded engine encapsulation as a solution for quieting the passenger compartment.

"We are trying to provide noise control directly at the engine so it's not just noise control that controls noise down the line just before it's into the interior," says Gordon Ebbitt, manager of advanced acoustics at Carcoustics. "We are not trying to absorb noise once it's inside the interior, but trying to reduce it closer to the source.

Ebbitt is seeing a lot of people ask about high temperature materials that can be put directly on the engine. "When you have something making noise, all of the energy is concentrated into that one spot. And then as it propagates away from there, it takes up more and more real estate as that noise propagates. If you want to get the noise where it is all concentrated, you have to go directly to the source."

Carcoustics offers acoustics throughout the entire vehicle, including the engine compartment, the interior and the trunk.

The company is currently exploring the wheel wells and the body board for additional noise reduction within the passenger compartment.

At the wheel well, Carcoustics has added an acoustical feature to the wheelhouse liner. "Wheel tire noise is an issue," says Dirk Hoffman, CEO of Carcoustics Inc. "If you compare European wheel-well systems to the U.S. systems, in the U.S. you have more or less injection molded, non-acoustical wheel house liners. In Europe, that same wheel house liner, thanks to Carcoustics, adds acoustical features that reduce road tire noise, water spray noise in rainy weather and also reduces noise from stones and other debris coming up. The principle is, by going closer to the noise source, in this case, the meeting point of road and tire, we absorb those noises there."

More popular in Europe, Carcoustics acoustical wheel well is being used in the U.S only in the rear of the Ford Focus, according to Hoffman.