Taking the Measure of Automotive Noise Standards, Part 2

When auto manufacturers set out to create award-winning vehicles, much consideration is given to interior sound quality. Ironically, the manufacturers have been so successful in mitigating road noise they have inadvertently caused a new problem for themselves: Apparently, the cars are too quiet.

Noise levels inside the vehicle’s cabin are affected by everything, from obvious sources such as engine, transmission, and exhaust to the not-so-obvious—such as the seat adjustment motor.

As reported by The Wall Street Journal, “Ten years ago most interior noises couldn’t even be heard because of the engine and road noise,” says John Tepas, vice president of engineering at Mahle Behr Troy, the Michigan-based subsidiary of Mahle GmbH, an auto-parts maker that produces such components as heating, air-conditioning, and ventilation systems. Recent progress in damping those sounds, he says, has forced manufacturers to lower noise levels “even on little parts like the tiny motor that runs the vent door that opens and closes in a heater.”

To improve interior sound quality, engineers and project managers rely on a combination of psychoacoustic preference ratings, industry standards like ISO 3741 and ISO 3745, and modern anechoic test chambers.

Faurecia headquarters in Auburn Hills, MI. Photo courtesy Faurecia

Faurecia is an automotive subassembly supplier with a large share of its global sales being to premium automotive manufacturers. Some of Faurecia’s products include those motors that have heretofore been cloaked in road-noise anonymity. Perceiving the need to take its noise and vibration testing to the next level, Faurecia went directly to the Eckel Industries team to design and install a hemi-anechoic test chamber for the Automotive Seating R&D Center in its new headquarters in Auburn Hills, Michigan. It is the largest chamber in any of Faurecia’s worldwide locations and can accommodate an entire vehicle.

The design process began in June 2013. In February 2014, Eckel got the call to head to Michigan to install the chamber, which has been operational since March. Faurecia began conducting testing in April as it transitioned employees and operations to the new headquarters.

Faurecia’s Auburn Hills sound chamber under construction. Photos courtesy Eckel Industries

During production of new vehicles, automotive manufacturers rely on Faurecia to test their seat frames and motor controls to achieve specific noise and vibration goals. Accordingly, Faurecia required the new hemi-anechoic test chamber to meet the ISO 3745 standard for transmissibility, including a target base-ambient sound level below 25 dBA (which is somewhere between rustling leaves and a whisper). The Eckel chamber exceeds the ISO specifications: It’s certified at 17 dBA.

Faurecia sound chamber in action. Photo courtesy Faurecia

“Our team is well-accustomed to responding to client alterations and revisions to anechoic-chamber layout and plans,” says Eckel’s vice president Jeff Morse. “Circumstances often and understandably change, especially when one of our chambers is part of a larger, ‘ground-up’ new construction project. In this case, we were able to address Faurecia’s needs through web meetings that allowed face-to-face online interactions, and through onsite design review meetings with key personnel on both sides. It’s part of our job to adjust to changing client needs, and we’re all invested in the ultimate goal: a chamber that is exactly what the customer requires.”

Faurecia’s engineering director Robert A. Parmann agrees. “Eckel worked very well with us and with our general contractor,” says Parmann. “Their exceptional planning, organization, and collaboration helped to keep our total project on schedule. The final test chamber was completed on time and exceeded the ISO specifications and our expectations.”

Even more interesting than the on-spec sound levels is what is referred to as psychoacoustic and preference ratings.

“Sound pressure level can only tell us the level of the sound we hear,” says Christopher Kus, project engineer and NVH specialist for Faurecia. “We rely on psychoacoustics to more accurately depict how the human ear perceives sound quality. Psychoacoustics such as loudness, roughness, kurtosis, and sharpness are used in our internal method to validate the sound quality of our seat motions and features. It is imperative that background noise be eliminated so as to not alter the signals used to study and evaluate results. The preference rating method was developed by Faurecia in 2001 and is used to replicate how the human ear perceives and evaluates sound quality. Preference rating uses multiple psychoacoustic metrics to take the subjective nature out of sound evaluation and make it objective and repeatable. It is a reliable way to measure acceptability of sound, make engineering design decisions, and measure quality performance. Because preference rating is an accurate measure of sound quality, it can be used to manage our noise evaluations with the OEMs, metal plant, and suppliers. It is currently accepted by all our customers including GM, Chrysler, BMW, and Nissan.”

In part 1 of this series, I asked how loud is too loud, but in this case the question is how quiet is too quiet?

“Motors and actuators that Faurecia use emit sound pressure levels of 35-45dBA on average,” says Kus. “This is relative to a low hum in the background that is loud enough to let consumers know it is functioning, but quiet enough as to not disturb them. This can be compared to the volume of the vibrate feature on your phone. The point of diminishing ROI for sound level falls below the point at which the consumer notices the seat feature is being operated.”

The common factor in meeting ISO, preference rating, and OEM standards seems to be a state-of-the-art anechoic chamber where such esoteric measurements are possible.

Paul Riehle, director of Roush Industries’ Noise and Vibration Testing Lab, agrees. “As cars have gotten quieter there’s been more and more effort to reduce the background noise in chambers,” Riehle says. “Anechoic chambers have gotten quieter, and performance is better from the sound absorption coefficient within the chambers.”

Roush’s Lavonia, MI campus head quarters. Photo courtesy Roush Enterprises

Roush Enterprises is widely known in the United States as an automotive after-market performance icon. Perhaps less known is that Roush provides comprehensive, integrated development services, from design through prototyping, testing, and manufacturing. In addition to the automotive industry, Roush serves a wide variety of sectors, including aerospace, renewable energy, defense, and entertainment. Roush has established more than 50 high-tech development centers throughout North America, and its Lavonia, Michigan, campus is a leading supplier of noise and vibration control engineering and consulting services. In order to offer laboratory testing as well as acoustic dynamometer testing of vehicles, engines, transmissions, brakes, and components, Roush also turned to Eckel industries.

Photo courtesy Roush Enterprises

“Eckel built the acoustic components of our chamber in Lavonia, Michigan.” says Riehle. “The background noise in our chamber is very, very low, and we’re able to measure noise at lower frequencies. Interior components have gotten more durable as well.”

According to Riehle, most of Roush’s work doesn’t involve ISO, ANSI, or SAE standards so much as customer requirements.

“A lot of what we do is tailored to OEM and supplier requirements,” he says. “We also conduct a lot of testing strictly for data acquisition. The resulting measurements can then be analyzed as needed by our customer. Not to say that we don’t do any standardized testing, because we do. We test engine accessory noises, including a/c pumps, alternators, etc. A lot of hybrids and electric vehicles use electric a/c compressors, and we do testing of those to SAE standards. We actually worked with an OEM on that testing methodology, which was then developed into the standard that is now commonly used across the industry.”

Roush’s lab director added a wrinkle to this writer’s grey matter as he explained one more way the company used its anechoic chambers: for acoustic holography.

“Acoustic holography is a way of getting a picture of the way a sound is distributed across a surface” says Riehle, “It’s quite an involved process, but sometimes it’s difficult to determine exactly where a noise is coming from due to reflected or environmental noises.”

Microphone array. Photo courtesy Roush Enterprises

Acoustic holography image overlay. Photo courtesy Roush Enterprises

“We use an array of microphones set up in a matrix configuration, and from the signal processing of the different microphone channels we get a ‘picture’ of the sound,” Riehle explains. “We then overlay that image on the area of concern and can see where specifically that noise is coming from.”

All in all, these are fascinating technologies being employed in creative and practical ways. Although if acoustic holography was used to map noise anomalies in my daily-driver pickup truck, I fear the resulting image would resemble a kindergartner’s over worked finger painting more than a useful overlay.

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