Rethinking Aluminum for NVH Abatement

Engineers, abandon those mastics! New “quiet” materials solutions are at hand.

June 1, 2019

Lindsay Brooke

Among the various semi-anechoic chambers in MSC’s Canton facility is this four-wheel chassis dyno, which represents about one-third of the facility’s investment. The dyno has full thermal-environment capability, is in non-stop operation and is often used for brake-noise investigations.

Engineers who have witnessed aluminum-intensive vehicles being “uncloaked” in a full competitive teardown relate the same story: After you’ve pulled the carpet out and stripped it down to the naked bodyshell, the “band-aids” are clearly exposed—typically on the floorpan, bulkheads, on the rear package tray of sedans, around the wheel- houses, and within the noise-critical dash panel and cowl plenum.

Band-aids are a form of NVH triage. These bake-hardenable butyl materials, cavity-injected foams and the like are added to the body structure and exterior panels to dampen noise. Such countermeasures are often added late in development, after production tools are released and the vehicle on-sale date looms. But they’re also employed after production launch, as a solution to customer complaints about unacceptable resonances and road noise.

The NVH band-aids aren’t unique to light-metal structures — they’re the typical “fix it” solution on steel vehicles, too. They don’t come cheap, and experts say the mass they add to the vehicle can easily range from about 50 lb. (23 kg) on a smaller vehicle, up to 150 lb. (68 kg) on large utilities. Also, when countermeasures are large—dash panel insulators, for example—they’re unwieldy and labor-intensive in the assembly plant.

MSC’s Matt Murphy (left) and veteran NVH principal engineer Karl Karlson in one of the Canton R&D center’s acoustic chambers.

“We’ve seen mastic patches all over the car,” observes Matt Murphy, VP Engineering Services at MSC Corp. Speaking with Automotive Engineering at MSC’s Application Research Center in Canton, Mich., he explained: “Aluminum structures typically need more of them, and they’re thicker than the patches used on steel, because the aluminum — even in heavier gauge — doesn’t have the mass to provide adequate noise and vibration dampening.”

Murphy noted that the floorpan in a certain high-volume aluminum-intensive vehicle in the lab during AE’s visit measures 1.8 mm, a .6-mm increase over the steel floorpan in the previous-generation model. “To get the same stiffness as steel with aluminum, they had to up-gauge,” he said. “In this case the floor is much thicker—but the resonances and radiated noise that come off the aluminum are much higher than they were in steel.”

Part of a holistic design

During recent customer demonstrations at MSC’s extensive R&D facility, which it shares with Brüel & Kjaer (see sidebar and article, p.20), Murphy provided insights into the company’s Quiet Aluminum, an innovation designed for both stamped- aluminum applications and integrating into steel-intensive structures. Readers familiar with MSC’s Quiet Steel (see AE June 2017) will understand the product’s holistic design concept, although the two innovations have different material characteristics.

Both are designed to eliminate need for add-on NVH treatments for resonance damping. Like its steel cousin, the new Quiet Aluminum uses an engineered viscoelastic layer that’s laminated, sandwich-style, between two sheets of aluminum. Noise is damped by micro-level shearing action: the aluminum panels move against each other, very slightly, when they are excited by various frequency inputs, whether an engine’s idle shake (on a dash panel) or the wide frequency bandwidth inputs of a audio subwoofer mounted on a rear parcel shelf.

Quiet Aluminum development is expanding from body components to other applications, including powertrain pans (right) to replace castings and composites.

The laminate is formable and weldable, supplied in coils or blanked sheet, with a variety of coatings. Its polymer core layer (measuring roughly .025-mm thick) likewise can be tailored for specific customer applications aimed at mitigating / eliminating NVH under a wide range of operating and thermal requirements, Murphy explained.

Compared with a monolithic aluminum panel of similar geometry, the Quiet Aluminum panel offers up to 45% lower mass and a slimmer profile compared with AL that carries a mastic patch. Vehicle applications include those areas that are natural sources of NVH including cowl plenums, dash panels, floorpans, transmission tunnels, truck-cab back panels, package trays, wheelhouses, and seat storage tubs in SUVs and minivans.

“Cavity boom is an ongoing area of NVH focus,” noted Gary Newton of B&K.

Low-NVH materials harmony

Dig into the instrument panel of a current-generation Ford F-150 and you won’t find all variety of mastics and add-on treatments. Put a magnet to the panel and it will stick—evidence that MSC’s Quiet Steel is peacefully co-habitating within the aluminum-intensive cab.

“Our analysis showed that on this vehicle the combination of aluminum structure with aluminum dash panel did not perform at the previous [steel intensive] F-150’s NVH level, which had a Quiet Steel dash,” Murphy explained. “Going to aluminum, even with the treatments, wasn’t producing the improved NVH performance the truck’s development team was targeting.”

Murphy’s team discussed the option of using a Quiet Aluminum dash panel, but for various engineering reasons it would have needed to be about 4 mm (.157-in) thick—nearly plate-gauge stuff that was not optimal for stamping or laminating.

“So the decision was made to go with Quiet Steel for this essentially aluminum application,” Murphy related. “We overcame any galvanic-corrosion issues by applying a .001-in.-thick coating, made by Henkel, at the same time we laminate. The coating can be welded through so the brake booster bracket can be welded to it, but when it’s inserted it’s riveted and adhesively bonded. This allowed us to run it through the line just once. The Quiet Steel panel is 100% damped and has been on the current aluminum F-150 since launch, and we’ve been on F-150 since 2004.

EV quieting opportunities

To further expand its range of on-vehicle applications — in more complex geometries including U-shaped cowl plenums, C-tubs, and wheelhouses — MSC continues to bring greater formability into both of its popular NVH products. Murphy displays the deep section rear floor of a new EV that’s in development.

“Customers who are interested tell us they need data up front to prove it out,” he said. “Typically, first we’ll run some CAE, then a stamping trial – we send them some material and they put it through their dies. Then we build a fixture to simulate where it would be in the vehicle.”

He shows AE a monolithic aluminum panel from a customer vehicle, coated in NVH-fighting mastic, and taps it to produce a very resonant, metallic ring. Then he picks up the Quiet Aluminum version—same strength and stiffness — and excites it the same way — the sound is dull, dead, neutral. NVH engineer Karl Karlson has instrumented and tested both examples; he reports a 7-dB delta between them. “Engineers will fight for 1-2 dB,” Karlson noted.

Old-school thinking in vehicle development and body engineering has favored band-aids to mitigate NVH, Murphy and Newton explain. They note, however, that a number of EV startups have chosen to optimize their structures with the MSC Quiet laminate materials from the design stage.

“In terms of lightweighting a vehicle, the old-school approach is to spend months on the design, reducing cost, while realizing when the program is completed that there is an immediate noise and vibration issue,” Murphy said. “The only response is to go into ‘fire-fight’ mode and add mastic, weight and cost.”

They argue that MSC’s products, in conjunction with B&K’s simulator and modal tools, allow engineers to reduce NVH up front, well in advance of the prototype stage.

“Usually it’s at the fourth or fifth prototype stage where you get to an ‘NVH-ready’ prototype,” Newton noted. “So if we can help eliminate one or two protypes at a half-million-dollars each, there’s our prototype position.” He adds: “In today’s world, the Body guys and NVH guys have got to be on the same page.”