Annoying cabin noise can be a deal-breaker for car buyers. Indeed, acoustics is a major factor in perception of quality, and many vehicles are marketed based in part on the quiet comfort of the interior. With so much riding on cabin quietness, the hit-or-miss method of shoving insulation into vehicle cracks is a thing of the past at most car companies. Rather, major automotive OEMs rely on acoustic specialists, often Tier 1 suppliers that provide particular sound insulation materials as well as the expertise in silencing bothersome noises quickly with minimal weight and cost—often while retaining smooth-running powertrain sounds that drivers are familiar with.
One of the global leaders in meeting these tough engineering challenges is International Automotive Components (IAC), a manufacturer of instrument panels, floor consoles, ductwork, and miscellaneous interior trim as well as door panels, overhead systems, complete flooring systems, and acoustics.
IAC’s NVH lab in Plymouth, MI, rivals capabilities at many car makers. Nearly 100,000 ft³ (2830 m³) in size, the environmentally controlled hemi anechoic dynamometer chamber is one of the largest in the auto industry for in-vehicle acoustic testing and passby noise studies. An advanced sound transmission loss suite—with a reverberation room on one side and a quiet receiver room on the other—is used for evaluating sound absorption of material samples and acoustic transmission through components and car sections. For smaller tests, an alpha cabin, impedance tube, and/or transmission loss tube are employed.
One of the most recent additions to IAC’s portfolio of NVH technology is the LMS Test.Lab High Definition Acoustic Camera. IAC was the first company in North America to purchase the tool. The device is a circular array of 36 or 54 microphones with a wide-angle camera for quickly identifying the location of sound sources up to 20 kHz. Output includes a color-coded sound pressure map superimposed on a photograph of the full surface of the test specimen—a full vehicle, major subsystem, or individual component. Sound fields can be visualized through animation as a function of time and/or frequency to clearly show acoustic hot spots.
At distances of 1 m (3 ft) or more, the device gives a far-field acoustic overview of the structure using beam-forming technology in which spatial resolution is proportional to sound wavelength and closeness to the array. If greater resolution is needed, near-field measurements can be taken simply by moving the antenna closer so the distance to the source is 30 cm (11.8 in) or less. A built-in laser measures this distance, and the tool automatically switches to the near-field high-resolution localization mode, resulting in more than double the precision of traditional beam-forming techniques.
Extremely high speed is the major advantage of the technology over other methods to identify noise sources. The tool provides engineers with a relative value of sound levels, which can then be measured more accurately later using an intensity probe for more detailed study if necessary.
“The LMS Test.Lab High Definition Acoustic Camera enables our engineers to easily identify noise sources in a few minutes compared to hours and days needed to do studies with an intensity probe,” said Timothy Carson, IAC NVH Lab Manager. Areas of red on the sound pressure map show concentrations of acoustic energy where insulation materials are applied and measurements quickly retaken until sound levels are satisfactory.
“Using the acoustic camera in this manner as a fast diagnostic tool, we can quickly pinpoint noise sources, evaluate numerous sound abatement alternatives, and find an optimal solution—all in the same day and often with the customer right by our side to see results firsthand,” Carson said. “The process provides tremendous insight into vehicle acoustic behavior, radically shortens turnaround times on acoustic projects, and significantly strengthens our value-added services by finding noise-reduction solutions at the least cost to customers.”
In one recent major development project, a prototype dashboard from an automotive OEM was studied with the LMS Test.Lab High Definition Acoustic Camera to benchmark the level of noise emissions through the various pass-through holes for the steering column, pedals, and other parts. Various thicknesses and shapes of different types of material were then evaluated to reduce sound levels to a minimum. The complete project from the start of testing to release of the final prototype to manufacturing took only two months—one third the time otherwise required without using the acoustic camera.
Likewise, IAC engineers were able to visualize sound emission levels from an air duct and devise a solution to lower noise by 4.2 dB. The work was done in less than a day compared to a week or more using other testing methods. In another application, acoustic hot spots pinpointed the crank pulley and accessory drive as sources of a troubling front-end noise, and engineers lowered sound emissions by 2 dB using a polyester fiber cover—all in just half a day vs. at least three days typically needed.
John Krouse, Senior Contributing Editor, LMS International, wrote this article for Automotive Engineering International.