Mercury Marine builds innovative acoustic testing facility

Mercury Marine commissioned Albert Kahn Associates to design and engineer a new noise, vibration and harshness test facility

By Peter G. Lynde, PE March 11, 2021

When an industry leader in the marine segment decided it needed a worthy noise, vibration and harshness (NVH) test facility — one that exceeded product development goals across a broad range of products — engineers at Mercury Marine quickly learned that designing a state-of-the-art facility to satisfy rigid acoustic noise and vibration criteria and facilitate collaboration among product development and engineering staff would not be easy.

Rather, the new facility had to overcome several challenges:

  • Undesirable low-frequency vibration levels from heavy vehicle traffic on nearby interstate highways, as well as snowplow activity in campus parking lots.
  • The same traffic, together with construction activity, facility service access roads and truck docks and small aircraft or helicopter flyovers created unpredictable, unavoidable exterior noise events that could impact the indoor test environment.
  • An unprecedented level of flexibility was needed to accommodate the entire Brunswick product line — test sources with a wide range of physical sizes and sound level signatures.

Establishing ground truth

The hemi-anechoic Plant 12 Sound Lab, located at the Mercury Marine PD&E Center in Fond du Lac, WI, had served Mercury Marine well for more than a decade. It was a cornerstone of the company’s leadership in NVH testing and development of marine products, particularly outboard and sterndrive motors. However, its use had become limited due to the relentless evolution of product technology and motor horsepower. To improve the NVH quality of its prop to helm marine propulsion solutions and test its future product mix, Mercury Marine commissioned Albert Kahn Associates (Kahn) to design and engineer a new NVH test facility.

The new facility needed to consolidate NVH testing and support areas to optimize team efficiency and promote collaboration between product development and engineering teams. Support areas are located around the test chambers with space for engineers and technicians to comfortably perform various duties. Nearby, a larger office space was incorporated with floor-to-ceiling windows, infusing a combination of workstations and an open collaborative area with natural light.

Figure 2: Rendering of the test facility lobby. Courtesy: Albert Kahn Associates Inc.[/caption]

Mitigating undesirable vibration

Marine product testing must replicate the open water environment to best simulate real world NVH characteristics. To do this successfully, the test environment must be controlled and external sources of noise and vibration are problematic.

Engineers suspected that low frequency vibration levels, caused by multiple sources including heavy vehicle traffic on nearby highways, and snowplow activity in campus parking lots, were evident in the soil. The facility’s structure could conduct these vibrations into the hemi-anechoic test chambers, potentially impacting the noise and vibration test results; engineers needed to confirm whether this would indeed be a problem.

The design team expanded a typical preconstruction geotechnical investigation to include ambient ground vibration measurements. That testing confirmed their suspicions: Engineered systems would be required to block these unwanted vibrations from disrupting NVH testing in the new facility.

Figure 4: Computational fluid dynamics (CFD) analysis optimized the depth and shape of the reservoir to mitigate unwanted turbulence and wake formation from the engine props’ thrust. Courtesy: Albert Kahn Associates Inc.[/caption]

In turn, the coiled steel springs in the vibration isolating mounts are placed between the foundation walls/piers and the underside of the test room floor. They isolate the test room floor and acoustic room structure from any remaining unwanted ground-borne vibration.

Meanwhile, the foundation walls bear the load of the test room concrete slabs — 6-inch-thick steel-reinforced concrete designed for added stiffness to mitigate the potential for floor resonance.

The acoustic panel test chamber and its steel structural frame are supported on the perimeter of the floor slab, where the slab transfers its dead and live loads through 42 steel-coil isolation mounts. Mount loadings vary considerably and range from 2 kips to nearly 12 kips [A kip is a U.S. customary unit of force. It equals 1,000 pounds-force and is used primarily by architects and civil engineers to indicate engineering loads where the pound-force is too small a unit. Although uncommon, it is occasionally also considered a unit of mass, equal to 1,000 pounds, i.e., one half of a short ton.]. Steel spring isolating elements afford 90% isolation efficiency from disturbing frequencies of 8 Hz and higher.

The high mass afforded by the foundation mat and walls mitigates the transmission of unwanted ground-borne vibration into the structure. To further attenuate unwanted ground-borne vibration, a three-foot band of sand material, in lieu of the native clay soils, was used for back fill around the entire depth of the perimeter foundation walls.

Mitigating exterior noise events

Exterior noise events from construction activity, truck noise from the nearby highways and roadways, as well as small aircraft or helicopter flyovers, could easily produce ambient noise levels in excess of 80 dBA. Because these events are inherently unpredictable, even careful test scheduling can’t mitigate them.

With test room noise floor targets nearing NC-10 [NC is the abbreviation for noise criterion. The NC rating can be determined by plotting the measured sound pressure at each octave band. The noise spectrum is specified as having a NC rating same as the lowest NC curve which is not exceeded by the spectrum.], a minimum sound transmission class of STC-56 was necessary to afford effective reduction of unwanted sound from surrounding site activities. Research into precast concrete products found panels offering sound transmission class ratings as high as STC-61.

With precast concrete a preferred choice — and part of the existing campus architectural vocabulary — the decision was made to construct the exterior perimeter walls with precast insulated concrete panels to prevent the unwanted external sound sources from reaching the indoor test environment.

Room-within-a-room configuration ensures the maintenance of targeted background sound levels. With ground-borne vibration a concern, and test room isolation essential, the inner test room was best constructed of metal acoustic wall panels to significantly reduce loading on the vibration isolation mounts.

Mercury selected Eckel Acoustics to supply the acoustic test rooms and perforated metal anechoic wedge system to line the interior. Eckel furnished and erected the test room as a complete assembly with structure, panels, wedges and doors, guaranteeing specified acoustic performance parameters were met.

Flexible test room sizing and ventilation

Originally conceived as a test facility exclusively for Mercury products, Mercury NVH Engineers advised the project team that Brunswick, Mercury Marine’s corporate parent, had stipulated the test rooms be configured for NVH testing of the entire Brunswick product line — a line that covers outboard and sterndrive marine engines, electric trolling motors and various other marine parts and accessories.

This required an unprecedented level of flexibility to be designed into the test room infrastructure to accommodate a very wide range of acoustic test sources, both in physical size and sound level signature.

Conventional approaches to sizing hemi-anechoic test rooms generally assume the need for sound pressure level measurements in the free field of the test source. The recommended size and configuration of the test source for this facility was established by Mercury to allow the majority testing of the Brunswick family product line.

Review of the products and their physical parameters yielded a source envelope measuring 90 inches long by 40 inches wide by 70 inches high. In addition to the test source envelope, room sizing was also made considering multiple guidelines, including:

  • Compliance with mandatory requirements of applicable ANSI/ASA S12.55/ISO 3745 standards
  • Established industry practice
  • Benefit of experience gained on previous projects
  • Allowable clearances and access requirements around the applicable test source
  • Wavelength at room cutoff frequency
  • Requirements associated with room ventilation systems.

The ANSI/ASA S12.55/ISO 3745 standard offers guidelines for determining the size of hemi-anechoic rooms. Interior room dimensions are a function of test source size, radius of the measurement hemisphere and distance to the reflecting plane based on the lowest cutoff frequency.

These parameters yielded a room with internal dimensions (wedge tip-to-tip) of 38 feet long by 38 feet wide by 19 feet high at the minimum desired room cutoff frequency of 60 Hz. However, the resultant square room does not satisfy industry practice for room proportions and could be subject to undesirable standing waves. Accordingly, room dimensions were adjusted to fall within guidelines as shown in Figure 5.

HVAC considerations

Size wasn’t the only factor in designing flexible test rooms. HVAC system engineering — specifically, the ability to scavenge exhaust from operating engines — also needed to be considered.

Marine outboards discharge engine exhaust underwater through the hub of the propeller when operating off idle. When idling, exhaust is discharged through bypass ports above the waterline. Ventilation systems needed to safely remove exhaust gases in both modes of operation, as well as manage the heat released from a wide range of engine products.

A single-pass 100% outside air HVAC system is used to ventilate the test room and mitigate CO and HC emissions released during active engine testing. The system is configured with three operating modes: setup, test low and test high.

  • In setup mode, systems operate at their lowest flow rates, providing the minimum amount of ventilation make-up air to the continuous scavenge exhaust system while moderating the test room temperature to its design setpoint.
  • Test low and test high modes are used with active engine testing, with increasingly higher ventilation rates used to manage added heat loads from increased engine horsepower.
  • Mercury testing engineers select the operating mode based on multiple testing parameters, including engine horsepower and anticipated thermal cycling.

With the test room constructed over a water reservoir, controlling temperature and relative humidity to required tolerances proved an added challenge to a task already made difficult by the single-pass, multi-step ventilation system. Custom air handling units were configured with several heating and cooling features to allow these challenging conditions to be met.

In addition, the combination of high airflow rates and single-pass ventilation demanded specialized temperature controls given winter to summer temperature gradients characteristic of Wisconsin.

With the facility’s stand-alone nature and desire for energy efficiency dictating the use of natural gas, the heating system uses three direct-fired gas burner sections arranged in a 1/3-2/3 split and equipped with 30:1 turndown control valves to allow maintenance of ±2 F variance from heating setpoint.

This same setpoint tolerance was required when operating in cooling mode and resulted in the use of direct expansion refrigeration for cooling cycles with two condensing units sized at 1/3 and 2/3 total system capacity. Each condensing unit stages multiple compressors to match capacity with test room heat loading.

Finally, fuel supply systems are interlocked with the stepped HVAC control, allowing fuel solenoids to open only when HVAC is in low-test or high-test mode and staying closed when in setup mode.

Mercury Marine provided measured sound spectra from its marine product line to establish a desired noise floor of approximately NC-10 (21 dBA) in the test rooms. During commissioning of the hemi-anechoic test chamber, background sound levels with ventilation systems off were measured at 16 dBA. The team engineered a test chamber that exceeded their requirements to accurately test the full spectrum of products.

Outside the test facility, general building HVAC systems optimize energy efficiency and provide Mercury with a comfortable indoor environment. High-efficiency (95%) condensing boilers generate low-temperature hot water (100 F to 120 F based on OA reset schedule) for use in both variable volume terminal unit reheat coils as well as in-floor radiant heating.

The low-temperature hot water allows reduced system equipment complexity and simplified control technology while eliminating the need for heat exchangers and mixing valves. Self-regulating variable speed hot water (HW) heating pumps keep energy consumption low.

Partnering efforts were of foremost importance on this project. Representatives from the engineering, construction and supplier communities worked together to ensure its success. The uniqueness of this facility proved inspirational to many suppliers and constructors, who engaged in preliminary engineering and coordination efforts with enthusiasm. This proved to be significant as their experience helped the entire team foresee problems and plan in advance for their resolution. On Dec. 6, 2018, Mercury Marine unveiled its new NVH Technical Center in a grand opening event. It now stands as a testament to the art and science of engineering a world-class test facility.

Author Bio: Peter G. Lynde, PE, is senior vice president and director of research and technology at Albert Kahn Associates Inc.