While developing a new hydrogen fuel cell module, Ballard Power asked BKL to assess the module’s noise levels and consult on possible noise mitigation options.
BKL visited Ballard’s facility and measured background noise and module noise to establish a baseline. Because the client had specified criterion, the goal was to find mitigation solutions that fit the module’s spatial and operational requirements and also kept noise levels below the target threshold.
During initial baseline tests, BKL found that the exhaust was the module’s primary noise source. Post-measurement analysis confirmed that exhaust noise exceeded the criterion by 9 dBA. For listeners, this difference could seem almost twice as loud. Since controlling the dominant noise source is the most effective way to reduce overall noise levels, BKL provided generalized noise mitigation recommendations and contacts for several suppliers of commercially available mufflers. The key was identifying a muffler that would provide enough mitigation while also limiting pressure drop. Yet most mufflers that limit pressure drop provide less acoustical benefit.
Testing showed that the module’s air intake was another significant noise source. After analyzing the design, BKL’s acousticians suggested several mitigation options including adding acoustical louvers and additional insulation on the module’s panels.
The client installed an intake silencer, designed in house, and off-the-shelf exhaust mufflers. The client also added materials—lead vinyl and closed-cell foam—to the module’s panels. The additional material provided two functions: increase the sound transmission loss properties of the panels by increasing the mass and absorb the sound inside the module with acoustically absorptive foam. BKL conducted a follow-up test and found that these initial mitigation efforts had a limited effect on noise levels. In fact, compared to BKL’s earlier tests, the unit was noisier. The data was clear: the standard mufflers weren’t doing enough to control exhaust noise.
BKL worked with the client to develop custom-built mufflers that would function within the confined space of the module, limit the amount of pressure drop, and prevent noise from exceeding the target criteria.
Once the client had installed the new mufflers and a full enclosure—previous iterations featured panels on only the top and sides—BKL returned to test the module again.
BKL’s acoustician also evaluated the test space to account for building reflections and other factors that could have contributed to the higher noise levels observed during the second test. For example, the testing location was near the Ballard warehouse to provide fuel for the fuel cell. By using an omni-directional noise source at both test locations (near the building and in the middle of the parking lot), BKL determined that there was less than 1 dBA difference between test areas.
Another challenge was presented by the noisy load bank used in the testing. BKL worked closely with Ballard engineers to take short sound measurements and ensure the load bank did not overheat, thus avoiding noise contamination from the load bank’s cooling fan.
Having ruled out the influence of reflective surfaces at the testing location, the acoustician set out to assess the effectiveness of each mitigation component: the custom-built intake silencer, new custom mufflers, and absorptive lining on the panels. To do this, BKL took 60 individual measurements, each with a different combination of noise control features.
After correcting for background noise, BKL found that the new mufflers, along with the full enclosure, significantly reduced the module’s noise levels, meeting the criterion.
Performing detailed noise measurements and collaborating with the client to design customized mitigation solutions, BKL demonstrated both its flexibility and acoustical expertise. BKL’s team of acousticians helped the client meet its goals for the project and contributed to the successful development of an innovative fuel cell module.