Noise experts tell commissioners Project Tango’s modeled contribution will fall near existing ambient levels; low‑frequency concerns debated

County commissioners heard two acoustical experts explain how Project Tango’s cooling and mechanical equipment were modeled and what residents might hear at receiving sites. Mark Bastash, a board‑certified acoustical engineer, said the team used ISO propagation standards and a three‑dimensional acoustical model fed by equipment sound‑power levels to predict off‑site levels. "In general, the existing sound levels fluctuate around 50 dBA," Bastash said, and the model predicts about 45 dBA or less at the western edge of Arden and at the school — levels he said are similar to normal office or business sound levels and within county code limits.

Dr. Norm Bridal, a low‑frequency specialist, explained C‑weighted metrics used to capture low‑frequency sound and said he had applied his published criterion to the project. He reported predicted C‑weighted results in the mid‑50s dBC at the school but noted that existing ambient dBC levels at the school can be higher due to roadway and plant noise; comparing to measured ambient levels, he concluded the project would not create new health risks at the predicted levels given the site’s setbacks and building design.

Commissioners repeatedly asked whether a representative "hum" or low‑frequency sample could be played for the board; the experts said replicating site‑specific propagation in the chamber is difficult but that they could measure or compare ambient levels. To illustrate, the acoustician used a calibrated sound level meter during a 10‑second quiet period in the chambers and reported 43.9 dBA and 59.8 dBC as a point of reference. The applicant has proposed conditions that include pre‑ and post‑construction monitoring and binding noise limits to prove compliance; commissioners requested the specific noise condition language that staff placed in the add/delete Exhibit C1B in the packet.