Workgroup weighs noise standards for 24/7 data centers, considers CNEL and low‑frequency limits

At the Jan. 30 workgroup meeting, staff presented two principal options for addressing noise from 24/7 data center operations: a community noise equivalent approach (CNEI/CNEL) measured over 24 hours that adds penalties for evening and night periods, and a property‑line standard using both A‑ and C‑weighted scales to detect low‑frequency 'drone' noise.

Moderator summarized the CNEL approach and read draft language: "the community noise equivalent level at the boundary shall not exceed 55 dBA," with higher numeric allowances at boundaries not adjacent to protected uses. The CNEL method applies penalties (for example an added 5 dB for evening and 10 dB for late night) to reflect increased nighttime sensitivity. The draft also proposes an additional 5 dB penalty where a "prominent discrete tone" (low humming drone) is detected.

The group discussed a second approach that measures both the A‑scale (human hearing) and C‑scale (low frequencies). If the difference between C and A exceeds a set threshold (e.g., 15 dB), that would flag persistent low‑frequency nuisance and require mitigation before occupancy. Participants flagged rooftop and height effects — tall multi‑story buildings can project low‑frequency energy differently than single‑story sites — and asked staff to research propagation at elevation.

Staff described enforcement and compliance steps drawn from comparable ordinances: a post‑construction sound audit by an independent acoustical engineer (48‑hour measurement period at maximum cooling capacity), annual compliance reports, a 30‑day mitigation plan after a confirmed breach, escalating fines (e.g., $5,000 per day for tiered violations and up to $10,000 per day for major breaches in the example draft), and permanent continuous monitors after repeat violations.

Workgroup members expressed concern about enforceability under the county nuisance code versus a measurable numeric standard, and discussed whether to adopt an existing CNEL/CNEL model used by some municipalities or to use the current industrial code with added nighttime protections. Research presented by a participant noted Jay County (example model for rural communities) uses a 50 dBA daytime/nighttime metric in some drafts and Lake County uses 55 dBA at the property line, underscoring large variation among jurisdictions.

The group assigned homework: decide measurement approach first (how to measure — CNEL vs A/C property line), research rooftop propagation and barrier effectiveness for tall buildings, and compare enforcement strategies. Staff will return with comparative analysis and suggested numeric limits ahead of the next meeting.