Yellowstone County, MT Noise Ordinances: Decibel Limits (2026)
Key Facts
- Decibel threshold
- 55 decibels within 3 feet of unit
- Covered equipment
- AC, air handlers, backup generators
- Screening required
- Full height of the equipment
- Trigger zone
- Within 50 ft of residential zoning
- Enforcing office
- County zoning coordinator
- Penalty
- Misdemeanor, fine or up to 6 months jail
Summary
In unincorporated Yellowstone County, mechanical equipment on industrial property within 50 feet of a residential zoning district, including air conditioners, air handlers, and back-up generators, cannot generate noise above 55 decibels measured 3 feet from the unit without sound abatement. The Yellowstone County Zoning Code sets this limit in Section 27-602.F.3, alongside a screening requirement for the same equipment.
3. Mechanical equipment, including but not limited to air conditioning units, air handling units, back-up power generators, installed at ground level or on a roof must be fully screened from view. The screening shall be at least the height of the mechanical equipment. Mechanical equipment that generates or expected to generate noise in excess of 55 decibels within three feet of the equipment location must provide sound abatement or suppression, which may require the equipment to be enclosed in a structure.
Full Breakdown
F applies wherever an I1 (Light Industrial) or I2 (Heavy Industrial) use sits within 50 feet of a neighborhood or residential zoning district, including a planned development that allows residential use. 3 requires that mechanical equipment, including but not limited to air conditioning units, air handling units, and back-up power generators, installed at ground level or on a roof must be fully screened from view, with screening at least as tall as the equipment itself. The same subsection sets the numeric noise trigger: equipment that generates, or is expected to generate, noise in excess of 55 decibels within three feet of the equipment location must provide sound abatement or suppression, which the code says may require enclosing the unit in a structure.
9 adds a flat ban relevant to the same buffer: no outdoor announcement system or music system, permanent or temporary, is allowed on the industrial side of the line. 10 routes existing nonconforming equipment into Section 27-1100, Proportionate Compliance, so a roof-mounted unit installed before the rule can still be required to add abatement when the property redevelops or expands. The Planning & Community Services Department's zoning coordinator, designated the zoning official under Section 27-1701, administers Article 27-600. This decibel standard is specific to industrial mechanical equipment near homes; it is not a general county-wide noise ordinance covering traffic, construction hours, or residential-to-residential complaints.
Violations & Fines
Operating an unscreened or unabated unit above 55 decibels within the residential buffer violates Section 27-602.F.3, which Section 27-1702.A treats as a zoning violation for using land in contravention of the code. Section 27-1704.G makes the violation a misdemeanor: a fine up to the state statutory limit, up to six months in county jail, or both, plus costs, with each day of continued noise counted as a separate offense.
Frequently Asked Questions
Does the 55-decibel rule apply to a residential generator or AC unit?
How is the 55-decibel level measured?
What counts as adequate sound abatement?
Is there a general noise ordinance for the rest of the county?
Sources & Official References
Other rules in Yellowstone County
Compare Yellowstone County to another location·View the Montana noise ordinances overview
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