Generator Noise Level by Distance Table: Outdoor dBA Estimates

Generator noise decreases by approximately 6 dB each time the distance doubles, provided the generator behaves approximately as a point source in an open, reflection-free sound field. The table below estimates outdoor sound pressure levels from an illustrative rating at 7 meters—approximately 23 feet. It does not predict noise inside a bedroom or establish compliance at a property line. Cummins describes the same approximate distance relationship in its generator-installation acoustics guidance.

Generator Noise Level by Distance Table

Choose the column matching your generator’s documented A-weighted sound pressure level at 7 meters. The 60, 70, and 80 dBA starting values are illustrative, not measured ratings for particular models. Each subsequent value is calculated using distance attenuation only, with the generator’s load, configuration, and direction toward the receiver assumed unchanged.

Calculation source: OSHA Technical Manual, Section III, Chapter 5: Noise, updated July 6, 2022, Appendix B.6. The table applies its free-field distance equation to A-weighted reference levels, consistent with Cummins’ approximate 6 dBA-per-doubling guidance. Values are rounded to one decimal place for reproducibility, not to imply that field predictions are accurate to 0.1 dB.

These are estimates of the generator’s contribution alone. They do not include background noise, reflections, barriers, ground effects, weather, or additional sound sources. Actual sound levels may therefore differ, particularly near buildings or over longer propagation distances.

Calculation Method

For a point source under free-field conditions:

[
L_2=L_1-20\log_{10}\left(\frac{r_2}{r_1}\right)
]

Where:

• (L_1) is the known sound pressure level at the reference distance, in dBA when using A-weighted input.
• (L_2) is the estimated sound pressure level at the new distance, using the same weighting.
• (r_1) is the reference distance from the source.
• (r_2) is the distance from the source to the receiver.

Both distances must be positive and use the same units—meters or feet. The equation depends on their ratio, so either unit works. It models geometric spreading, not the effects of walls, enclosures, or changing operating conditions.

Check the Rating Before Using It

Use a documented sound pressure level at a stated distance, not simply a number labeled “noise level.” Confirm:

• The exact generator model, enclosure, and exhaust configuration.
• The load or operating mode during the test.
• The reference distance and measurement method.
• Whether the result represents one measurement position or an average around the generator.

A sound power level, often identified as (L_W) or (L_{WA}), is not a sound pressure level at a receiver and cannot be inserted directly as (L_1). Cummins’ Sound Attenuation and Test Strategies for Generator Set Installations, dated April 10, 2018, explains these distinctions and the importance of matching test conditions.

If the documented reference distance is not 7 meters, use the equation with its actual distance rather than treating the table as applicable unchanged. Do not use this simplified relationship close to a large generator where its separate sound sources cannot reasonably be treated as one point source.

Worked Example: Noise at a Property Line

Suppose a generator’s documented sound pressure level is 70 dBA at 7 meters, and the relevant property-line receiver is 20 meters from the same source reference. Assume unchanged operation, comparable receiver direction, and approximately free-field propagation:

[
L_2=70-20\log_{10}(20/7)=60.9\ \text{dBA}
]

The estimated generator contribution is therefore about 61 dBA at that receiver. This is a calculated example, not a measured result for a specific generator.

Before comparing it with a local noise limit, establish the required measurement location, operating conditions, time period, and sound metric. A single overall dBA estimate may not answer a requirement involving frequency-band limits or a specified site-test procedure. Cummins specifically identifies receiver distance, frequency requirements, background noise, and test conditions as relevant to generator sound assessment.

Property Lines and Bedroom Walls

A Property-Line Estimate Is Not a Compliance Result

A nearby wall can reflect sound toward a receiver, while a suitably designed barrier can reduce the direct sound path. Generator orientation also matters. The distance-only table assigns no fixed correction for these effects; use site-specific acoustic assessment or measurements when the decision depends on the actual level at a boundary.

Background noise must also be considered. Decibel levels are combined logarithmically, not by ordinary addition, so the table’s generator-only estimate is not necessarily the total level a meter will read.

Outdoor Noise Does Not Directly Predict Bedroom Noise

The calculated level at the distance of a bedroom wall is not the level inside the bedroom, nor does it include sound reflected from the façade. Indoor noise depends on transmission through the wall, windows, and other paths, as well as the sound’s frequency content and the room’s acoustic response. There is no universal “subtract this many dB for a wall” adjustment. OSHA’s noise-control guidance distinguishes sound transmission, absorption, and receiver treatment rather than treating them as a single distance correction.

Do not select a generator location from acoustic distance alone. For an actual installation, have the installer check the exact model’s installation manual and applicable local requirements for exhaust, ventilation, fire separation, and service access. Any proposed sound barrier or enclosure must preserve those requirements; an acoustic estimate does not establish a safe installation clearance.