Emergency Generator

Description and Noise Criteria:
The dimensions, plans and elevations used in this noise analysis are taken from drawings supplied by the architect.

The potential noise issue is stationary noise from two generator sets.

The generators are each 500 kW Cummins Genset model 500 DFEK with a “Quiet Site Level 2” enclosure package.  Manufacturer’s reported sound pressure level is 73 dBA at seven meters distance.

In addition to the manufacturer’s sound enclosure, a masonry noise barrier wall with a total height above grade of 9 feet 4 inches is proposed.

The generators are intended to be used only as emergency standby generators in case of PG&E power failure. A reasonable maintenance schedule for the generators would consist of running each generator individually, 15 minutes during one daytime hour, each month.

The stationary noise contours, without background contributions from nearby transportation sources, are shown in the accompanying figures. All noise contours are assumed to be 5’ 6” above grade. Sound levels for stationary noise sources are customarily expressed as LEQ values, normalized to one hour, meaning that the average sound pressure level over a one hour period of operation is shown in the following figures. Five scenarios are projected. The first four scenarios assume noise barrier gates are in place. The fifth scenario assumes there are no noise barrier gates in place.

1. One generator is operated for maintenance for 15 minutes.
2. Two Generators operate 15 minutes in a one-hour period
3. Two Generators operate 30 minutes in a one-hour period
4. Two Generators operate 60 minutes in a one-hour period (continuous operation)
5. Two Generators operate 60 minutes in a one-hour period (continuous operation), no noise barrier gates in place

partial site plan

one generator set running 15 minutes each hour

two generator sets running 15 minutes each hour. 
Units are Leq 1 hour, dBA

two generator sets running 30 minutes each hour. 
Units are Leq 1 hour, dBA

two generator sets running continuously. 
Units are Leq 1 hour, dBA

Noise Contour Modeling Noise contours incorporating the measured sound level values were generated using CADNA/A, an acoustical modeling program that incorporates Federal Highway Administration TNM 2.5 algorithms, and which was developed to predict hourly LEQ values for free-flowing traffic conditions. This computer modeling tool, made by Datakustik GmbH, is an internationally accepted acoustical modeling software program, used by many acoustics and noise control professional offices in the U.S. and abroad. The software has been validated by comparison with actual values in many different settings. The program has a high level of reliability and follows methods specified by the International Standards Organization in their ISO 9613-2 standard. The computer modeling software takes into account source sound power levels, surface reflection and absorption, atmospheric absorption, geometric divergence, meteorological conditions, walls, barriers, berms, and terrain variations. The CADNA A software uses a grid of receivers covering the project site, based on the characteristics of a generator set, with vertical area stationary noise sources on four sides.