Fundamentals of Environmental Acoustics and Noise Metrics
Domain 1: Fundamentals of Environmental Acoustics and Noise Metrics
This domain tests whether you can translate a physical sound field into the metrics used in permits, impact assessments, and enforcement. Expect calculation items, descriptor definitions, and questions that require choosing the right metric for a stated limit.
Core rules
- Sound pressure level is Lp = 20 log10(p/p0), with p0 = 20 micropascals. Decibels are logarithmic, so levels cannot be added arithmetically.
- A-weighting approximates human loudness response and strongly attenuates low frequencies; community limits are usually stated in dBA.
- Leq is the energy-equivalent level over a stated duration. Lmax and Lmin are extrema. L10, L50, and L90 are exceeded 10, 50, and 90 percent of the time; L90 is often used as a background descriptor.
- SEL normalizes event energy to one second. Ldn applies a 10 dB night penalty; Lden applies 5 dB evening and 10 dB night penalties; CNEL uses 5 dB evening and 10 dB night penalties.
- Fast weighting is 125 ms, slow is 1000 ms, and impulse is 35 ms.
Worked example
Two fans produce 62 dBA and 58 dBA at a receptor. Combine by energy: 10 log10(10^6.2 + 10^5.8) = 63.5 dBA, not 120 or 60. If the total measured level is 65 dBA and background is 60 dBA, the source contribution is 10 log10(10^6.5 - 10^6.0) = 63.4 dBA. The correction is small but can decide a compliance case near the limit.
Traps
Averaging dBA readings arithmetically; treating Leq as a median; comparing an Lmax to a Leq limit; ignoring tonality or impulsiveness; and forgetting that A-weighting understates low-frequency fan and transformer noise.