Method, sources and limits
DutyNode applies published screening methods exactly as written, and says what each one can and cannot tell you.
Quick screen: the gap between dBC and dBA
C-weighting keeps most low-frequency sound; A-weighting removes much of it. A large gap therefore suggests low-frequency content.
- More than 10 dB: the World Health Organization's Guidelines for Community Noise (1999, chapter 4) recommend a frequency analysis of the noise.
- More than 15 dB: Kjellberg and colleagues (1997) proposed adding 6 dB to the A-weighted level, as reported in the 2003 DEFRA review by Leventhall (section 8.2.7).
- More than 20 dB: the same review says "there is the potential for a low frequency noise problem".
Limits. The review also says the gap is of limited value, cannot be used to predict annoyance, and breaks down when levels are low. It is a prompt to measure properly, not a finding.
Full screen: the DEFRA NANR45 reference curve
The University of Salford developed this procedure for the UK Department for Environment, Food and Rural Affairs (Moorhouse, Waddington and Adams, 2005, revised 2011) so that local authorities could assess low-frequency noise complaints consistently, and field-tested it with them. Readings are one-third-octave Leq levels, taken indoors at the position where the noise is heard.
| Band (Hz) | Reference (dB Leq) |
|---|---|
| 10 | 92 |
| 12.5 | 87 |
| 16 | 83 |
| 20 | 74 |
| 25 | 64 |
| 31.5 | 56 |
| 40 | 49 |
| 50 | 43 |
| 63 | 42 |
| 80 | 40 |
| 100 | 38 |
| 125 | 36 |
| 160 | 34 |
The curve may be relaxed by 5 dB in every band if the noise occurs only during the day, and by 5 dB if the noise is steady rather than fluctuating. A reading above the curve indicates a sound that may cause disturbance; the procedure then requires checking that the sound comes from the source and is present at the times complained of.
Why a UK procedure for U.S. homes?
There is no single U.S. federal limit for low-frequency noise, and most local ordinances set limits in dBA. The DEFRA procedure is one of the few published, field-tested methods written for exactly this kind of complaint, and acousticians use it internationally as a reference.
The tracker
Entries come from court records and reliable reports, and from a weekly automated search of federal court dockets through CourtListener (Free Law Project). Every entry is checked against its source by a person before it is published. Corrections: [email protected].
Sources
- Moorhouse, Waddington and Adams. Procedure for the assessment of low frequency noise complaints (NANR45). University of Salford for DEFRA, 2005, revised 2011.
- Leventhall. A Review of Published Research on Low Frequency Noise and its Effects. DEFRA, 2003.
- World Health Organization. Guidelines for Community Noise. 1999.
- Institute of Acoustics. Briefing note: low frequency noise annoyance.
Research in progress
DutyNode's founder is developing a separate model for weighing evidence about low-frequency noise. It is not used on this site and will not be until it has been tested on real measurements and published for comment.