Sound level at a distance
Used on the ASP, CSP and CHST. The worked example below is computed by code and checked against the source.
Lp2 = Lp1 + 20 × log10(d1 ÷ d2)
Lp1 = level measured at distance d1; Lp2 = level at distance d2. Point source in a free field: about 6 dB less each time the distance doubles.
Worked example
OSHA example: 120 dBA at 50 ft; find the level at 80 ft.
120 + 20 × log10(50 ÷ 80) = 116 dBA
The source prints this result, and our code reproduces it.
Indoors, reflections mean the level falls by less than 6 dB per doubling of distance.
Run your own numbers: decibel addition calculator.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Blueprint: ASP11 Domain 1, task 5: noise hazards
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Blueprint: CSP11 Domain 6, task 1: occupational exposures (noise)
Practice: Occupational Noise
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Blueprint: CHST5 Domain 1, knowledge 18: noise exposure
Source
The formula comes from
OSHA Technical Manual, Section III, Chapter 5 (Noise), Appendix B.6(opens osha.gov in a new tab)“If a sound is generated at a point source in a free field, meaning there are no walls or other obstructions, the sound pressure level, Lp, will be reduced by 6 dB each time the distance from the noise source is doubled. ... Lpd2 = Lpd1 + 20 × log(d1/d2) ... Lpd1 is 120 dBA, d1 is 50 ft, and d2 is 80 ft. Therefore, Lpd2 is 120 + 20 × log(50/80), which is 116 dBA.”
Questions
- What is the formula for sound level at a distance?
- Lp2 = Lp1 + 20 × log10(d1 ÷ d2). Lp1 = level measured at distance d1; Lp2 = level at distance d2. Point source in a free field: about 6 dB less each time the distance doubles.
- Will this formula be given on the BCSP exam?
- BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.
BCSP on formulas at the exam
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
Related
- Noise dose
- Noise TWA from dose
- Reference duration for a sound level
- Adding decibels
- Hearing protector NRR (estimated exposure)
- ASP formula sheet
- CSP formula sheet
- CHST formula sheet
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.