Referentni proračunski list · samo geometrijsko širenje
Ideal Free-Field Point-Source SPL Transfer
Prijenos jednog eksplicitno identificiranog deskriptora razine zvučnog tlaka između dviju udaljenosti koristeći idealno sferno širenje. Rezultat je uvjetan – nije potpuna predikcija buke na otvorenom, buke u zatvorenom, izloženosti ili usklađenosti.
Zabilježeni ulazi
Nema unaprijed učitanih primjera vrijednosti. Unesite mjerenje ili dokumentiranu osnovu izvora koju zapravo imate.
Uvjetni rezultat
Model i dimenzijska provjera
Za sferne valove od idealnog točkastog izvora u bezgubnom slobodnom polju, amplituda zvučnog tlaka u daljinskom polju proporcionalna je 1/r. Stoga je razlika razina tlaka:
L₂ = L₁ − G
ΔL = L₂ − L₁ = −20 log₁₀(r₂/r₁)
Ovdje su L₁ i L₂ isti specificirani deskriptor razine zvučnog tlaka, u decibelima; r₁ i r₂ su pozitivne udaljenosti u istoj jedinici. Omjer r₂/r₁ je bezdimenzijski, pa je uzimanje njegovog logaritma dimenzijski ispravno. Povezani idealni omjer amplitude tlaka je p₂/p₁ = r₁/r₂.
Udvostručenje daje G = 20 log₁₀(2) = 6.0206 dB – ne točno 6 dB. Ako je r₂ < r₁, G je negativan i model predviđa porast; proračunski list izvještava o predznaku umjesto da ga prikrije kao „prigušenje”.
Popis primjenjivosti
| Stanje | Zahtijevano za ovaj proračunski list | Ako nije zadovoljeno |
|---|---|---|
| Source and operation | Same stable source power, spectrum and operating state | Do not attribute the observed difference solely to distance |
| Acoustic quantity | Same named level descriptor, weighting/band and time basis | Levels are not directly transferable |
| Geometrija | Point-like at both distances; both points in the validated far field | Use a source/geometry-specific method |
| Smjer | Same source directivity sector | Include documented directivity |
| Propagation | Free field; no relevant reflecting or screening objects | Model reflections, ground and barriers |
| Medium and weather | Their change is negligible over the path for the stated purpose | Use frequency-dependent atmospheric and meteorological modelling |
“Far field” is not established here by a universal distance preset. It depends on source dimensions, wavelength, radiation behaviour and measurement purpose.
What this worksheet deliberately does not calculate
- It does not calculate sound power level from sound pressure level.
- It does not model a road, railway, pipe, façade or arbitrary “line source.” A universal 3 dB-per-doubling rule is not assigned to those cases.
- It does not combine octave bands, A-weight a spectrum, or convert among instantaneous, maximum, peak, time-weighted, equivalent-continuous or exposure levels.
- It does not include atmospheric absorption, ground effect, reflections, screens, foliage, buildings, terrain, wind or temperature gradients.
- It does not establish a safe distance, worker dose, community-noise rating, legal limit or regulatory compliance.
Standards boundary and sources
Classification of the equation: general acoustical/physical relation for ideal spherical geometrical spreading. It is not presented as a formula prescribed by ISO 9613-2.
- OSHA Technical Manual, Section III, Chapter 5 — official explanation of the point-source free-field distance relation and limits of real sound fields.
- ISO 9613-2:2024, Edition 2 — current outdoor engineering method; its public scope lists geometrical divergence, atmospheric absorption, ground effect, reflections, screening and meteorology. A draft Amendment 1 is under development and is not used here.
- ISO 9613-1:1993, Edition 1 — published, currently under review; frequency- and weather-dependent atmospheric absorption. No coefficients from its licensed text are reproduced here.
- ISO 1996-1:2016, Edition 3 — current basic environmental-noise quantities and assessment procedures; its public note warns that the underlying acoustic quantity must be identified.
- ISO 80000-8:2020, Edition 2 + Amendment 1:2025 — trenutni nazivi, simboli, definicije i jedinice za akustične veličine.
- IEC 61672-1:2013, Edition 2 — sound-level-meter specifications distinguish frequency-weighted, time-weighted, time-averaged and sound-exposure measurements.
Official public records checked 13 July 2026. Clause-level ISO propagation algorithms and coefficients require the controlled standard text and are outside this worksheet.
Questions that prevent common misuse
Is “90 dB” enough information?
No. Record what level it is—for example a Z-weighted band level, LAeq,T, or another defined quantity—plus the relevant time/frequency basis and operating state. ISO 1996-1 explicitly warns that different acoustic quantities can all be expressed in dB and normally differ.
Can I use an A-weighted level?
Only as the same fully specified A-weighted descriptor at both positions and only when every propagation assumption is met. This worksheet applies the same frequency-independent geometrical term to the unchanged source spectrum; it does not calculate frequency-dependent losses or A-weight a spectrum.
Does the equation work indoors?
Not as a prediction of the total room level. Reflected/reverberant sound generally prevents the total level from following ideal spherical spreading. A separately isolated direct-field component may be analysed only with a defensible measurement model.
Why is there no line-source switch?
Because selecting “line source” does not prove cylindrical spreading. Real extended sources can be finite, incoherent or directional and can transition with distance; roads additionally require traffic, spectrum, ground, screening and meteorology. A generic switch would produce unjustified precision.
Can this choose a hearing-safe distance?
No. Hearing risk depends on the applicable A-weighted or peak descriptor, duration, temporal pattern, multiple sources and governing requirements. Verify exposure using suitable measurements or a complete validated assessment.
