Plant Noise Lab
Add up the noise from rooftop plant, AHUs, chillers, cooling towers and generators at a boundary or a neighbour's window. Each source spreads with distance, can be screened by a barrier and is summed band by band, giving dB(A) against day and night limits and the NR rating inside the room.
How to use: list the plant with its sound power in dB(A), the distance to the receiver, how it sits (directivity) and any barrier. Tap i for help.
1. Inputs
2. Sound at the receiver
Octave band spectrum against the NR curves, a 3D site with sound rings, and the day and night levels.
3. Checks
4. Each source at the receiver
Levels in dB(A) with all sources running. Biggest contributor first.
5. Octave bands at the receiver
6. How it works
Sound power and sound pressure
Makers quote sound power level Lw, the total acoustic power of the machine. What you hear at a point is the sound pressure level Lp, which falls by 6 dB for every doubling of distance from a point source. A machine on a roof radiates into a half space, so it is 3 dB louder at the same distance than in free field (directivity Q = 2), and 6 dB louder against a wall (Q = 4).
Adding sources
Decibels add on an energy basis: two equal sources give +3 dB, ten give +10 dB. The loudest source dominates, so treat that one first.
Barriers
A solid barrier that breaks the line of sight adds a path difference. The attenuation depends on that path difference compared with the wavelength (Maekawa), so barriers work well at high frequencies and poorly at low frequencies. Generators and large fans have strong low frequency noise, so barriers alone rarely fix them. Single barriers are capped at about 20 dB.
Limits
Outdoors, councils and authorities set day and night limits in dB(A), for example the Auckland Unitary Plan, NZS 6802 and BS 4142 assessments, or Dubai Municipality and Trakhees environmental limits. Inside rooms, designers use NR or NC curves: about NR 25 to 30 for bedrooms and NR 35 to 40 for offices.
Screening tool using ISO 9613-2 style spreading, air absorption and a single barrier. Ground effect, reflections and tonality penalties are not modelled. Use an acoustic consultant for planning submissions.