AirSelect3D Blog
Octave-Band Sound Power in AHUs — How LwA Totals Are Really Summed
An AHU's LwA is built by logarithmically summing eight octave-band sound power levels — Lw_total = 10 · log₁₀ (Σ 10^(Lwi/10)) — after A-weighting each band, never by quoting the fan's dBA alone. A consultant asks for "the dBA" and a sales engineer reads one number off a fan curve; that single figure is already the end of a calculation, not the start of one — and if the octave bands behind it were never checked, the number can be technically correct and still wrong for the space it's going into.
Why one number isn't enough
Sound power in an AHU is generated and attenuated differently at every frequency. A plug fan's blade-pass tone sits in a narrow band; duct-borne noise from a coil face falls off faster at high frequency than low; casing breakout is dominated by the low octaves where panel damping is weakest. Collapse all of that into one dBA figure too early and you lose the information a consultant actually needs — whether the unit will trip a low-frequency criterion (NR/NC curve) even while its overall dBA looks fine.
That's why sound power is measured and reported per octave band, typically the eight bands from 63 Hz to 8 kHz (EN ISO 3744 / EN ISO 9614 measurement basis), before it is ever reduced to a single number.
How do the octave bands actually combine?
Sound power levels in decibels cannot be added arithmetically — they're logarithmic. Two components each radiating 80 dB do not sum to 160 dB, or even 80 dB; they sum to 83 dB. The correct combination across N bands or sources is:
Lw_total = 10 · log₁₀ ( Σ 10^(Lwi/10) )
Applied across octave bands, this is how a fan's raw spectrum and the casing's radiated spectrum combine into a single AHU sound power spectrum, band by band, before any weighting is applied.
From octave-band Lw to a single LwA
Once the eight band levels are logarithmically summed into a broadband Lw, the A-weighting correction is applied per band to reflect human hearing sensitivity — subtracting roughly 26 dB at 63 Hz down to a few dB at 4 kHz — and then the corrected bands are logarithmically summed again to produce the LwA figure that ends up on a technical data sheet.
Skipping straight from "fan manufacturer's dBA at one operating point" to "AHU dBA on the datasheet" throws away two real physical additions: the casing's own radiated noise, and the attenuation (or amplification, if a resonance lines up) of any silencer or duct section between the fan and the measurement point.
A worked example
| Octave band (Hz) | Fan Lw (dB) | Casing radiated Lw (dB) | Combined Lw (dB) | A-weighting (dB) | LwA contribution |
|---|---|---|---|---|---|
| 63 | 78 | 74 | 79.6 | −26.2 | 53.4 |
| 125 | 82 | 70 | 82.4 | −16.1 | 66.3 |
| 250 | 85 | 66 | 85.2 | −8.6 | 76.6 |
| 500 | 83 | 60 | 83.3 | −3.2 | 80.1 |
| 1000 | 80 | 55 | 80.1 | 0 | 80.1 |
| 2000 | 76 | 50 | 76.1 | +1.2 | 77.3 |
| 4000 | 70 | 45 | 70.1 | +1.0 | 71.1 |
| 8000 | 63 | 40 | 63.1 | −1.1 | 62.0 |
Summed across all eight bands: LwA ≈ 84.7 dB(A) — the number that goes on the datasheet. A figure that only means what it claims to mean because it was built up band by band, not read off a fan-only curve and relabeled. Casing size and face velocity set the flow-generated part of this spectrum before the fan is even chosen.
What to check on a quote
- Ask for the full octave-band table, not just the LwA — a consultant checking against an NR curve needs the 63/125 Hz bands specifically, since that's where AHU-driven complaints usually originate.
- Confirm whether the quoted figure is fan-only or system Lw (fan + casing breakout + duct fittings). The gap between the two grows as casing insulation gets thinner or panel spans get wider.
- Check the measurement or calculation standard referenced (EN ISO 3744 sound power basis is standard practice) rather than accepting a bare number with no method behind it.
Low-frequency mismatches are also where Eurovent/ECP-05-2026 compliance reviews most often surface acoustic non-conformance — SFP and casing class get checked early, acoustics gets checked once someone complains.
Where the tooling matters
AirSelect3D carries the octave-band sound power spectrum through the full chain — fan selection, casing radiation, and any attenuator — and sums it logarithmically into the LwA figure on the technical data sheet, band by band, the same way a consultant's own check would. Nothing gets collapsed to a single number until the physics says it's time to.
See the full octave-band spectrum behind every AHU selection →
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