Italian differential noise criterion

Note for non-Italian readers. This differential criterion is specific to Italian regulation (Law 447/95 + DPCM 14/11/97). It is an additional check on top of the absolute zone limits and applies to the indoor environments of disturbed receptors. It has no direct equivalent in most other European frameworks.

How do you use the tool?

The criterion checks that the noise from a specific source does not cause an excessive increase over the background, measured indoors in the dwellings of the disturbed receptor. It does not apply to transport infrastructure, non-productive activities, shared building services for disturbance in the same building, Class VI areas, or when ambient noise is below the negligibility threshold (Art. 4 §2).

The differential criterion checks that the noise produced by a specific source (plant, activity) does not cause an excessive increase over the background noise, measured indoors in the dwellings of the disturbed receptor.

It does not apply:

• to transport infrastructure (roads, railways, etc.), governed by dedicated decrees (DPR 142/2004, DPR 459/1998);
• to activities and behaviour unrelated to productive, commercial or professional needs;
• to shared building services, for the disturbance perceived inside the same building where they are installed;
• in Class VI areas (exclusively industrial);
• when ambient noise is below the negligibility threshold (exemptions Art. 4 §2).

The first four exclusions are the tool’s two preliminary checks; the last is handled in the per-period computation.

How is the differential criterion computed?

The differential is ΔL_r = L_r,ambient − L_r,residual. The limits of DPCM 14/11/97 Art. 4 §1 are ΔL_r ≤ 5 dB in the daytime period (06:00–22:00) and ≤ 3 dB at night (22:00–06:00). The Art. 4 §2 exemptions rule the criterion out below threshold: with windows open 50/40 dB(A) day/night, with windows closed 35/25 dB(A).

Difference:

$$\Delta L_r=L_{r,\text{amb}}-L_{r,\text{res}}$$ $$\begin{array}{rl}{L}_{r,\text{amb}}& =\text{residual level}+\text{disturbing source, inside the dwelling}\\ & (\text{with corrections }{T}_R+K_T+K_I+K_B)\\ L_{r,\text{res}}& =\text{residual level WITHOUT the disturbing source}\end{array}$$

The ΔL_r difference is not explicitly defined by the decree: it is technical practice derived from DM 16/03/1998 and from interpreting L_r as the rating level. DPCM 14/11/97 sets the limit values, not the formula.

Limits — DPCM 14/11/97 Art. 4 §1:

$$\begin{array}{rl}\text{daytime}(06\text{:}00-22\text{:}00)& :\Delta L_r\le 5\text{dB}\\ \text{night-time}(22\text{:}00-06\text{:}00)& :\Delta L_r\le 3\text{dB}\end{array}$$

Exemptions Art. 4 §2 — the criterion does not apply if ambient noise is below threshold (the decree states “inferiore a”, a strict comparison):

$$\begin{array}{rl}\text{windows open:}& \text{noise}<50\text{dB(A) daytime}\\ & \text{noise}<40\text{dB(A) night-time}\\ \text{windows closed:}& \text{ambient noise}<35\text{dB(A) daytime}\\ & \text{ambient noise}<25\text{dB(A) night-time}\end{array}$$

Terms

L_r — A-weighted equivalent continuous level, the “rating noise” including any corrections for tonal (K_T), impulsive (K_I) and low-frequency (K_B) components. The reference time TR is the daytime or night-time period.

Ambient level — noise present in the receiving environment, produced by all sources together, including the disturbing one.

Residual level — noise present in the receiving environment when the disturbing source is switched off or not operating.

Acoustic class — class of the municipal acoustic zoning (I–VI) assigned to the territory; it sets the absolute limits and, in Class VI, makes the differential criterion inapplicable.

What are the tool’s limitations?

The differential criterion is an additional condition on top of the absolute zone limits; it does not replace them. The tool assumes the supplied levels are already corrected L_r: identifying the reference and measurement times and applying the K_T, K_I, K_B corrections must happen before the comparison. The applicability checks cover common Art. 4 §3 cases; borderline situations require a technician’s assessment.

• The differential criterion is an additional condition on top of the absolute zone limits (immission/emission); it does not replace them.
• Identifying the reference and measurement times (TR and TM) requires careful analysis of the source’s operating cycle: this tool assumes the supplied levels are already expressed as corrected L_r values.
• The K_T, K_I, K_B corrections must be applied before the comparison, during sound-level processing.
• The applicability checks cover the most common Art. 4 §3 cases; borderline situations (mixed use, multiple sources) require a technician’s assessment.

Standards & references

• Italian Law 26/10/1995 no. 447 — framework law on noise pollution.
• DPCM 14/11/1997 — Italian Prime Ministerial Decree on the limit values of noise sources, Art. 4 (differential criterion, §3 exclusions) and Annex A (corrections TR, K_T, K_I, K_B).
• DM 16/03/1998 — Italian Ministerial Decree on noise-pollution detection and measurement techniques.

Learn more: Differential immission criterion wiki →