Electroacoustic Calculation for Outdoor Civil Alert Coverage (RASTsO): STI ≥ 0.6 Intelligibility per GOST R 42.3.01-2021

Outdoor public address (PA) coverage is calculated differently from indoor systems: sound propagates in an open direct field, and intelligibility is eroded by distance and the site's ambient noise. Below is how to estimate coverage and the signal level above noise to reach STI ≥ 0.6 — and how the VPA online calculator performs this calculation for you.

How an outdoor calculation differs from an indoor one

Indoors, intelligibility is heavily affected by reverberation from walls and ceilings. Outdoors there are almost no reflections — the direct sound field dominates: the level drops by roughly 6 dB with every doubling of distance from the loudspeaker. This simplifies the picture but adds two new challenges — large open areas and variable ambient noise (traffic, wind, industrial surroundings, crowds).

The goal of the calculation is to deliver, across the entire protected area, a sound pressure level at which the voice message reliably cuts through the background and stays intelligible. The benchmark under GOST R 42.3.01-2021 (Russian standard for public civil alert systems) is a Speech Transmission Index of STI ≥ 0.6 (IEC 60268-16 scale), with the signal level exceeding the measured ambient noise by a margin. Exact thresholds and zones are tied to the specific site at the design stage.

VPA supplies the equipment; the design and installation are carried out by a licensed contractor — the calculation below helps procurement specialists and design engineers estimate the equipment list and quantities in advance; it does not replace design documentation.

Signal margin above ambient noise: the key input parameter

In open areas, it is the ambient noise that determines the level the loudspeakers must produce. The logic is simple: first take (or measure) the background level at the noisiest points, then specify a signal margin above that background — only then will speech remain intelligible instead of drowning in the noise.

• Site background noise. A quiet park is one thing; a station square or a highway is quite another. The calculation uses the worst realistic scenario.
• Signal margin. The message must consistently rise above the background across the whole zone, not just next to the loudspeaker mast.
• Upper limit. An excessively high level close to the source is harmful and degrades intelligibility through overload — so the level is capped from above as well.

Specific margin values and level limits are tied to the site type at the design stage; what matters here is the principle itself. The VPA online calculator lets you enter the ambient noise and instantly see the coverage and intelligibility calculation point by point.

Distance, directivity, and the number of speaker points

Since the level falls off with distance, the key question is how densely to place loudspeakers and where to aim them. Three factors are at play here.

• Throw distance. A horn with high sensitivity and a narrow dispersion pattern carries farther than an omnidirectional source of the same power.
• Directivity. Aiming the axis at the protected zone saves power and reduces spill onto neighboring areas and residential windows.
• Zone overlap. Adjacent loudspeakers must overlap so that no coverage dips below the required level appear between them.

Recalculating all of this by hand across a large site is laborious and error-prone. That is why the calculator includes a dedicated outdoor scenario: it lays out the speaker points, accounts for distance attenuation, and builds a coverage heat map. You can upload a site plan and get the layout automatically.

Horn loudspeakers vs. column loudspeakers: which to choose

Two main types of loudspeakers are used outdoors, and the choice depends on the geometry of the site.

• Horn loudspeakers. High sensitivity and long throw, narrow directivity, weather resistance. Optimal for large open squares, highways, and industrial sites where speech must carry for tens of meters. For model selection, see the horn loudspeakers section.
• Column loudspeakers. A more uniform, softer dispersion pattern and smoother speech timbre; vertical-plane directivity keeps the sound focused on the listener zone. Convenient for courtyards, pedestrian areas, and grounds around buildings. Models are in the column loudspeakers section.

In practice, a site is often covered with a combination: horns for long open throws, columns for nearby zones where people gather. All VPA outdoor models are rated for outdoor installation and certified to EAEU (Eurasian Economic Union) requirements; final selection and mast placement are done in the design project.

From acoustics to electrical design: power, cable, backup supply

Once the type, quantity, and layout of the loudspeakers are set, the calculation continues on the electrical side — this determines whether the calculated level actually reaches the farthest point.

• 70/100 V distribution lines. Outdoor runs are long, so a high-voltage (constant-voltage) distribution scheme is used — as covered in the article on 70/100 V lines.
• Amplifier power. Total load plus a 25% margin for losses and future expansion.
• Cable cross-section. Sized from line length and power, accounting for voltage drop; cable type per GOST 31565 (Russian fire-safety cable standard).
• Backup power. A UPS and batteries sized for the required operating time in standby and alert modes.

The calculator runs this whole chain for you: based on the selected loudspeakers, it computes amplifier power, cable cross-section, and battery capacity. Amplification equipment is in the mixer amplifiers section.

How this fits into wider civil alert (GOChS) tasks

Outdoor site coverage is part of a broader framework of public civil alert and facility protection. The acoustic calculation (coverage + STI ≥ 0.6) answers the question of whether the message is audible and intelligible, while the organizational and technical requirements for the alert system as a whole are a separate set of tasks, which we cover in our civil alert (GOChS) material.

It is important not to confuse the acoustic calculation with system interfacing and the communication channel: here we are talking only about sound on the site. Formal compliance verification by calculation and the final design are performed by a licensed contractor — at that stage, VPA supplies the selected equipment and the source calculation data.

Frequently asked questions

Estimate outdoor coverage and intelligibility in a few minutes: define the site and the ambient noise, and the VPA online calculator will lay out horns and columns, build a heat map, and check STI against GOST R 42.3.01-2021 — open the outdoor calculation in the calculator. Once the equipment list is set, place your order via the "Request a quote" button (for business customers): the VPA brand, a 24-month warranty, a Moscow warehouse, and delivery across Russia.