---
title: "Near field / far field"
description: "The near field is the acoustic zone immediately surrounding a sound source — typically within one wavelength — where pressure and particle velocity are out of phase and SPL does not follow a clean 1/r² fall-off. The far field is the region beyond, where the wave behaves as a simple radial expansion and the inverse-square law applies."
canonical_url: "https://sylio.co/glossary/near-field-far-field"
last_updated: "2026-06-28T02:29:22.467Z"
---

The **near field** is the acoustic zone immediately surrounding a sound source — typically within one [wavelength](/glossary/wavelength) — where pressure and particle velocity are out of phase and SPL does not follow a clean 1/r² fall-off. The **far field** is the region beyond, where the wave behaves as a simple radial expansion and the [inverse-square law](/glossary/inverse-square-law) applies.

## Why the distinction matters for cleaning

Cleaning targets immediately adjacent to a horn's [bell](/glossary/bell-horn) are in the near field. The pressure environment there is intense and irregular and is what actually does the cleaning. Further targets sit in the far field, where the simpler radial model predicts the SPL.

For a 60 Hz horn (λ ≈ 5.7 m) the near field extends several metres. For a 400 Hz horn (λ ≈ 0.85 m) the near field is much smaller. Multi-horn arrays in large vessels deliberately overlap near-field zones so every target surface sees high-intensity coverage.

## Why it matters for measurement

Nameplate SPL is normally measured at 1 m — close enough to the source that the result depends on whether that point falls in the near or far field for the horn's frequency. Apples-to-apples comparisons between vendors require knowing where the measurement was taken.

## Related terms

- [Wavelength](/glossary/wavelength)
- [Sound pressure level](/glossary/sound-pressure-level)
- [Inverse-square law](/glossary/inverse-square-law)