Acoustics and physics
Near field and far field
Also known as acoustic near field, acoustic far field.
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.
Why the distinction matters for cleaning
Cleaning targets immediately adjacent to a horn's bell 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
Related terms
- WavelengthWavelength is the distance a sound wave travels in one cycle. At 60 Hz in air a wave is 5.7 m long; at 400 Hz it is 0.85 m. Wavelength governs how far a sonic horn's cleaning reach extends.
- Sound pressure levelSPL is the logarithmic measure of sound pressure in decibels relative to a 20 µPa reference. Industrial sonic horns operate at 140–180 dB SPL.
- Inverse-square lawIn free-field conditions sound intensity falls as 1/r². Sound pressure level drops by approximately 6 dB for each doubling of distance from the source.