Near field and far field

The near field is the acoustic region close to a sound source where pressure, particle velocity and phase relationships are complex. The far field is the region farther away where the wave behaves more like a simple propagating sound field and distance-based rules such as the inverse-square law become more useful.

For industrial sonic horns, the boundary is not a single sharp distance. A practical rule is that near-field effects are important within about one wavelength of the horn, modified by bell size, directivity and nearby surfaces.

Why the distinction matters for cleaning

The highest acoustic forces often occur near the bell horn, where the field is intense and non-uniform. Deposits close to the horn may see strong pressure fluctuations that are not predicted by simple distance calculations. Farther away, sound level usually falls with distance, but reflections inside vessels can preserve energy and create a more diffuse field.

Approximate wavelengths in air at 20 deg C:

Low-frequency horns therefore have large near-field regions. In a small hopper or ESP compartment, much of the target volume may be acoustically close to the horn.

Why it matters for measurement

Nameplate SPL is often reported at 1 m. For a low-frequency horn, 1 m may still be inside the near field, so measurements depend strongly on microphone position, bell axis, reflections and test setup. Comparing two horns requires the same distance, operating pressure, orientation, weighting and frequency-band data.

For environmental or worker-noise assessment, far-field measurements and octave-band data may be more useful than a single near-field number. For cleaning design, near-field intensity and vessel reflections may be more relevant.

Acoustic-cleaning design

Multi-horn layouts intentionally overlap acoustic coverage so dead zones do not become deposit anchors. Engineers consider near-field reach, far-field attenuation, obstructions, acoustic shadowing, access doors, refractory, catalyst layers and whether horns fire together or in sequence.

Measurement and design detail

Near-field and far-field behaviour affect both cleaning performance and noise assessment. Close to a horn, sound pressure can vary strongly with position because the source has a finite size and a directional bell. A microphone moved a short distance may show a large change that is not representative of worker exposure or vessel coverage. Farther away, the sound field becomes more stable, but reflections from plant steel, walls and ducts can still dominate.

For acoustic cleaning, the near field can be useful because high cyclic pressure near the bell may disturb deposits on nearby ledges, screens or hoppers. Farther into a vessel, wavelength, reflections and obstruction density determine whether energy reaches hidden surfaces. Multi-horn systems are arranged to overlap these regions and avoid persistent dead zones.

For measurement, engineers should state microphone position, distance, height, weighting, octave-band data and whether the source is in free air or coupled to a vessel. Worker exposure should be measured where people stand. Environmental noise should be assessed at receptors. Cleaning effectiveness should be checked through process response and inspection, not inferred from a single near-field reading.

Acoustic-cleaning interpretation

The near-field zone is also where small installation details matter most. A partially blocked bell, a flange lip, a nearby baffle or a refractory step can change the pressure pattern before the sound has spread into the vessel. That is why visual inspection of the mounting and penetration is part of troubleshooting a weak cleaning result.

In the far field, the question changes from source detail to coverage. Engineers ask whether enough acoustic energy reaches the deposit after losses from distance, absorption and obstruction. Low-frequency horns can maintain useful coverage farther into a vessel, while higher-frequency devices may clean strongly near the source and fade behind tube banks. For this reason, acoustic layouts should be reviewed in section drawings, not only on plan views. The difference between near and far field helps explain why a loud local reading can coexist with poor remote cleaning.

Related terms

• Wavelength
• Sound pressure level
• Inverse-square law