Probe Frequency Guide

What Does Probe Frequency Mean?

The frequency of an ultrasonic probe determines how the sound beam behaves inside the material. As frequency increases, the wavelength becomes shorter, allowing smaller flaws to be detected. However, higher frequencies lose energy more quickly, reducing penetration. Lower frequencies travel deeper into the material but cannot resolve very small defects.

Penetration Depth
Defect Resolution
Sensitivity
Surface Noise
Material Suitability

The Effect of Frequency

Lower Frequency

  • Greater penetration depth
  • Better in coarse grain material
  • Less attenuation (energy loss)
  • Longer wavelength
  • Detects larger flaws only
  • Lower resolution

Higher Frequency

  • Detects smaller flaws easily
  • Higher resolution
  • Better in fine grain material
  • Shorter wavelength
  • Reduced penetration
  • Higher attenuation

Frequency Selection Table

Frequency Penetration Resolution Typical Applications Recommended Crystal
0.5 MHz Excellent Low Thick castings, composites, concrete, coarse grain materials Single Crystal
1 MHz Very High Low Heavy forgings, large castings, thick steel Single Crystal
2 MHz High Medium Pressure vessels, thick welds, coarse grain materials Single Crystal
2.25 MHz High Medium General weld inspection, thick sections, corrosion mapping Single Crystal
3.5 MHz Med-High Good General purpose UT, welds, forgings Single Crystal
5 MHz Medium High Standard weld inspection, plate, pipe, forgings, flaw detection Single or Twin
7.5 MHz Medium Very High Thin sections, aerospace parts, machined components Single Crystal
10 MHz Lower Excellent Precision flaw detection, thickness testing, fine grain materials Single or Twin
15 MHz Limited Extremely High Thin materials, tubing, laboratory inspection Twin Crystal
20 MHz+ Very Limited Maximum Micro defects, electronics, specialist inspection Twin Crystal

Single Crystal vs Twin Crystal Probes

Single Crystal Probes

Uses one piezoelectric element to both transmit and receive ultrasonic energy.

Advantages
  • Higher inspection range
  • Better sensitivity at greater depths
  • Most commonly used UT probe
  • Suitable for angle beam inspections
Best For:
Weld inspection Forgings Pressure vessels General flaw detection Angle beam

Twin Crystal Probes

Contains two separate elements (one transmits, one receives) angled slightly toward each other.

Advantages
  • Excellent near surface resolution
  • Significantly reduced dead zone
  • Better on rough/corroded surfaces
  • More stable thickness readings
Best For:
Corrosion surveys Wall thickness Pipe & Tank inspection Coated materials Thin walls

Material Recommendations

Material Typical Frequency
Carbon Steel2.25 to 5 MHz
Stainless Steel2 to 5 MHz
Aluminium5 to 10 MHz
Titanium5 to 10 MHz
Plastics2 to 5 MHz
Composites0.5 to 2.25 MHz
Cast Iron1 to 2 MHz
Thick Cast Steel1 to 2.25 MHz

Typical Industry Applications

Industry Common Frequency
Oil & Gas2.25 to 5 MHz
Power Generation2.25 to 5 MHz
Petrochemical2.25 to 5 MHz
Aerospace5 to 10 MHz
Manufacturing5 MHz
Rail2 to 5 MHz
Marine2.25 to 5 MHz
Mining2.25 MHz

The "Dead Zone" Explained

Every ultrasonic probe has a dead zone immediately beneath the contact surface where defects cannot be reliably detected. This occurs because the probe is still transmitting energy and cannot immediately receive returning echoes.

  • Lower frequencies often produce a larger dead zone.
  • Twin crystal probes significantly reduce the dead zone, making them perfect for thin walls.

Quick Pick: Which Probe Should I Choose?

Weld Inspection
Single Crystal
Angle Beam
Corrosion Mapping
Twin Crystal
Straight Beam
Thickness Gauging
Twin Crystal
High Freq
Lamination
Single Crystal
Straight Beam
Forgings
Single Crystal
Mid Freq
Castings / Composites
Low Freq
Single Crystal
Precision Inspection
High Freq Single Crystal

Helpful Tips for Technicians

Lower frequencies provide deeper penetration but lower detail.

Higher frequencies reveal smaller discontinuities but have reduced penetration.

Twin crystal probes are the preferred choice for corrosion surveys and wall thickness measurements.

Single crystal probes are generally preferred for weld inspection and conventional flaw detection.

Selection Rule: Probe frequency should always be selected based on the material type, thickness, grain structure, and the size of defects that need to be detected.

Disclaimer: The suggested frequencies and wavelengths are intended as a theoretical starting point based on general acoustic principles. Final probe selection should always be verified against the applicable inspection code, material calibration blocks, and specific procedure requirements.

Interactive Calculator

An interactive frequency selector calculator is coming soon!