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Air-Coupled Ultrasonics. Principles and Applications. Jinying Zhu. Contents. Introduction Benefits and Limitations Transducer Design Applications Reflection Plate Wave Through Transmission Products Information. Introduction - Theory. Use air as couplant Non-contact measurement

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air coupled ultrasonics

Air-Coupled Ultrasonics

Principles and Applications

Jinying Zhu

  • Introduction
  • Benefits and Limitations
  • Transducer Design
  • Applications
    • Reflection
    • Plate Wave
    • Through Transmission
  • Products Information
introduction theory
Introduction - Theory
  • Use air as couplant
  • Non-contact measurement
  • Detect air pressure change
  • Detect leaky waves leaked from materials surface

Reflection Inspection

Guided wave Inspection

Through-transmission Inspection

benefits and limitations
Benefits and Limitations
  • Non-contact Measurement
  • Allow for rapid inspection of large structures
  • Avoid contamination of materials
  • Foam and paper-based products which are not suitable for water coupling detection
  • Inexpensive and less complexity compared with laser technique
  • Large acoustic impedance mismatch between air and solid
  • Increasing absorption with frequency
  • Velocity depends on temperature


  • Maximize acoustic output
  • Minimize path losses
  • Maximize sensitivity of the receiver
  • Signal processing


  • Large sound impedance mismatch between air and detected materials
    • Energy transmission
  • Several air-solid interfaces for a complete test system
  • Energy loss >100dB compared to water couplant (i.e. energy transmission about 10-8)
piezoelectric transducer
Piezoelectric transducer
  • Un-damped Resonant Piezoelectric transducer
  • Use Paired receiver
  • Use an intermediate acoustic matching layer (lightweight polymer)
  • Use toneburst excitation

QMI AS400C Air-coupled probe*

piezoelectric transducer1
Piezoelectric transducer
  • using an acoustic lens to focus the sound energy
  • Limitation
    • Narrow frequency range
    • Ring effect

curved piezoelectric element

flat piezoelectric element and external optics

plastic lens bonded to a flat piezoelectric

capacitance transducer
Capacitance transducer
  • Much better match to air compared to piezoelectric transducer
  • Broadband frequency response (0kHz – 2MHz)
  • High sensitivity
  • No ‘ring’ effect, allow for high-frequency imaging at small distance

Structure of capacitance transducer*

applications reflection inspection
Applications -- Reflection Inspection
  • Automation of manufactuering processes and control, quality assurance
    • Thickness determination of paint coating
    • Automation of wool shearing process
  • Guidance and positioning in robotics

Topographic imaging of a PCB board with 4mm diameter center hole and 2mm diameter holes

applications through transmission inspection
Applications-- Through transmission Inspection
  • Defect detection in composite materials
  • Detect disbonds in multi-layer structures
  • Limitation: need to access both sides

Detect delaminations in aerospace materials using piezoelectric transducer

*Grandia, et al. ‘Airscan Transducers, Terchnique and Applications’

applications lamb rayleigh wave

Laser generation and air coupled detection

Using surface acoustic signals for non-contact inspection of composites

Applications -- Lamb/Rayleigh wave
  • Low attenuation in long distance
  • Suitable for large structures and sheet materials
  • Suitable for surface/subsurface defects (surface wave)
application concrete detection1
Application-- Concrete Detection
  • Concrete wall
    • Thickness: 915mm
    • P-wave Velocity : 3812m/s
    • R-wave velocity : 2100 m/s

The Non-dispersive wave velocity is

Between 2000 to 2100 m/s

  • Non-contact NDE technique
  • Increasing application in composite material detection (Aerospace Industry)
  • Inexpensive Instrument
  • Possible extend application to civil engineering
product information
Product Information
  • Sonatest Plc
  • AirStar Inc.
  • QMI Inc
  • MicroAcoustic Instruments Inc
  • Useful Links
  • Buckley J., 15th WCNDT, Roma 2000.
  • Castings M., Hosten B., NDT&E International, 2001; 34:249-258
  • Lanza di Scalea, J. Comp. Mater., 2000;34:1860
  • Schindel D.W., et al., IEEE trans. Ultrason.Ferroelect. Freq. Cont., 1995;42:42-58
  • Zhu J., Popovics J., 28th QNDE, 2001