Air coupled ultrasonics
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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

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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 detection

Application-- Concrete Detection

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

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