Enhancing Air-Coupled Impact-Echo with Microphone Arrays

Combining the impact-echo (IE) method with air-coupled sensors has high potential to implement an efficient non-destructive testing system for concrete structures. By using microphones as sensors air-coupled IE offers significant advantages in terms of equipment costs and acquisition speed since no direct coupling of delicate and expensive contact-transducers is necessary. However, problems related to direct impact noise and ambient noise can hamper signal processing and final data interpretation. These problems are addressed in this study by introducing a modified air-coupled sensing concept. Instead of using a single microphone we use a disc shaped array of MEMS microphones to sense the acoustic waves above a concrete structure during IE testing. The concept of using the summed signal of distributed microphones arranged in an array takes advantage of the spatial extent where the wave field excited by a mechanical impact is causing a coherent surface displacement. We investigate on the spatial coherence of the dominant waveform through numerical simulations. Results from the simulations lead to the design of an appropriate microphone array for IE measurements on plate like engineering structures. The microphone array is tested on real world structures and the signals are compared to conventional contact sensing techniques and single microphone measurements. With the microphone array the frequency of the zerogroup-velocity S1 Lamb wave – which is usually interpreted in IE measurements – is more evident in the recorded spectra than with single microphones. Furthermore, we combine microphone array measurements with solenoid driven automatic impactors. In this way measurements in continuous movement and areal scans become feasible. This is an essential prerequisite for developing efficient field applicable testing devices.