**2. Applications of AE techniques**

Application areas of AE span numerous fields, including aerospace, automotive, biomedical, manufacturing, civil, and materials engineering fields.

Monitoring the condition and predicting the life of the main structures of an aircraft play significant role in guaranteeing the flight safety. AE techniques are successfully employed in damage and crack identification and monitoring in aircraft composite and steel structures [3, 4].

AE is extensively used in the automotive industry in fault diagnosis of internal combustion engines (ICEs). Reference [5] deals with advanced techniques based on vibro-acoustic signals that can diagnose and monitor ICE malfunctions under vehicle operating conditions. Reference [6] develops new AE models and

effective wavelet-based AE signal processing techniques for monitoring lubrication conditions.

The primary use of AE technique in biomedical field is bone condition assessment under different loading conditions, in osteoporosis and in fracture healing process monitoring [7]. AE can be used for detecting defects in tissues and materials, predicting failure, and monitoring damage progression in real time [8].

AE has been used as a widely applied technique in manufacturing process monitoring due to its sensitivity to process parameters. The use of AE as a monitoring technique for machining operations comes with more advantages, one of which is its ability to detect machine vibrations from those of AE signals due to highfrequency range and sensitivity of AE signals, thus preventing it from interfering with the cutting operation [9, 10].

Structural health monitoring (SHM) in civil engineering involves AE technique for detecting cracks in structures. This technique relies on the high-frequency ultrasonic waves generating energy that is rapidly emitted from a material throughout from the initiation to growth progression of cracks. The wide applicability of AE technique is evident in several metal piping system evaluations and fiberglassreinforced plastics (FRP) and concrete bridges [11, 12].

AE originates from stress waves generated as a result of the growth or movement that takes place in solid defects. When a composite material is subject to a mechanical load, it can experience matrix cracking, debonding, and delamination. AE is a powerful technique capable of detecting these damage types in composites [13, 14].
