## 1 Introduction  
Concrete is commonly used in many types of construction and in a variety of configurations. Although materials scientists have been trying to improve the capabilities of concrete in recent years [1; 2], the shrinkage of concrete, the differential settlement in a building's foundations, and the impacts of temperature stress and loading may cause cracks. In addition, the corrosion of reinforcing bars in concrete may lead to the development of longitudinal cracks along the bars. Cracks are the most common form of deterioration in concrete objects [3; 4]. Moreover, Taiwan is located on the boundary of the Philippine Sea Plate and Eurasian Plate, where geotectonic movements are frequent, leading to severe or partial damage to the internal structure of buildings. The severity of such damage cannot be identified by sight, and, without repair, damaged structures may lead to large numbers of injuries and deaths. For example, several structures collapsed in recent years, including Houfeng Bridge in 2008, the Weiguan Building in 2016, and the Marshal Hotel in Hualian city (located on the east coast of Taiwan) in 2018, resulting in several casualties [5; 6] and demonstrating the importance of structural safety inspections. Among the evaluation methods for structure damage, nondestructive structural safety evaluations have become increasingly popular. The advantage of a nondestructive testing method is that it does not cause damage to a building, and its implementation (online testing) does not interfere with the regular usage of the building. It is mobile, convenient, and can evaluate the internal deterioration condition of a structure in a short period of time. To date, the most common nondestructive test in civil engineering was developed using the theories of electromagnetic waves and stress waves. For example, ground-penetrating radar uses electromagnetic testing technology [7; 8; 9; 10], whereas the ultrasonic method and the impact-echo method involve stress wave testing technology [11; 12; 13; 14; 15]. The electromagnetic testing technology involves expensive equipment, namely transceiver signal devices, transceiver antennas, signal recorders, and analysis systems; consequently, it is typically used in geotechnical engineering or oil-mining engineering. When the testing objects are concrete structures, such as those in the present study, testing technologies based on stress wave theories are more suitable. In addition, the ultrasonic method involves preburied boreholes that cause damage to the structure of a building. Therefore, the impact-echo method has gradually become the leading technology in civil engineering for conducting concrete structure testing. In recent years, the implementation of the impact-echo method has become increasingly diverse. It is used to test internal anomalies in rod structures (e.g., beams and columns) or in the concrete lining structure in tunnels, to examine the quality of a concrete structure, and to test the minimum width and the depth of cracks in a concrete structure. However, the implementation of the impact-echo method typically involves a device generating an impact on the testing object, and subsequently using a digital receiver to acquire the echo wave signals and signal acquisition devices to transmit the signals to the computer for analysis [16; 17; 18; 19]. For disaster prevention, however, fixed-point automatic detection and rapid signal processing are critical. To improve the latencies, our lab developed a low-cost, labor-reducing, and real-time nondestructive testing system that can be applied as a mechanical impact method to determine whether a concrete object contains defects. The sound card used in our system transforms the data collected through the audio software interface, supplemented with MATLAB software [20], into the soundwave of the echo signals. Subsequently, the soundwaves are connected to the computer; the echo waves are acquired and reconstructed on a personal computer (PC).