# Defect Detection in Solid Timber Panels Using Air-Coupled Ultrasonic Imaging Techniques
## 2 Materials and Methods
Then, the signal was processed by a band-pass filter with a frequency range between 190 kHz and 210 kHz and amplified by an in-house low-noise amplifier with a gain of 70 dB. According to the experiments, the differential amplifier circuit composed of instrumentation amplifier AD620 was used. The filter and amplifier designs were essential to the whole process since the signal captured by the receiver was very weak due to the reflection and attenuation. Finally, the post-processing was performed by the main controller of the system and sent to a PC for imaging if necessary. The scanning assembly was also driven by the main controller with fully automatic data acquisition and post-processing. During the scan, the transmitter and the receiver were clamped in a frame attached to a stepping motor driven over an XY-stage while the samples were held in a fixed position. The transmitter and the receiver moved together over the surface of the sample in a meandering pattern, while the main controller captured the signal amplitude and the TOFs of individual pulses with a peak detector and a time synchronizer. All the information was then sent to a PC and combined into composite images called C-scan and D-scan, respectively. In C-scan images, the received signal amplitude of each sample position was normalized and colored representing attenuation. In D-scan images, TOFs of each sample position were recorded and colored representing flight time. Experiments suggested that the orientation of the transducers should be perpendicular to the surface of the samples. A gap between the transmitter and the receiver of 150 mm was adopted based on past experiments. The samples were scanned radially/tangentially to their surface. The adopted scan grid was 2 mm but it can vary depending on the resolution required. Figure 3 shows the integrated measurement system. The receiver is fixed on the back of the frame. The signals captured by the receiver were weak and vulnerable to interference because of the significant impedance mismatch between the air and solids. One way to improve the accuracy of detection is to apply pulse compression technology [25; 26]. The successful application of pulse compression technology in radar and medical fields [27] provided an effective reference for solving problems such as low signal-to-noise ratio (SNR) and poor  
Figure 3: Integrated measurement system. Figure 2: Received waveforms of sine-wave ultrasound. defect detection accuracy in timber air-coupled ultrasonic testing area without increasing the instantaneous peak power of the transmitter. Barker coded method was a typical phase-coded pulse compression method [28]. It was used in this application, since it required only a single transmission signal, making it easier to generate compared to other modulation methods. Barker code is a binary code group with values of only \(+1\) and \(-1\), and cannot be directly used as the excitation signal for ultrasonic transducers. It requires high-frequency carrier modulation to be used as the excitation signal for transducers. For example, taking the sine signal as a carrier signal, it is multiplied by the Barker code signal in the time domain as the excitation signal. The principle of its modulation process is shown in Figure 4.  
In this paper, 13-bit Barker code {+1, +1, +1, +1, \(-1\), \(-1\), \(-1\), +1, +1, \(-1\), +1} was used as a modulating signal for ultrasonic transducers because it had highest Peak Sidelobe Level (PSL) compared to other Barker codes. Figure 5a showed a sinusoidal carrier signal, whose frequency was the resonant frequency of the transmitter. The Barker code signal is shown in Figure 5b. Multiplying the Barker code signal with the carrier signal in the time domain yielded the modulated excitation signal as shown in Figure 5c. At the receiver end, the matched filter was used for pulse compression of the received signals. This process is the cross-correlation between received signals and excitation signals.