###### Abstract  
The transfer function method is a common method for establishing a traveling wave field in a sound tube to measure the reflection and transmission coefficient of underwater material. The voltage applied to the secondary sound source can be calculated in accordance with the transfer matrix between the sound sources and hydrophones, then a traveling wave field can be established in the sound tube. However, the transfer function must be remeasured when the measurement frequency needs to be changed. A checking procedure of the traveling wave field in the sound tube is essential before measuring underwater acoustic material. If it is not an accurate traveling wave field, the secondary sound  
tube method [19; 20; 21; 22; 23], and traveling wave tube (TWT) method from the perspective of the sound field formed in the tube. Among them, the pulse tube method is suitable for high frequency. As the test frequency decreases, the required length of the acoustic tube increases. Thus, the reflection and transmission coefficient of materials are difficult to test in a low frequency. When the acoustic properties of materials are tested in a standing wave tube, only the acoustic reflection coefficient can be obtained, and the acoustic impedance value of the sound tube's downstream is necessary to obtain [24; 25; 26; 27; 28; 29; 30; 31]. If the TWT method is used to measure the reflection and transmission coefficient of materials, the sample is placed in the middle of the acoustic tube, the main sound source is installed upstream of the acoustic tube, and the secondary sound source is installed downstream of the acoustic tube to absorb transmission-reflection sound waves. Continuous plane traveling waves are generated using the main sound source. In an ideal case, the traveling wave field can be formed in the half part of the acoustic tube under the material, that is, the length of the acoustic tube can be regarded as infinite. Therefore, the TWT method can be used to obtain the low-frequency acoustic reflection and transmission coefficients of materials at the same time. The control methods for TWT include passive and active control [32; 33; 34; 35; 36; 37]. The principle of passive control is adding a sound-absorbing material downstream of the acoustic tube to ensure that the transmission-reflection sound waves are absorbed. However, reflected sound will exist in the right part of the tube because the sound absorption performance of the sound-absorbing material decreases with the decrease in frequency. If the active control method is adopted, the relationship between the secondary sound source and its surface reflection coefficient shall be established first [38], then the secondary sound source loading signal shall be controlled in accordance with the change in the reflection coefficient to change its surface reflection coefficient. When the reflection coefficient reduces to a certain value, a traveling wave field can be generated. Although this method can accurately establish a traveling wave field in a sound tube, some shortcomings, such as the complex calculation and the cumbersome control process, are not inevitable. The establishment of a traveling wave field will also require considerable time. Consequently, this method has gradually been abandoned. Currently, the transfer function method is immensely popular. This method can obtain the desired voltage value, which will be applied to the secondary sound source by calculating the matrix between two sound sources and hydrophones. Then, the establishment of a wave field in a sound tube is completed. Lastly, the reflection and transmission coefficients of the material to be tested will be obtained. The transfer function method can rapidly determine the desired voltage value of the secondary sound source. Nevertheless, when the measurement frequency changes, the transfer matrix between the sound sources and hydrophones should be remeasured, which increases the workload of researchers.