## 5 Conclusions  
On the basis of the tests carried out, which included ultrasonic measurements on prepared samples and analysis of the results, the following conclusions can be drawn.  
The results of ultrasonic tests carried out with the use of a longitudinal ultrasonic wave are influenced by both the structure of the tested element (estimated by its hardness) and the roughness of its surface, which is particularly evident in ultrasonic tests in industrial or operational conditions in an element of an installation, a machine, or mechanism with difficult access to the tested part.  
The attenuation of an ultrasonic wave, estimated by the decrease in the gain of successive reflection pulses of this wave, imaged on the screen of an industrial or laboratory defectoscope, is sensitive to the surface roughness at high frequencies of ultrasonic transducers. For the frequency range used in industry and the customary surface roughness, there is an acceptable level of unambiguousness in the results. The propagation velocity of the longitudinal ultrasonic wave can be a measure of the mechanical properties of the materials of the tested elements, but on the condition that there is good access to the element under test and that its thickness is known. This requirement is usually met in the laboratory but is difficult to achieve in real test conditions.  
Ultrasonic waves with wavelengths from 0.3 to 3 mm were used. Measurement errors were small, ranging from about 0.1-0.2 dB for ultrasonic probes with frequencies from 2 to 12.5 MHz and 0.95 dB for 20 MHz. Differences in measurement results for different roughnesses (Figure 8) were, for example, for the frequency of 2.75 MHz from 3.8 dB for Ra = 0.32 \(\upmu\)m to 11.24 dB for Ra= 384.69 \(\upmu\)m (a difference of 7.44 dB). For the highest frequency used (20 MHz) for Ra = 0.34 \(\upmu\)m, the decibel drop in pulse height was 13.45 dB while for Ra = 348.69 \(\upmu\)m it was 19.30 dB (a difference of 5.88 dB). The differences obtained are definitely larger than the measurement errors (Figure 7). In the hardness tests, differences in ultrasonic measurement values of up to 5 dB were obtained for the 20 MHz frequency.  
In the case of limited access during ultrasonic testing to the reflective surfaces of the tested element and one-sided access of the transducer to the element, as well as in the case of insufficient information concerning the dimensions, properties, structure, and mechanical parameters of the machine parts tested under real conditions, using an ultrasonic measure of these properties and structure the decibel drop amplification of successive impulses of a longitudinal ultrasonic wave may be a better methodological solution than using the wave propagation velocity as a measure of the properties of the ultrasonically tested part. This is because the decibel gain of the pulses is permissibly sensitive to the surface roughness and less dependent on the size of the tested part.  
Conceptualization, J.K. and M.J.; methodology, J.K., M.J. and D.W.; software, K.S.; validation, J.K. and D.W.; formal analysis, M.J.; investigation, J.K.; resources, K.S. and M.N.; data curation, J.K.; writing--original draft preparation, M.J. and J.K.; writing--review and editing, J.K., D.W. and M.J.; visualization, M.J. and K.S.; supervision, M.J. and M.N.; project administration, K.S.; funding acquisition, M.N. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Polish National Centre for Research and Development under the grant--decision no. DWP/TECHMATSTRATEG-III/136/2020.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.