**4. Conclusions**

In the paper, an attempt is made to identify the acoustic signature of a high-speed railway vehicle, moving at a speed of 200 km/h on a straight section and a curve. As part of the experimental research, test fields were determined, measurement equipment was selected, and a methodology for carrying out measurements was specified, including the assessment of noise on a curve and straight line for electric multiple units of Alstom type ETR610-series ED250, the so-called Pendolino. The measurements were made using an acoustic camera and a 4 × 2 microphone array. As a result of the conducted experimental research, the main sources of noise coming from the studied object were identified and the dominant amplitude–frequency characteristics in the range from 20 Hz to 20 kHz, divided into one-third octave bands, were determined.

Additionally, measurements of the equivalent continuous sound level were made for 20-s passes of high-speed railway vehicles, at all points in the measurement cross section. The conducted experimental research made it possible to compare acoustic phenomena recorded separately for a straight section and a curve. The study confirmed that in the case of electric multiple units travelling at 200 km/h, the dominant noise source is the rolling noise resulting from vibrations at the wheel–rail interface. In the case of high-speed railway vehicles travelling along a curve with a radius of about R = 4000 m, the noise resulting from wheel–rail vibrations does not clearly increase the maximum sound levels compared with a straight section.

The highest acoustic pressure levels from high-speed railway vehicles travelling on a straight section, for heights above rail-head level, were recorded in the low (20 Hz to 160 Hz) and medium (1000 Hz to 4000 Hz) bands. For heights 4 m above rail-head level, the highest levels occurred in the range of 20 Hz to 100 Hz (for a distance of 20 m) and 1000 Hz–4000 Hz.

For vehicles travelling in curves, at rail-head level, the highest values were recorded for frequencies in the low bands from 20 Hz to 100 Hz (for a distance of 40 m) and additionally from 125 Hz to 200 Hz (for 20 m) and in the medium bands from 630 Hz to 3150 Hz (for a distance of 20 m). Additionally, at a distance of 40 m, high levels were recorded at 1600 Hz and 2000 Hz. For a height of 4 m above rail-head level, high values of acoustic pressure levels were recorded in the low bands from 20 to 100 Hz (for a distance of 20 m), and for a distance of 40 m in the range of 50 Hz–100 Hz. High values of sound levels were also recorded for the medium bands in the range from 1000 Hz to 3150 Hz.

On the basis of differences in the measured levels of acoustic pressure for the straight section and the curve, the dominant frequencies in the one-third octave bands (at the level of the rail head) characterising the passage of high-speed railway vehicles along a curve of the radius R = ~4000 m were indicated.

Based on the differences in the measured acoustic pressure levels, the dominant frequencies in the one-third octave bands (at the rail-head level) characterising the passage of high-speed railway vehicles over a curve of radius R = 4000 m were identified. Differences in acoustic pressure levels relative to the straight section were recorded for the bands in the range from 315 Hz to 2000 Hz.

Significant differences in amplitude of exposure acoustic pressure level were recorded between the straight section and the curve (at the level of the rail head) for close distances from railway line (5 m and 10 m from track axis). The divergences from the straight section were 4.2 dB and 6.3 dB, respectively, which may indicate an increased contribution of rolling noise due to the additional friction of the lateral surface of the wheels against the rail head.

In this study, results were developed and presented for the author's model including band noise correction, which was also verified to reproduce the propagation phenomenon. The model was characterised by good accuracy for obtained levels of noise propagation in the range of results obtained experimentally in individual measurement points. The analysis of the results obtained for this model (no abrupt changes of levels, linear character of decreasing levels) indicates that there is a high probability of correct representation of the phenomenon of propagation of noise from high-speed railway vehicles by the author's model, also beyond the measurement points.

Detailed recognition of the acoustic signature of high-speed railway vehicles travelling at 200 km/h will make it possible to minimise the acoustic impact more effectively. By identifying the main sources of noise and knowing the dominant amplitude–frequency characteristics of the studied object, it will be possible to optimally select solutions and measures limiting the negative impact of noise on the environment, including appropriate selection of noise barriers (acoustic screens) aimed at the reduction of particular frequency bands. The methodology presented in this paper to carry out experimental studies based on the measurement of acoustic phenomena using an acoustic camera and microphone matrix can also be used for other railway vehicles. Due to the differences in speed and shape of the locomotive/traction unit, it should be assumed that the recognised acoustic signal will have different amplitude and frequency characteristics. Further work directions include carrying out acoustic measurements for other railway vehicles in order to verify and compare characteristics of particular research objects.

**Author Contributions:** Conceptualisation, K.P. and J.K.; methodology, J.K.; validation, J.K. and K.P.; formal analysis, J.K. and K.P.; investigation, K.P.; resources, K.P.; data curation, K.P.; writing—original draft preparation, K.P.; writing—review and editing, J.K.; visualisation, K.P.; supervision, J.K.; project administration, K.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
