Speed-confounded before–after assessment of tramway pass-by noise under track-renewal field conditions

Abstract

This study investigates the system-level acoustic response of tramway track renewal under real urban operating conditions, with emphasis on the confounding role of operating speed in before–after comparisons. Pass-by noise measurements were conducted on a tram route in Poznan before (2022) and after (2025) renewal, when a conventional ballasted track was replaced by a ballastless system. During the same period, mean pass-by speed increased from 25.9 to 58.1 km/h. Energetic, psychoacoustic, and spectral indicators were analysed using direct comparisons, quartile-based speed sensitivities, and an integrated year–speed–tram type regression model. The direct comparison showed an increase in LAeq of +3.37 dB, while SEL remained nearly unchanged (Δ ≈ −0.06 dB), indicating that higher instantaneous levels were largely offset by shorter pass-by durations. Spectral analysis revealed lower measured levels of approximately 4–11 dB in the 125–200 Hz range and localised increases of up to 7–11 dB around 1.6–2 kHz. Quartile-based per-km/h derivatives indicated lower 2025 local sensitivities for L_Aeq, SEL, and loudness N in the pooled dataset. However, speed-doubling sensitivities showed that this apparent flattening was partly scale-dependent and not uniform across quantiles. The integrated model provided supportive evidence for changed metric–speed relationships, particularly for L_Aeq, although common-reference slope estimates were partly extrapolative because of limited speed overlap. The results demonstrate the usefulness of speed-sensitive analysis for interpreting before–after tramway noise measurements under changing operating regimes and should be regarded as system-level field-condition responses rather than isolated causal effects of track renewal.