Acoustic performance of micro-perforated panel sound absorbers with paralleled coiled-up cavities for noise reduction in traffic tunnels

Traffic tunnels are facing severe noise problems due to multiple sound reflections between parallel walls and vehicle noise emissions, especially for speech intelligibility of public address systems at low frequencies. Moreover the poor ventilation and high humidity in tunnels require noise reduction materials to be moisture-proof and environmentally friendly. This study introduces a novel fibrous-free sound absorbing material to improve acoustic environment at low frequencies in traffic tunnels, which is a micro-perforated panel absorber (MPA) with paralleled coiled-up-cavities of different-depths (PCD). The acoustic performance of PCD-MPA was investigated through in-situ measurements, scale model experiments (1:32), and numerical simulations with ray tracing technology in geometrical acoustics. Results from these three approaches show excellent agreement, demonstrating that the tunnel without absorption treatment exhibits problematic acoustic conditions: long reverberation time (RT) of 12.62 s at 250 Hz, low speech transmission index (STI) of 0.11 at 250 Hz representing poor speech intelligibility, and much low noise attenuation of 12.00 dB at 100 m source-receiver distance in the tunnel. After implementation of PCD-MPA on one sidewall in the tunnel, results from scale model experiments and numerical simulations show that the tunnel acoustic environment can be significantly improved. The mean RT decreases by 66 % to 4.26 s at 250 Hz, STI increases by 191 % to 0.32 for acceptable speech intelligibility, and noise attenuation in the tunnel is improved by 17.06 dB to 29.06 dB at 100 m source-receiver distance and is close to that in the semi-free acoustic field. Four different absorber arrangements in tunnels were investigated in this study. It is revealed that continuous PCD-MPA arrangement near sound sources is most effective for noise control in traffic tunnels. The introduced PCD-MPA with excellent moisture resistance and environmental friendliness demonstrates considerable effectiveness on noise reduction in tunnels, making it a promising solution for sustainable acoustic treatment in traffic tunnels for enhanced urban environment.