The Speech Intelligibility Aboard Metros in Different Running Conditions

Abstract

In this paper an investigation about the influence of noise on speech intelligibility (SI) inside metros under two different driving conditions (running in tunnel straight route (TS) and in curve(TC)),for female and male speakers with four voice levels (normal, raised, loud and shout)ispresented. Perceptual laboratory tests were carried out where the noise of 11 metros wasmixed with words of Italian language. The noise wasalways reproduced with the same level, while the speakers' voice amplitude differed, thus permitting to evaluate several signal-tonoise ratio (SNR)c onditions to findo ut the SNR providing best intelligibility aboard. SI wasq uantified as the percentage of disyllabic words correctly understood (%WCU). Results showed, that %WCU washighly correlated with SNR. Poor intelligibility rates corresponded to "normal" voice condition. Fair SI can be guaranteed for SNR values greater than -6 dB in TC up to -3 dB in TS. Considering the use of raised vocal effort of 68,3 dB, the target noise levelaboard, providing good SI, should be less than 71 dB. Besides, the results showed that the %WCU wasinfluenced by the route conditions (driving in TC resulted in lower SI), the voice amplitude (%WCU rose with the increasing of the speech volume), the metro sound characteristics (SIscores differed in diverse metros), the gender of speakers (the voices of male speakers generally were more intelligible than female ones)and by the interaction of all the considered factors. PACS no. 43.50.Rq, 43.71.Gv Received12September 2011, accepted 3April 2012. ACTA ACUSTICA UNITED WITH ACUSTICA Maffei et al.:S peechintelligibility in metros Vol. 98 (2012)ing conditions (engine RPM,accelerations, decelerations, curves, tunnel crossing, open/closed windows, etc.). Passenger activities during the trips are partially responsible for the loudness increase. These activities, consisting in speaking, using cell phones, headsets for music, portable gaming devices, strongly vary,d epending on the social habits of populations. In manys ituations sellers, singers or musicians could constitute annoying noise sources.During the years 2008 and 2009 several acoustic measurements and registrations have been performed in 17 European and not European metro lines in Naples, Rome, Milan, Turin, Paris, Berlin and NewY ork. The results of this noise recordings campaign have demonstrated that often very high noise levels were noticed inside the metros during the rides [23]. These levels can cause stress effects, acoustical discomfort and, moreover, prevent the speech communication of passengers and require additional strenuous vocal efforts to reach fair intelligibility rates.The Speech intelligibility (SI) is the principal measure for the assessment of human communication and communication systems efficiency [ 24]. It is the percentage of speech units that can be correctly identified by al istener overagiven acoustic environment or the degree to which speech can be understood within givenc onditions [25]. Thomas Brand define SI as a" proportion of the spee...