On the adequate sound levels for acoustic signs for the visually impaired: A basic study for barrier-free soundscape designs

Acoustic traffic signals and the entrance chimes of public buildings are the two most common auditory signals for the visually impaired in Japan. The importance of these sounds is well known to even non-impaired citizens, and the number of these sounds has been increasing in particular in urban districts. However, there are currently no standards for the sound levels of these signals, and thus they are adjusted through ad hoc rules. Though signals which have been set up with adequate sound levels can be a great help to the visually impaired, those set up with inadequate sound levels are largely ineffective. They can cause noise problems when too loud and can be ineffective, or even useless, when too low. It is therefore necessary that adequate sound levels for these sounds be determined. In this study, adequate sound levels were investigated in a psychoacoustical experiment. Environmental sounds and auditory signals for the visually impaired were recorded with a Head And Torso Simulator (HATS). The recorded environmental sounds and auditory signals were then played simultaneously via headphones, and the visually impaired participants were required to adjust the sound level of each acoustical sign to an adequate level. Results showed that the adjusted sound levels of both types of signals were different among the participants, and the maximum difference was about 20 dB. However, participants could be divided into two groups depending on their chosen strategy for adjusting the sound levels. One group adjusted the sound levels of the auditory signals through comparison with the maximum environmental sound, while the other group adjusted the sound level according to the normal conditions of the acoustic environment.