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World Health Organization – Guidelines for Community Noise is a systematic review of scientific evidence regarding noise, annoyance, speech and hearing impairment. The document provides also relevant guideline values.

Extract 1 Interference with communication Speech (3.3)
“Longer reverberation times, especially when combined with high background interfering noise, make speech perception more difficult. Even in a quiet environment, a reverberation time below 0.6 s is desirable for adequate speech intelligibility by vulnerable groups. For example, for older hearing-handicapped persons, the optimal reverberation time for speech intelligibility is 0.30.5 s (Plomp 1986)”
Extract 2 Schools and preschools (4.3.2)“For schools, the critical effects of noise are on speech interference, disturbance of information
extraction (e.g. comprehension and reading acquisition), message communication and annoyance. To be able to hear and understand spoken messages in classrooms, the background sound pressure level should not exceed 35 dB LAeq during teaching sessions. For hearing impaired children, an even lower sound pressure level may be needed. The reverberation time in the classroom should be about 0.6 s, and preferably lower for hearing-impaired children. For assembly halls and cafeterias in school buildings, the reverberation time should be less than 1 s
Extract 3 (Noise) Interference with communication (4.2.1)
“Speech signal perception is of paramount importance, for example, in classrooms or conference rooms. To ensure any speech communication, the signal-to-noise relationship should exceed zero dB. But when listening to complicated messages (at school, listening to foreign languages, telephone conversation) the signal-to noise ratio should be at least 15 dB. With a voice level of 50 dBA (at 1 m distance this corresponds on average to a casual voice level in both women and men), the background level should not exceed 35 dBA

Acoustic Standards 77 Educational Environments 178

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