gillyflower In his study in 2003, Fernandes reported that protective devices improve speech intelligibility in an environment with 80-90 dB background noise; at lower levels (60-70 dB), however, these devices decreases decrease ( the ) intelligibility. Fine as shown above. I consider "the" optional at the end.
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gillyflowerIn his study in 2003, Fernandes reported that protective devices improve speech intelligibility in an environment with 80-90 dB background noise; at lower levels (60-70 dB), however, these devicesdecreasesdecrease (the) intelligibility.