Tonality masking system: Design and implementation of an industrial noise masking application

Abstract

A noise source that exhibits tonal qualities is often deemed to be more annoying than one which is broadband in nature. In environmental noise assessments, it is common to apply a penalty for tonality (typically up to +5dB). Tonality is often objectively classified as a function of the spectrum shape (typically in one-third octave bands), where a prominent band relative to its adjacent bands indicates the presence of tonality. A potential means of removing this tonality is to introduce masking noise, altering the spectrum shape to cover up the peaks. By adding targeted masking, in principle, it is possible to not only remove the tonality classification and penalty but also minimise the increase in overall noise level to less than a few decibels. This paper discusses the practical implementation of a tonality masking system for a low-frequency industrial noise emission problem occurring at neighbouring residential properties. The masking system was designed to detect tones generated by a large piece of factory equipment and calculate the optimum masking signal using custom-made software. The masking signal was reproduced through a subwoofer located inside the factory. With the system successfully commissioned, the resultant noise levels at the residential neighbours were no longer classified as being tonal.