Critical methodological assessment of the distraction distance used for evaluating room acoustic quality of open-plan offices

Abstract

ISO 3382-3, the international standard for evaluating the acoustics of open-plan offices, includes metrics based on measuring spatial decay, over workstations, of speech sound pressure level, and speech intelligibility. The speech intelligibility metrics, out of which distraction distance (rD) has recently been shown to be most useful, are derived from speech transmission index (STI) calculations. This study examines how the ISO 3382-3 metrics, especially rD, are affected by certain aspects of STI calculation, by using acoustic measurements of 20 offices. Results show that rD values calculated using ISO 3382-3 speech spectrum were significantly different to those calculated using other standard and proposed speech spectra. The choice of standard octave-band transmission index weights, e.g., male/female, used in standard STI calculation as per IEC 60268-16 (2011), did not significantly affect rD values. However, compared to the standard STI calculation, the pre-standard weights from the STI model from 1985 that is used in some important studies cited in ISO 3382-3 yielded much shorter rD values; RASTI yielded an increase in rD of less than 1 m; whereas STIPA (IEC 60268-16: 2011) did not affect rD. The effect of auditory spectral masking in standard STI calculation was negligible overall, but there were cases where the auditory masking reduced rD by up to 0.5 m. The effects of speech spectrum on two other relevant metrics in ISO 3382-3, namely spatial decay rate of speech, and A-weighted sound pressure level of speech at 4 m distance, were not significant. Overall, sensitivities of rD to several aspects of STI calculation are highlighted, which should contribute to the interpretation of studies that informed ISO 3382-3 development, support the current implied practice of using male STI weights for genderless speech in the calculation of rD, and provide guidance on potential refinements.