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Investigating sound decrement per doubling of distance as a universal room acoustics parameter Ng, Denny

Abstract

The suitability of Sound Decay per Doubling of Distance (𝐷𝐿₂), as a universal room acoustics rating parameter was investigated. 𝐷𝐿₂ was combined with the Speech Transmission Index (𝑆𝑇𝐼) to rate acoustical room quality. This follows the methodology of ISO 3382-3, allowing evaluation of room quality using speech intelligibility as a foundation quantity. To prescribe rating criteria, all rooms where unamplified speech is present were postulated to be categorizable into one of three room types: Case 1 (intelligibility) rooms where intelligibility is required at all distances, Case 2 (distraction) rooms where distraction is permissible until a defined distance, or Case 3 (privacy) rooms where privacy is expected beyond a defined distance. For rating metrics, Case 1 used listening effort, Case 2 used loss of productivity, and Case 3 used percentage of speech intelligible. An idealized initial tool was developed which calculates 𝑆𝑇𝐼 at all points along a 𝐷𝐿₂ curve. The tool calculated 𝑆𝑇𝐼 based only on reverberant speech and was therefore applicable only to rooms where direct speech is impeded by obstacles. Assumptions used in the initial tool were checked using experimental data collected in 62 rooms of varying case classifications from 22 buildings. The data were used to evaluate the accuracy of regressions using sound pressure level measurements over a limited range of 1 – 16 m (𝐷𝐿2,𝑠,𝐴,1βˆ’16π‘š,), the octave band variation in 𝐷𝐿2, and the sound pressure level at 1 m from a sound source. The maximum regression error for 𝐷𝐿2,𝑠,𝐴,1βˆ’16π‘š, was 5.6 dB, and 2.5 dB on average. DL2 trends observed in the experimental data were then implemented in the DL2 tool and the STI calculation model was updated to include direct speech contributions. The updated tool was used to evaluate theoretical rooms for each case using the developed rating schemes. Room reverberation time (RT, 𝑇) and background noise levels (BNL, 𝐿𝑛) were modeled using values recommended in standards. Due to the variability of RT and BNL within rooms of similar types, standardized rating schemes based on 𝐷𝐿₂ were deemed unfeasible. However, 𝐷𝐿₂ and the tool developed provide valuable insight on how to optimize rooms acoustically. Supplementary materials available at: http://hdl.handle.net/2429/73596.

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