Measurement and Prediction of the Spatial Decay of Speech in Open-Plan Offices

This thesis consists of seven publications that presented results of room acoustic research studies. Room acoustic measurement and prediction methods for work spaces are studied. Measurements are made in industrial workrooms, open-plan offices and two significantly different laboratory spaces of Finnish Institute of Occupational Health, Turku. This thesis presents a new room acoustic measurement method for open-plan offices. The method is published in ISO 3382-3:2012 standard. The method is developed from a method originally used in room acoustic measurements of industrial workrooms. An omnidirectional loudspeaker and wide band noise signal are used to measure spatial decay of sound pressure level (SPL) of speech from one workstation to other workstations on a measurement line. SPL of masking sound and speech transmission index (STI) are also measured in the workstations. The measurement results are used to determine single-number quantities: spatial decay rate of the A-weighted SPL of speech, D_2,S, the A-weighted SPL of normal speech in the distance of 4 m from the speaker, L_p,A,S,4m, and distraction distance, r_D. Target values are determined for the single-number quantities. They are based on the distribution of measurement results in Finnish open-plan offices. Simple prediction models are developed for D_2,S and L_p,A,S,4m. The models are based on linear regression analysis of empirical data from 16 different open-plan offices. The effect of room acoustical changes on spatial decay of speech in open-plan offices can be estimated using these simple models. A procedure to calculate STI using the simple models and to determine r_D is described. The predicted single-number quantities are compared to measured quantities in 26 open-plan offices. The effect of room acoustic changes on STI and spatial decay of speech is studied in two series of laboratory experiments. The first study focuses on two adjacent workstations in a test room where all the walls are sound-absorbing. The effects of ceiling, floor and screen absorption, screen height, room height and masking sound level are investigated. The second study examines the effect of ceiling, wall and screen absorption, screen height and masking sound level on the A-weighted SPL of speech, the spatial decay rate of speech and STI in an open-plan office laboratory with 12 workstations. The laboratory experiments provide measured evidence on the effects that typical room acoustic changes cause to SPL of speech and STI in open-plan offices. The effect of ceiling, wall and screen absorption, screen height and SPL of masking sound are studied exhaustively. The results emphasize the importance of all-inclusive design of room acoustics in open-plan offices.