The speech privacy between locations in an open plan space is determined by the degree to which intruding speech sounds exceed the ambient sound pressure levels at the listener's ear; a classic signal-to-noise ratio situation.

The sound pressure levels at the listener's ear from intruding speech depend upon:

The individual vocal effort and orientation of the talker,

The attenuation of speech signals due to distance or intervening barriers, and

The reinforcement of speech signals due to reflections from surfaces such as the ceiling, furniture panels, light fixtures, walls, or windows.

The ambient sound pressure levels will often be controlled to mask intruding speech. This is accomplished by means of a masking sound system. However, in certain positions and frequency ranges, heating, ventilating, or air conditioning equipment (HVAC) may contribute significantly to ambient sound pressure levels.

The primary purpose of this test method is to measure the speech privacy for an average speech spectrum using the standard Articulation Index method. This requires measurement of the relevant acoustical characteristics discussed in 5.2 and 5.3 for a pair of locations and calculation of the Articulation Index using an average speech spectrum. The average speech spectrum is for male talkers speaking with normal voice effort.

The Articulation Index ranges from 0.00, where speech is unintelligible, to 1.00, where all individual spoken words can be understood. Caution should be exercised in interpreting the numerical results of this test method. There is a need for further research to establish the relationship of Articulation Index to speech privacy. One purpose of this test method is to encourage the measurement of data and further research on this topic perhaps leading to development of well-documented speech privacy categories and criteria.

This test method can be used to:

Compare the relative privacy afforded between different locations within open plan spaces.

Evaluate how changes in open plan components (barriers, furniture, ceilings, masking sound, or wall panels) affect speech privacy.

Measure speech privacy objectively for correlation with subjective responses.

This test method could be one element of a performance or acceptance test procedure. However, many additional items would need to be specified to use this test method for performance testing of an open plan environment, such as, the number of locations to be tested and method of selecting those locations, and the method of averaging the results. Specifying a numerical criterion in terms of the Articulation Index is also necessary for acceptance testing; however, the selection of such a criterion and permissible deviations should be undertaken with care in view of the present state-of-the-art as discussed in 5.5.