Creating acoustical equity

Figure 1 The person (blue) in Room 1 speaks at a “Casual” level, while the person (orange) in Room 2 uses a “Normal” level (i.e. per Pearsons). Despite the latter’s elevated vocal effort, they enjoy speech privacy due to the higher and consistent background sound level within Room 1. Contrastingly, the person in Room 1 does not have speech privacy due to the lower and variable nature of the background sound in Room 2; however, they believe they have privacy, by virtue of the fact they cannot hear the person in Room 2. Photos courtesy KR Moeller Associates Ltd.

Speech privacy, on the other hand, is both well-defined and measurable (e.g. using articulation index or speech privacy class). Therefore, it is a psychoacoustic metric which can be used in both theoretical (i.e. to illustrate the concept of acoustical equity) and practical ways (i.e. to set expectations during design and estimate occupants’ subjective impression of the built space). In this case, evaluation of acoustical privacy is effectively a review of the signal-to-noise ratio; it considers an intruding “signal” (speech) and its level relative to the background “noise” (or, rather, sound) in the receiving space.

By way of example, see the rooms and occupants in Figure 1:

  • Room 1: The orange arrows depict an elevated level of intruding noise, compared to the blue arrows. This case represents a well-designed space where the combination of the insulating properties of the wall (STC-45) and the constant background sound level of 40 A-weighted decibels (dBA) ensures the noise is not intelligible and/or audible.
  • Room 2: The blue arrows depict a lower level of intruding noise. This case represents a space that fails to consider occupant needs and/or expectations. The combination of the insulating properties of the wall (still STC-45) and the existing background sound level (only 30 dBA or less) in the receiving room is insufficient to ensure acoustical privacy. Although the intruding level of the blue source is lower than the orange example, it remains intelligible and/or audible.
The person in Room 1 briefly has speech privacy—and the person in Room 2 a corresponding reprieve from disruption—when the background noise produced by the HVAC system temporarily reaches the level required for privacy, highlighting the need to establish minimum background sound levels in addition to maximum noise thresholds.

If one were to assume the orange and blue signals are people speaking, the orange talker’s voice carries into Room 1; however, it is masked by the background sound. The listener in the room cannot identify and/or understand speech and the orange talker enjoys speech privacy. The blue talker’s voice is carried into Room 2; however, it is not masked by the background sound and the listener can identify and/or understand speech. The blue talker does not have speech privacy.

Sounds equal in overall level can be perceptibly different, depending on their frequency content. Differing spectrums also impact speech privacy. Here, a masking system is tuned with varying degrees of precision. Despite the fact the resulting sounds are at the same overall level (i.e. 47 dBA), note the impact on comprehension (i.e. privacy) when the frequencies defined by the National Research Council (NRC) masking spectrum are not met.

There are impacts beyond the one-way speech privacy. It is understood the orange talker has speech privacy because the background sound in the adjoining room masks the received level of their voice. However, the orange talker’s “perception of privacy” is violated because they can hear the blue talker. This discrepancy can cause reactive behavioural changes on the part of the orange talker (e.g. lowering of voice, avoiding confidential topics). It is also accepted the blue talker does not have speech privacy because the background sound in the adjoining room does not mask the received level of their voice. However, the blue talker has a false perception of privacy engendered by the fact they are unable to hear the orange talker. This discrepancy can result in breaches of confidentiality, the implications of which can run the gamut—or gauntlet, depending on the consequences—from embarrassment to legal proceedings.

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