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Figure 1. Speech privacy between two adjacent offices. Red line, voice level; gray area, background noise level; green line, intruding voice level that has been reduced by the architectural path (through the wall and the ceiling) due to appropriate design choices. See text for details.
Speech Distraction
Speech distraction, also called “freedom from distrac- tion,” is from the perspective of the listener whose desire is to be free from the distraction of other talkers that could impede focus, worker productivity, and/or overall comfort. The paper called this “normal privacy.” However, many ASA members believe the term “normal” is insuf- ficient for practical use because when working with the architectural community (e.g., architects, building speci- fiers, general contractors, and building owners), terms must be clear and have legal meaning. Thus, the term normal would be bad practice for legal documents such as building specifications. Speech distraction is the term whereby the occupant expects that speech can be both audible (heard) and intelligible (understood) by unin- tended listeners but minimally distracting.
Confidential Speech Privacy
This kind of privacy is from the perspective of the talker whose desire is to limit the information shared via speech. One might argue that the typical occupant within a closed room has a reasonable expectation that the infor- mation shared (spoken) will be contained within the boundaries of the room. Consider how your expectation for “confidential speech privacy” changes with the door open versus with the door closed. For confidential speech privacy, a talker’s speech may be heard by unintended listeners in adjacent rooms but not understood by them.
Speech Security
This kind of privacy is once again from the perspective of the talker who requires limitations to information
shared aurally. In this case, the desire is that speech must be infrequently heard, if at all, and not understood by unintended listeners outside the source room.
Fundamentals of Speech Privacy
Speech privacy depends on the signal-to-noise ratio of the intruding speech level (signal) and the background noise at the listener’s location. The intruding speech level in the listener’s location is determined by the source talker’s effort (how loud) and the noise reduction provided by the archi- tectural design (walls, ceilings, etc.) that might lower the sound level before it gets to the listener’s location.
The overall principals are illustrated in Figure 1. These rooms could be medical treatment/examination rooms or adjacent enclosed private offices. Figure 1, left, shows the source room with two people having a conversation. The voice is well above the background noise because it needs to be to achieve clear intelligible speech within the room. Figure 1, right, shows the receiving room occupied by an unintended listener. Note that the intruding voice (Figure 1, green line) is within the background noise (Figure 1, gray area); thus, the intruding voice is “masked” by the background noise.
Speech privacy is a complex problem that involves the frequency content of the source voice, the wall and ceil- ing attenuation, and the background sound level and spectrum. Figure 2 shows a plot of frequency versus amplitude level (in decibels). In this plot, the relative
Figure 2. Room-to-room speech privacy is plotted as frequency versus amplitude level. Red line, source voice; green line, intruding voice level into the listener’s space (receiving room); gray area, background noise level. See text for details.
Fall 2020 • Acoustics Today 13