Introduction to Acoustics

420 Part C Architectural Acoustics
Part C 11.6
25
50
75
Approximate % of people satisfied
0
100
1.00
Normal speech
privacy
Confidential speech
privacy
0.75 0.50 0.25 0.00
Articulation index (AI)
Fig. 11.69 Correlation between articulation index and
speech privacy (adapted from [11.1])
When privacy is desired, it is necessary to have a low
AI. When communication is desired, it is necessary
to have a high AI so people can clearly understand
speech.
Figure 11.69 shows how AI relates to speech privacy.
This graph provides general guidelines from experience.
Speech privacy goals are divided into three categories –
minimal distraction, normal speech privacy, and confidential
speech privacy. Minimal distraction corresponds
to an AI of 0.35 or less. Normal speech privacy, in
which the average person can work without distraction
although occasional parts of outside conversations
can be heard, corresponds to an AI of 0.20 or less.
Confidential privacy, where the average person can
carry on discussions with assurance that he or she
will not be understood by neighbors, corresponds to
an AI of 0.05 or less. There is no privacy when AI
exceeds 0.40.
Speech Intelligibility Index
AI has been replaced by the speech intelligibility index
(SII) in recent versions of ANSI S3.5. SII is still, like AI,
a weighted speech-to-noise ratio. However, it is somewhat
more complex to calculate than AI and includes
revised frequency weightings and the masking effect of
one frequency band on nearby frequency bands. Like
AI, it has values that range between 0 and 1, but for the
same conditions SII values are slightly larger than AI
values.
a)
b)
2
1
Plan view
Cross section
Fig. 11.70a,b The most common paths for sound leakage
between closed offices. (a) Plan view; (b) cross section
(1-diffraction between openings, 2-transmission through
partitions, 3-structure-borne transmission, 4-transmission
through air gaps in wall seals, 5-diffraction through plenum
when partition does not seal to structural deck, and 6transmission
through ductwork)
3
4
5 6
Privacy Index
Low AI values indicate a higher degree of privacy. Since
this may be confusing to laypersons who want to focus
on better privacy conditions, a metric call privacy index
(PI) has been published in ASTM E 1130. The Privacy
Index (PI) is defined as:
PI = (1 − AI) × 100 , (11.9)
presented as a percentage. Higher PI values indicate
more privacy; lower PI values indicate less privacy. For
example, an AI of 0.10 corresponds to a PI of 90%.
Generally accepted practice for the design of openplan
offices refers to two levels of speech privacy.
Confidential privacy is defined as a condition in which
speech may be detected but not understood. Normal privacy
allows modest amounts of intelligibility, but normal
work patterns are usually not interrupted. These terms
are standardized in ASTM E 1374.