Privacy index
Privacy index (PI) is the inverse of AI, expressed as a percentage. The higher the PI, the better the speech privacy:
- 95 to 100 per cent represents confidential privacy;
- 80 to 94 per cent represents normal privacy; and
- values less than 80 per cent represent minimal or no speech privacy.
Speech intelligibility index
Similar to AI, the speech intelligibility index (SII) is calculated using speech spectrum level, background noise level, hearing threshold level, and a modulation transfer function that better accounts for distortion of the speech signal due to reverberation.
Speech privacy class
A single-number value developed to define speech privacy in closed rooms, private offices, and conference rooms, the speech privacy class (SPC) accounts for the sound transmission loss properties of the intervening construction as well as ambient background noise levels to predict average time intervals between lapses in speech intelligibility and audibility between rooms.
Speech transmission index
Speech transmission index (STI) is a single-number value on a scale from 0.0 to 1.0. Rating speech intelligibility, it takes into account the spectrum of the sound source, reverberant conditions, and background noise. STI is especially useful in determining speech intelligibility in reverberant spaces.
Articulation class
A single-number rating of the acoustical performance of products (e.g. acoustic ceiling tiles) used in open-plan office environments with cubicle partitions, articulation class (AC) is dependent on the product’s ability to absorb sound, particularly at higher frequencies, carrying the consonant sounds that are critical for speech intelligibility.
Sound-reflective surfaces, such as gypsum board, may have an AC value of 120, while ceilings best-suited for open-plan offices may have AC values of 180 or higher for moderate levels of privacy. For maximum privacy, AC values greater than 200 are typically recommended.
Speech intelligibility and privacy summary
If the previous terms and definitions appear quite similar to one another, it is because they are. With the exception of AC, which is a simplified value assigned to a product, all the speech privacy and intelligibility criteria listed are closely related. The origins of the study of speech intelligibility go back to the 1930s with AI becoming the first standardized calculation method of its kind in 1969 (i.e. via ASTM E1130, Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index). Since then, it has been a steady process of iterative improvements, adjustments and adaptations with the development of STI, SPI, and SII, and other quantifiers less frequently used than those listed above.
Myriad factors playing a role in speech privacy and intelligibility, including speech sound source levels and spectra, room acoustic conditions, background noise levels and spectra, and the sound transmission properties of the wall/floor/ceiling systems between open-plan and closed spaces. Therefore, speech privacy and intelligibility should be developed as part of a holistic acoustic approach with architecture, interior furnishings and finishes, structure, and mechanical/electrical/plumbing (MEP) systems.
Conclusion
For the sake of presentation and discussion, this article’s authors have segmented different areas of acoustics: sound transmission, room acoustics, and speech privacy and intelligibility. However, these segments are all enmeshed, intimately tied together in the fast-flowing movement of those sound pressures wave travelling through a fluid medium—much like those waves in water.
Andrew Schmidt, ASA, is an acoustical consultant experienced in a wide array of commercial and residential construction types. With an engineering degree from the acoustics and music program at University of Hartford, he spent his early years as a consultant at Jaffe Holden Acoustics, under the mentorship of the late Chris Jaffe. Schmidt’s expertise spans all areas of architectural acoustics, including sound isolation, room acoustics, and mechanical systems noise and vibration control. He joined USG Corporation (the parent company of CGC Inc.) in 2017 as a senior researcher with the Building Science & Technology Commercialization group. Schmidt can be reached at aschmidt@usg.com.
Ajith (AJ) Rao, PhD, WELL AP, is a senior researcher with the Building Science & Technology Commercialization group at USG, based at its Corporate Innovation Center in Libertyville, Ill. He has a PhD in architectural sciences from Rensselaer. Rao’s core expertise lies in development and application of codes, standards, and technologies related to high-performance buildings. He can be reached at arao@usg.com.
