February 2016: The Alphabet Soup of Office Acoustics

The language of office acoustics can be very confusing.  Add to that, Green Building rating systems like LEED and FGI list speech privacy requirements, but some of the criteria doesn’t make much sense. This month, we will help de-mystify rating systems so it doesn’t sound so much like alphabet soup.

Enclosed rooms are intended to render speech unintelligible to casual listeners outside the room. Here are some of the more common acronyms used to describe isolation of enclosed spaces.

STC – Sound Transmission Class. This commonly used rating describes the performance of a partition between two enclosed spaces and is a laboratory measurement. If measured in the field, it is denoted ASTC where the A stands for Apparent.

NIC – Noise Isolation Class. This is a field measurement of the isolation between two spaces. Whereas the ASTC attempts to rate the performance of the partition itself, the NIC rates the isolation between two rooms including flanking paths.

SPC – Speech Privacy Class. This rating system can be either measured or calculated and is the value of the threshold of audibility and intelligibility of enclosed rooms. A rating of 75 and above indicates speech privacy while anything below 70 is poor.

Sound transmission, latteral





Open floor plans provide the most productivity when there is a minimum of distractions from voices and office equipment. Here are some of the metrics used in open office acoustics.

AI – Articulation Index. This rating is a measured metric and ranges from 0 to 1. Ratings under 0.20 are acceptable and above 0.40 are undesirable.

PI – Privacy Index. This is the opposite of AI and is also a measured metric; it is simply 1-AI X 100, so it is a percentage from 1 to 100.

STI – Speech Transmission Index. This is an objective, physical measure of speech transmission quality. It is on a 0 to 1 index whereas perfectly intelligible speech has a rating of 1 and as the rating approaches zero, all information is lost.

SII – Speech Intelligibility Index. This rating system is based on either measured values or calculated predictions utilizing estimates of speech spectrum level. It can be used in both enclosed rooms and open plan spaces.

Open Office

The issue with the AI, PI and STI is that LEED and FGI use these as design criteria, but they are solely measured metrics. There is no standard method to calculate these ratings from a set of drawings, which poses problems. Also, LEED V4, which is the most recent version, references FGI 2010 in the Healthcare section which is the predecessor to FGI 2014, the latter of which has been updated to address some of these problems by eliminating the STI and replacing it with the SII. Confused? We are, too.  It is our hope that the rating systems can be updated for the subsequent versions of LEED, FGI and other Green Building standards.

As we have moved into our new office at 8 Penn Center in Philadelphia this month, we are putting our expertise to task. We hope to see you soon – look for an open house later this Spring! But, in the meantime if the alphabet soup of office acoustics has your head spinning, we are just a phone call away!

alphabet soup

March 2016: Rain Rain Go Away
January 2016: Let’s Get Weird Volume III