Speech privacy – open plan
The degree of speech privacy attained in an open plan space is indicated by its privacy index (PI). It takes into account the acoustical performance of everything in the space, including ceiling, walls, floorcovering, furniture, and background noise and voice levels. It is expressed as an integer value between zero and 100.
The recognized levels of speech privacy as defined by ASTM E1130, Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index, are as follows.
Confidential
Represents a PI rating of 95 to 100. Nearby occupants may hear muffled sounds but the meaning of spoken words is unintelligible, and occupants are not distracted from what they are doing.
Normal or non-intrusive
Represents a PI rating between 80 and 95. Nearby occupants may hear some of the conversation but the loudness of speech is not distracting, and occupants can generally continue what they are doing. Non-intrusive speech privacy is a common goal for open-plan environments, especially where ‘knowledge worker’ productivity is a key issue.
Marginal or poor
Represents a PI rating of 60 to 80. Nearby occupants can understand most words and sentences, and the loudness of speech can be distracting to them.
No privacy
Represents a PI rating of 60 or less. Nearby occupants can understand all words and sentences and the loudness of speech can be a constant distraction.
Speech privacy – closed plan
The degree of speech privacy attained in a closed plan space is indicated by the following.
Speech privacy class (SPC)
It is calculated as the sum of factors related to the sound isolation provided by the room construction and furnishings and the ambient background sound. It is determined by a test procedure conducted in an existing space according to ASTM E2638, Standard Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room. SPC values normally range from 70, which represents ‘minimal speech privacy,’ to 90, which is speech security.
Noise isolation class (NIC)
It is a single-number rating indicating how much sound is blocked between adjoining closed spaces, taking into account all transmission paths and acoustic properties of the source and receiving rooms. This includes walls, door, windows, ceiling, lights, etc. It is a field test to assess actual installed conditions. It is conducted according to ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings. The higher the number, the better the sound isolation.
Speech privacy potential (SPP)
It is a single-number rating serving as a rule-of-thumb prediction of speech privacy between two spaces. It is a function of two factors: the designed ambient background sound level in the receiving space and the designed sound isolation performance of the total construction between the two spaces.
SPP is simply the sum of the two factors (sound isolation and background sound). An SPP rating of 75 is minimally acceptable speech privacy and greater than 80 is high speech privacy.
Acoustical parameters
To attain a specific level of speech privacy for a space, it is important to be familiar with several acoustical performance parameters influencing it.
Noise reduction coefficient
The noise reduction coefficient (NRC) indicates the ability of a material to absorb sound from all angles. It is expressed as a number between 0.00 and 1.00, and indicates the average percentage of sound the material absorbs. An NRC of 0.60 means the material absorbs 60 per cent of the sound striking it. The higher the number, the more sound the surface absorbs. A ceiling with an NRC less than 0.50 is considered low performance, and one with an NRC that is greater than 0.70, high performance.
