There are many counselling service professionals (professional counsellors, therapists) that need speech privacy in their offices (examination rooms):
Many of the professional counselors and therapists operate from home offices, or from small clinics. They need a high level of privacy in the rooms where they talk to clients.
Quantitative Sound Insulation Objectives for Speech Privacy
Levels of speech privacy in closed rooms are specified in ASTM standard E2638-10. This standard defines Speech Privacy Class (SPC) as a single-number metrics for quantifying degrees of speech privacy as follows:
Minimal speech privacy
One or two words will be intelligible at most once each 3 minutes, and speech sounds will be frequently audible (at most once each 0.6 minutes)
Standard speech privacy
One or two words will be occasionally intelligible (at most once each 18 minutes), and speech sounds will be frequently audible (at most once each 2 minutes)
Enhanced speech security
One or two words will be rarely intelligible (at most once each 2.3 hours), and speech sounds will be occasionally audible (at most once each 12.5 minutes)
High speech security
Speech essentially unintelligible (at most once in 16 hours) and very rarely audible (at most once each 1.5 hours)
Very high speech security
Speech not intelligible and very rarely audible (at most once each 11 hours)
It is important to note that SPC defines speech privacy (unintelligibility), not speech audibility. As per information in reference , SPC must be over 82 to ensure that 80% of listeners will consider the speech inaudible.Most professional counsellors (therapists) are looking for at least “standard speech privacy” SPC 75, or if possible, “enhanced speech security” SPC 80.
Speech privacy in therapists' offices
As per the ASTM standard, speech privacy is governed by two components:
- 1Sound attenuation of the office walls and door.
- 2Background noise level at the unwanted listener’s location.
The sum of background noise level in dB at the unwanted listeners position, and sound insulation in dB of the wall separating the listener from the talker, adds up to the required SPC.
An acoustical consultant will measure the sound insulation of the therapist’s office walls and doors and decide if improvement in soundproofing is required or not. In many cases it may be possible to ensure speech privacy by increasing background noise level at the unwanted listener’s location, usually the waiting room.
Door in a partition wall is an acoustically weakest point. Even a well sealed, good quality door will provide sound insulation of only about 33 dB. This is only adequate if background noise level near the door is at least 42 dB for standard privacy, or 47 dB for enhanced privacy. Both levels are achievable with professionally installed sound masking system.
 ASTM metrics for rating speech privacy of closed rooms and open plan spaces. NRC research paper by B. Gover & J. Bradley
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