Sound Masking for Speech Privacy
Many open office areas employ electronic sound masking systems to create an even distribution of background noise that is just loud enough to help mask distracting noises (particularly from other conversations) without being loud enough to be a nuisance. The system typically consists of special loudspeakers mounted in or above the ceiling and a head-end unit that generates and shapes the noise.
Another useful application of sound masking is to improve speech privacy between rooms. Privacy is traditionally assessed simply by using the Sound Transmission Class (STC) of the partition between two rooms. However, this is only part of the picture. Privacy is dictated both by how much sound passes through the partition (STC) and the background noise level in the receiving room. Obviously, it is easier to eavesdrop on a conversation if the receiving room is quiet.
There are many situations where privacy is important and the background noise level can be used to our advantage. Speech privacy is important in healthcare where HIPAA requires that “reasonable safeguards” be taken to protect the privacy of patients’ personal healthcare information. The 2014 FGI Guidelines for Hospitals and Outpatient Facilities acknowledges the effect of background noise on privacy in the STC requirements between Exam Rooms. Table 1.2-6 requires STC 50 between Exam Rooms unless there is an electronic masking system, in which case only STC 40 is required. A 10-point difference in the STC rating is significant, and it may be less expensive to build STC 40 walls and install a sound masking system in an exam area than to build STC 50 walls and no masking.
Sound masking can also be useful for privacy in offices, particularly where there is a closed office or conference room where sensitive information is being discussed located next to an open office area. Sound masking can help keep private discussions confidential.
To demonstrate the effect of sound masking on privacy, we put together a series of recordings. The demo uses an example of a sensitive conversation that starts out at normal voice levels and then escalates to raised voice levels, which further demonstrates the effect of the masking sound level versus voice level. The conversation was played back in a small conference room with the door closed and recorded in the open office area outside the door to simulate the office example described above. The background level in the open office area was then increased with a sound masking system. You could also imagine that you are a patient in an exam room listening to the doctor discuss sensitive medical info with another patient in the next room. Please note that the masking sound in the demo audio is much more noticeable than it should be when permanently installed in real office environment.
Clip #1: Conversation inside the conference room
(Use this clip to set the volume of your speakers or headphones to a comfortable conversation level as if you were inside the conference room and leave the volume there for the other clips. If you are listening in a noisy environment, you might have to turn the volume up a little more to hear Clips #2 and #3 better.)
Clip #2: Listening in the open office with the door closed – 29 dBA background noise (no masking system)
(Both normal and raised voice levels can be easily understood even with the door closed.)
Clip #3: Listening in the open office with the door closed – sound masking system set at 43 dBA (Normal voices are unintelligible, and raised voice sounds muffled.)
This graph shows the background noise levels used for the demonstration. As you can see, the masking noise is much louder at speech frequencies (500 to 4,000 Hz) without being so loud that it would bother people in the open office.
One advantage to sound masking systems is that they are relatively easy to retrofit. For example, if you have a healthcare clinic where the isolation between exam rooms is not quite good enough and background noise is low, you might be able to install a masking system to achieve a little more privacy and meet the FGI or other guidelines. Adding masking is usually faster and less disruptive to the facility operations than making architectural improvements like adding layers of gypsum board to the walls, extending walls to deck above the ceiling, adding door seals, or treating duct crosstalk.
Adding sound masking is not a substitute for building good partitions and doors, but it can be part of a comprehensive solution to achieving good privacy.
Want to learn more about sound masking systems for speech privacy or open offices?
[Special thanks to 20th Century Fox for the use of the clip from the movie Office Space and to Cambridge Sound Management for the use of a masking demo kit for the recordings.]