Sound masking

Sound masking is the addition of sound created by special digital generators and distributed by normally unseen speakers through an area to reduce distractions or provide confidentiality where needed. The sound is broad band random that conveys no information about itself to a listener. It is often referred to erroneously as white noise or pink noise; the sound spectrum and level is specially shaped to provide the degree of privacy desired by occupants. Masking operates by covering up or masking unwanted sounds, similar to perfume that covers up other odors. This is in contrast to the technique of active noise control which attempts to eliminate the unwanted sound. Sound masking is used in homes, commercial offices, medical facilities, court rooms, and in secure facilities to provide secrecy.

Based on Sound Masking Done Right .

A seminal work covers the subject in some detail. Noise is defined as unwanted sound. It can have three effects depending mostly on level. At high levels, there are mechanical changes in a person, such as heating of the skin, rupture of the eardrum, or vibration of the eyeballs or internal organs. At lower levels, there are physiological (biological) changes in a person, such as elevation of blood pressure, or stress. At still lower levels, the changes are psychological (subjective) such as annoyance and complaints. Annoyance is based on factors such as the person's evaluation of the necessity of the noise, or whether it can be controlled, or whether it is normal for the environment (see Common Opinions about Sound). The levels of sound masking are sufficiently low that they have no known physical or physiological effects on people. One aim of sound masking design is to make the sound be "normal", i.e., acceptable.

Since most sound masking is used in offices, a number of cognitive psychology studies have been made that relate specifically to the office environment. One study found that there was a modest stress (physiological) increase and diminished motivation caused by typical office noises, including speech. It is recommended that the use of sound masking is under the control of the worker. Another study suggested that changes in level are an important factor, but that habituation to the noise can occur. In the office, habituation can be interpreted to mean "I’ve grown used to the noise and it no longer distracts me" or "Since I cannot do anything about it, I will have to live with it." Another study point out that the specific information within the speech intrusion is not important nor is the "intensity" (level) of the sound between 48 and 76 dBA. Since the energy level of the louder sound was 1,000 times that of the least, one must assume that distraction occurred for all levels. For arithmetic tasks, both speech and non-verbal intrusive noises caused significant performance decreases. For "prose tasks" it was found that speech caused a greater performance decrease than nonverbal noises. In another study the author added several significant observations. It was found that "during a serial recall task, the accuracy of report decreases 30 to 50%." When the intrusive speech was increasingly filtered to a meaningless mumble, there was a monotonic increase in performance. Finally, the author states: "Perhaps the single feature that makes the irrelevant speech phenomena so fascinating is that the processing of sound is obligatory; it appears beyond the individual’s control." Within the references cited above are further references to earlier works on this subject. There are several implications for a sound masking system. The masking must reduce the difference between the steady background level and the transient levels associated with both speech and other sounds. Motivation and productivity are improved when this is accomplished. The masking sound itself must not change rapidly and should be as meaningless as possible.

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