Page 24 - Fall 2007

innumerable articles on the negative effect of hospital noise on both nurses and patients. Yet very little has been done to solve the problem beyond administrative controls (please talk quietly, do not page so often), which are of limited effec- tiveness. The passage of the Health Insurance Portability and Accountability Act,2 includes requirements for speech priva- cy associated with medical information. This legislation has provided an unprecedented opportunity to solve both the disturbance and privacy problems.
There is an active group of acousticians helping hospital designers provide good acoustics in medical facilities. The struc- tural solution is well in evidence with those complying with the law. For example, anyone visiting a pharmacy is asked to stand back six feet behind the yellow line. However, there is a glimmer of hope for a dynamic (masking) solution in hospitals.
Schieber3 found that the amount and rate of increase in the
sound level from a constant background was the main contrib-
utor to full awakening, or changes in the stages of sleep. He
determined that the magnitude of the change in level, regardless
of its median value, was more significant than the level of a steady
A test was done in the patient room of a modern hospi- tal. Night time hourly percentile levels were calculated and the results are shown in Fig. 1. As is usual, the door was kept open during the night. It is clear that there are several peri- ods with levels sufficient to cause awakening in the patient.
Although masking has been applied in doctors’ offices for over twenty years, nursing homes, pharmacies, hospitals and medical providers (public health authorities, life insurers, billing agencies, and service organizations) have yet to appre- ciate the effectiveness of sound masking. All standard com- mercial masking equipment can be used in medical facilities.
Speaker locations
There are criteria commonly used to insure that a mask- ing system performs well. They are: (1) spectrum contour; (2) overall level; and (3) spatial uniformity of the overall level. Experienced professionals might include spectrum smoothness. In most cases, these criteria can be met success- fully. However, it has been found that another criterion is needed to improve both performance and acceptability—dif- fuseness of the masking sound field. For those familiar with lighting this is similar to Equivalent Sphere Illumination, an accepted lighting criterion for reducing glare. A diffuse sound field reduces “acoustical glare” that, in practical terms, means that the source of the sound cannot be located. But it also means that listeners will accept higher masking levels. Most applications have speaker arrays above suspended ceil- ings but masking can be applied successfully in other loca- tions. The criterion of diffuseness can be used to develop a hierarchy of preference for those speaker locations.
sound of the same median value. This conclusion was support- 45
ed by an Environmental Protection Agency document. Suter expanded this finding by stating “it is clear that intermittent and impulsive noise is more disturbing than continuous noise of equivalent energy, and that meaningful sounds are more likely to produce sleep disruption than sounds with neutral content.” Griefahn6 noted that the difference between the ambient and the single event levels should not exceed 8 to 10 dB.
Fig. 1. A large difference between two important percentile levels in the patient room of a modern hospital will awaken patients. Significant awakening events occurred during the night time hours for this sample.
1. Speakers under raised floors. Because the loss of sound through a raised floor is considerably higher than that through a suspended ceiling, the amount of sound reaching the listener from other directions is relatively higher, resulting in a high degree of diffuseness. Field expe- rience has shown that the diffuseness is sufficiently high that neither the source nor the direction of the sound can be identified.
2. Speakers in a plenum above very high suspended ceil- ings. The sound reaching the listener from above arrives from a broad range of angles improving diffuseness. Because it is still possible to determine that the sound is coming from above, it is rated second in preference.
3. Speakers in an open ceiling plenum without a sus- pended ceiling. The sound reaching the listener from above arrives from a broad range of angles improving diffuseness. Because it is possible to determine that the sound is coming from above and because the speakers can be seen, it is rated lower in preference.
4. Speakers in a plenum above a normal height suspend- ed ceiling. This location has been the standard for many years. Because suspended ceilings are normally near nine feet high, the direction from which the masking comes can be identi- fied, and diffuseness of the sound is reduced. With very high plenum depths the diffuseness is better.
5. Speakers mounted face-down in a suspended ceiling. Because the listener is in the direct sound field of the speak- ers, diffuseness is considerably reduced, so the source of masking can be identified acoustically, and in some cases, visually. Shadowing of the masking can occur with higher open office furniture panels. Although this is the least desir- able location for a masking speaker, it can be beneficial when the plenum above is very small.
Determining the amount of diffuseness for each location would make an interesting study. When choices are available, the above hierarchy is recommended. Masking speakers for any of the above locations are currently available, including those that fit into a small plenum or floor cavity. Most mask- ing systems can accommodate the required difference in spectrum contour and overall level.
22 Acoustics Today, October 2007