Creating Quiet: Sound Masking in Patient Rooms

Increased noise has become a concern for patients as more noise sources, such as new medical instruments or the layout of patient rooms as they relate to the nurses’ station, have been added to the hospital environment. Numerous studies have been conducted to evaluate the impact of these noise levels on the hospital environment, but few have considered the type of noise source. Surely, the sound of a fan will be very different from an alarm, even if they measure at the same sound levels. Without clear objectives on the type of noises studied, there is a false impression that a quieter environment is always a better one.

Granted, some of these noise sources are necessary in a modern working hospital. The trick is to take a different look at these noise sources and develop more efficient methods in reducing the disturbance to patients. Medical clinicians provide a very important perspective. Florence Nightingale understood that some noise in a working environment was necessary, but that it was important to keep these noises to a minimum. “Unnecessary (although slight) noise injures a sick person much more than necessary noise (of a much greater amount),” she wrote. She also documented that impulsive sound was more disturbing than constant noise. “But intermittent noise, or sudden and sharp noise, in these as in all other cases, affects far more than continuous noise — noise far more than noise without.”

More than 100 years later, the World Health Organization (WHO) and Environmental Protection Agency (EPA) guidelines for hospitals require low noise levels in patient rooms. The WHO guideline for continuous background noise level in patient rooms recommends 35 dBA during the day and 30 dBA at night, with nighttime peaks in wards not to exceed 40 dBA. The EPA guideline values for continuous background noise levels are 45 dB (A-weighting is assumed to be implied) during the day and 35 dB during the night in patient rooms. Considering that “casual” sound pressure levels for speech at a distance of 1-meter range from 50-58 dBA, limiting peaks to 40 dBA, these guidelines would preclude conversation in corridors with patient doors open. This requirement conflicts with nurses’ needs to see patients and to discuss patient care. These guidelines from WHO and EPA are also based mainly on transportation noise, which has a character that is quite distinctive and bothersome to building occupants.

Unfortunately, most sleep disturbance studies do not document the differential between the background noise level and the impulsive noise level. One exception was a study by Harvard Medical School that was conducted with acousticians in which impulsive sounds were studied in the context of low background sound (between 32 and 35 dBA). To address distraction in an office environment or a library, introducing a constant noise source helps to reduce the impact of impulsive tonal noises such as speech. It stands to reason people would react similarly to impulsive noises asleep as they would awake.

The typical background noise level can be considered a constant noise with full frequency content. These noise sources include air movement from the building’s HVAC system and cooling fans, as well as electronic sound masking systems. Tonal and impulsive noise sources typical of healthcare activity include conversation, alarms, paging systems, pneumatic tubes, phones and movement of carts. One study conducted on healthy patients simulating ICU noise found that increasing the ambient noise level with white noise reduced the chance of awakening by these impulsive noise sources. Unfortunately, the study had a relatively small sample set (four participants) and used relatively high noise levels (62 dBA) as background sound, which is significantly louder than the low noise levels recommended by design guidelines.

Regardless of the background noise level used for the sound masking experiment, most studies do indicate that impulsive noise events should be limited as much as possible and that excessive constant background noise levels will cause sleep disturbances. What may be counterintuitive is that the quieter the background noise level, the more intrusive the impulsive noise source. This is because the key is the difference in levels between the background noise and the impulsive noise. Subjectively, the higher this difference, the more our ears distinguish the impulsive noise.

Since increasing the background noise will reduce the impact of impulsive environments, it stands to reason that increasing the background noise level in a controlled manner with sound masking will reduce the impact to occupants during sleep as well. Keep in mind that too high of a background noise level, regardless of whether it is broadband or tonal, will still reduce sleep quality. To determine the appropriate average noise levels in a hospital, facilities can consider the following methods for evaluation:

• Measure background noise levels in a large sample set of patient rooms. Evaluate the Hospital Consumer Assessment of Healthcare Providers and Systems (HCAHPS) survey question: “How often was the area around your room quiet at night?” The hospital’s rating on this question is based on the average noise level in the patient room as well as the peak noise sources’ amplitudes and frequency.

• Once it is determined that sound masking can help the HCAHPS rating, increase background noise levels electronically.
A background sound level between 45 and 48 dBA is ideal in a patient room. Implement a patient-controlled sound masking system in a number of single-bed patient rooms for an extended period of time. The sound-masking system would consist of a speaker in the ceiling plenum, ceiling or in the room, and generates a full-frequency noise source. A control device allowing the patient to adjust the level of the masking sound in various degrees would be provided. The noise in the hallway, the door’s open/closed status, the noise in the patient room, as well as the noise level selected by the patient, should all be noted and documented.

• Once the sound masking system is added, reassess the HCAHPS quietness scores along with patient outcome metrics for the rooms in which sound masking was deployed.

In addition to adding a higher level of background sound level to the health care environment for the purpose of sound masking, acoustically absorptive materials should be included in the hospital design to reduce the amount of sound energy that is bouncing around the occupied areas. Television speakers are for the most part now kept at the bed of the patients rather than far away on a wall. In addition, hospitals should limit overhead pages, turn pagers to vibrate and inform staff to be cognizant of their conversational level. Careful design of the nurses’ stations will also help mitigate conversations and noise sources to patient rooms.

Aaron Bétit, Ben Davenny and Rose Mary Su are senior consultants in acoustics at Acentech Inc., a multi-disciplinary acoustics, audiovisual systems design and vibration consulting firm with offices in Cambridge, Mass.; Trevose, Penn.; and Los Angeles.