Patient bed areas and the spaces opening onto them shall be designed to produce minimal ambient noise and to contain and absorb much of the transient noise that arises within the nursery. Overall, continuous sound in any occupied bed space or patient care area shall not exceed: (1) an hourly Leq of 50 dB and (2) an hourly L10 of 55 dB, both A-weighted, slow response. The 1-second duration Lmax shall not exceed 70 dB, A-weighted, slow response.
The recommended criteria apply to every bed space in occupied, newly constructed or renovated nurseries; they do not apply to existing nurseries. They were developed by the Sound Study Group of the National Resource Center: The Physical and Developmental Environment of the High-Risk Infant.1 Subsequently, they were reviewed by a panel of experts in audiology, acoustical engineering, developmental psychobiology, hearing science, hospital design, infant development, nursing, and neonatology. The criteria have been adopted by the Committee to Establish Recommended Standards for Newborn ICU Design and have been endorsed by a number of professional associations and at least one state regulatory agency as part of the Committee's Recommended Standards for Newborn ICU Design?
RESEARCH SUPPORTING A GENERALLY QUIET ENVIRONMENT FOR PRETERM INFANTS
The human auditory system undergoes the majority of its development before term gestational age, with cochlear function and hearing commencing by —22 to 24 weeks.3 For humans, as for other mammals, the “interactions between auditory structures and their environment are critical to normal development.”3 Prematurely born infants lack the genetically appropriate stimulation of the uterine sound environment. This environment has both protective and adaptive functions.4-8 It is moderately loud with low-frequency background sounds predominating and the maternal voice providing a distinctive and clear signal against the background.9 Despite attention paid to maternal heartbeat sounds in older studies, contemporary equipment and methods fail to detect consistent sounds from maternal or fetal heartbeats.10 Unlike intrauterine sounds, traditional nursery sounds are airborne, comprise a wide range of frequencies, and are continuous, unpredictable, and strong, even by adult standards.11-13
The long-term effects of nonexposure to the biologically expected environment of the uterus and of exposure to the sounds of a hospital nursery are difficult to study in humans. Animal studies, however, indicate that normal sensory development relies on moderate stimulation during periods of rapid brain growth and organization. They also indicate that unusually strong or atypically timed stimulation to one sensory system may interfere with perceptual organization in the same and other sensory systems.14-17 For example, strong auditory stimulation during a period of rapid growth in the visual system may interfere with the organization of visual perceptions.16,18 There is, however, some evidence in animals for a lessened effect of moderate, yet out of sequence, stimuli.19 Such findings argue for moderation in sensory stimulation during a period of rapid brain growth such as occurs during the last trimester of human gestation.
Detrimental physiological responses of term newborns to hospital nursery sounds include apnea and bradycardia as well as abrupt fluctuations in heart rate, blood pressure, perfusion, and oxygen saturation.20-22 These changes are associated with brief auditory stimuli at ≥80 dBA.21,23,24 Changes in heart and respiratory rates, transcutane-ous oxygen tension, and intracranial pressure have been documented after exposure of preterm infants to bursts of sound at >70 dBA.20 These events may alter blood oxygenation and, therefore, can affect all the vital organs. The infant residing in a traditional nursery can experience many such fluctuations daily over weeks of rapid brain growth. Potential consequences include increased risk of weakened vessel walls in the cerebral vasculature.25-27
Maternal Speech Discrimination by Infants
The third trimester fetus is inferred to be learning the particular features of maternal speech, because term newborns make fine discriminations among the features of their own mother's vocalizations shortly after birth.28,29 Therefore, the opportunity to hear the mother's voice as a relatively clear signal may be an important aspect of the fetal sound environment.
Although human studies are not available, it is more difficult for an animal with immature hearing to discriminate particular features of a signal in the presence of masking sounds than it is in a quiet field, particularly if the masking sounds have a similar frequency and pressure to the signal.30 The frequency of human speech is between 200 Hz and 6000 Hz, the range of most acute hearing for humans.31 A mother's speech to her infant has the same frequencies as other speech in the nursery, monitor alarms, and telephone ringers and could, therefore, be masked and rendered less intelligible by these sounds. Furthermore, it is known that sound levels typical of the traditional nursery interfere with speech comprehension in adults. If persons with fully matured language systems have difficulty discerning speech content accurately in an environment, it is reasonable to assume that persons just beginning to organize sounds into a language system will also have difficulty making fine discriminations in that environment.
Potential for Noise-Induced Hearing Loss
Studies of nursery noise confirm that sound levels do not exceed Office of Safety and Health Administration or National Institutes of Health standards for adults in industry.32-36 Not surprisingly, therefore, although nursery noise is often suggested as an agent in the loss of hearing acuity in preterm infants, causative relationships have not been demonstrated consistently.37-39 The recommended criteria do not address the potential relationship between nursery noise and hearing loss.
RESEARCH SUPPORTING SPECIFIC NOISE CRITERIA Sleep Disruption
The primary objectives of the criteria are to preserve a large portion of each hour for infant sleep. Sound at the pressure levels, frequencies, and durations commonly encountered in the hospital disturb sleep in healthy term infants, either by causing a change in sleep state or by awakening the infant.40,41 Preterm infants who have difficulty maintaining stable behavioral states experience the same or greater sleep disruption as do term infants to similar stimuli.42-44 Frequent disruption of behavioral states and forced, abrupt transitions between states (such as occur with rapid onset, loud sound) may interfere with the development of stable behavioral states and of well-regulated transitions between states.42,43,45
Interference with Speech Intelligibility Among Adults
An additional objective is to improve the intelligibility of spoken communication in the nursery. Noise at levels typical of intensive care nurseries is known to interfere with adult communication as well as with the performance of complex tasks, and to increase work-related stress.31,46 Interference with conversational speech is defined by signal-to-noise ratios. The background sound intensity (i.e., the noise) in a traditional nursery is typically in the 50 dBA to 70 dBA range. The sound intensities of conversational human speech, particularly the consonants (i.e., the signals) are exceeded by these background sound intensities.
RATIONALE FOR SPECIFIC NOISE CRITERIA Hourly Leq of 50 dBA
The hourly Leq is intended to preserve sleep for most healthy, term infants most of the time. In a carefully controlled study of 126 infants, Gadeke et al.40 determined that <5% were disturbed or wakened by 12 minutes of broad band noise at 50 dB. The percentage of disturbed or wakened infants rose sharply with each 5-dB increase so that 5% of infants were affected by a 55-dB stimulus and 20% were affected by a 60-dB stimulus. The linear measurements used in this study are roughly equivalent to A-weighted measurements at the frequencies and intensities of the experimental stimulus.
Hourly L10 of 55 dBA
The L10 ensures that, regardless of the hourly Leq, sound levels may exceed 55 dBA only 10% of the time or a total of 6 minutes of any hour. The L10 limit is supported indirectly by several studies, including that described above by Gadeke et al.40 Similarly, Wedenberg47 showed that term newborns were wakened from a light sleep by a mean sound level of 55 dB. These stimuli were 2- to 5-second pure tones at 500 and 3000 Hz repeated irregularly for no more than 1 minute from the first to last tone.
The L10 of 55 dBA translates to a nursery that interferes with normal speech intelligibility at a distance of 12 feet for a maximum of 6 minutes of each hour.48 Conversely, this L10 enables caregivers at nearby bedspaces to speak at normal conversation levels and be clearly understood 12 feet away ∼90% of the time.
Lmax of 70 dBA
Bursts of noise will also rouse babies or cause startle responses. These bursts require control even though they contribute marginally to both the Leq and L10. The Lmax is conservative in that it describes sounds of ≥1 second in duration. The hospital nursery produces many sounds that are both more brief and more intense than this limit. However, there are no studies to support a limit for a newborn's exposure to these transient sounds.
Using 3-minute exposures to a broad band sound of 100 Hz to 7000 Hz, Gadeke et al.40 observed that 25% of infants were disturbed or wakened at 65 dB (linear) and 45% were disturbed or awakened at 70 dB (linear). A startle reflex nearly always accompanied this change in behavioral state with a stimulus of 70 dB to 75 dB (linear). Steinschneider et al.49 studied the motor responses of term newborns to 5-second bursts of white noise covering a uniform spectrum from 200 counts per second to 20,000 cycles per second. The infants were in a light sleep state and ambient background noise was 47 dBA. They found that 55% of the infants startled 2% of the time in response to a 55-dBA stimulus, 78% startled 10% of the time in response to a 70-dBA stimulus, and 100% startled 25% of the time in response to an 85-dBA stimulus. All of the infants in this study had motor responses to each level of stimulus. The latency to the startle response decreased and the amount of general motor response increased significantly with increasing sound intensities. Miller and Byrne41 showed that newborns were wakened from a light sleep 40% of the time by a 2-minute stimulus at 78 dBA against a 58-dBA ambient background. Although not tested, it is possible that the response in this case was due to the 20-dBA difference from the background rather than the absolute dBA of the test tone itself. Wedenberg47 showed that term newborns in deep sleep were wakened by pure tones at mean sound levels of 70 dB to 75 dB lasting from 2 to 5 seconds. These pure tones were presented at various intervals but for no more than 1 minute from first to last tone.
COMPARISON WITH OTHER CRITERIA
The recommended criteria are similar to or more lenient than criteria set by various cities, U. S. government agencies, and the American Academy of Pediatrics.50 Unlike the others, however, these criteria specify level limits for hourly intervals as well as for upper tenth per-centiles and transient events, thereby reducing possibilities for large excursions above the average, which can be permissible if limits are set for 12- or 24-hour periods.
For example, the New York City Noise Control Code provides that the 10 PM to 7 am hourly Leq at the property line of a property impacted by construction noise may not exceed 50 dBA in a low-density residential area.51 Similarly, the U. S. Federal Highway Administration stipulates that highway noise may not cause interior sound intensities to exceed an Leq of 52 dBA and an L10 of 55 dBA in residences, churches, libraries, schools, and hospitals.51 The American Academy of Pediatrics recommends following the 1974 U.S. Environmental Protection Agency proposal for an averaged daytime level of 45 dBA and an averaged nighttime level of 35 dBA.50,52 Note that the Environmental Protection Agency proposal was intended for an unoccupied building.
DESIGNING, EDUCATING, AND MONITORING TO MEET THE CRITERIA
Professional architects and engineers with experience in building acoustics and noise control can assist hospitals in design and construction to meet the criteria. Because human behavior influences the way the building functions when occupied, regular monitoring will determine ongoing compliance. The nurseries currently meeting the criteria contain many soft surfaces, and nursery staff receive ongoing education regarding the need for a quiet environment. Clearly, there is a role for equipment manufacturers to assist in supporting a quiet nursery through product design.
The methodology of measuring nursery sound is technically complex and is addressed elsewhere.52 In brief, the measuring device of choice is a programmable dosimeter, which can be set to avoid accidental or purposeful tampering and which has sufficient memory to accumulate data for several consecutive days. A dosimeter measures sound and integrates the measurements into dose equivalents over time.
As each nursery has unique physical dimensions, mechanical systems, surface materials, and equipment, sound levels can vary considerably from one bedspace to another. Consequently, measurements are taken at or near the head of individual infants in each crib or incubator. The bedspaces shown over time to be most noisy can serve as proxy indicators for the whole nursery in a quality assurance program. Although sound levels in staff work areas may exceed the criteria, attenuation may not be necessary if individual bedspaces remain within the recommended limits.
LIMITATIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH
The recommended criteria are an early attempt at guidance, based on the available scientific literature. They address only sound levels. However, frequency, pattern, rise/fall characteristics, and cyclicity are also important features of the acoustic environment. The criteria should evolve to reflect the best knowledge at a given time.
Research is needed to determine an auditory environment that moves beyond interfering with normal functions (e.g., sleep and speech intelligibility) toward one that actively supports and facilitates all aspects of development. Both animal and human studies are needed to determine the effects of biologically expected and atypical environments on the short- and long-term function of auditory and other sensory systems, as well as on functions related to auditory processing such as physiology, communication, and reading skills.
Note: definitions are given here as an aid to the nontechnical reader. Technically specific definitions and further explanations of terms and concepts can be found in the texts cited here and in literature provided by the American National Standards Institute.*
A-weighted - A frequency-weighting filter that approximates “the frequency response of the human ear, which is less sensitive to low-frequency than to high-frequency sound. A scale decibel levels are expressed as dBA.”53 “There is an impressive degree of correlation between A-weighted sound level, speech interference level, loudness level, [and] noisiness level…”;54
Ambient sound - Ongoing sound in a defined space usually distinct from a specific signal of interest. Also “The sound… levels associated with a given environment. Ambient noise is usually a composite of sounds from near and far sources none of which are particularly dominant.”55
Broad band noise - “A spectrum [of sound] consisting of a large number of frequency components, none of which is individually dominant.”55
Leq - Equivalent sound level. “… the equivalent steady [dBA] noise level which in a stated period would contain the same noise energy as the time-varying noise during the same time period.”31
Lmax - The maximum dBA sound level of <1 second in duration during a measurement period.54
L10 - “That time-varying dBA [sound] level which will be expected [i.e., exceeded] 10% of the time [during a specific measurement period].”56
Masking - The effect of raising a listener's hearing threshold by the presence of another sound. Also “… the number of decibels by which a listener's threshold of audibility for a given tone is raised by the presence of another sound.”57
Noise - “… any undesired sound.”57
Slow response - A measurement period of 1000 msec.54 This response time approximates that of the human ear, which does not respond to extremely brief sounds.
Sound (noise) level - “… the sound pressure level in decibels measured by the use of the A… frequency weighting and… slow exponential-time-averaging [response]… constant.”54
Tone - A discrete frequency or narrow band of frequencies that is easily perceived as different from broader band sounds even though close in frequency and level.
Transient sound - “ [A sound]… which occurs only once and, hence, cannot be analyzed at leisure. Some… [transient sounds]… exist for as long as several seconds in a quasi-steady state. Others are of very short duration.”54
Expert Review Panel
Robert M. Abrams, PhD, University of Florida; Heidelise Als, PhD, Harvard Medical School; Jack B. Evans, PE, Jack Evans and Associates, Inc.; Kenneth J. Gerhardt, PhD, University of Florida; Cara Krulewitch, CNM, PhD, National Institute of Nursing Research; Robert Lickliter, PhD, Virginia Polytechnic Institute and State University; Donald Nielsen, PhD, Central Institute for the Deaf; Constantine Trahiotis, PhD, University of Connecticut, Farmington; Robert White, MD, Memorial Hospital of South Bend
Sound Study Group of the National Resource Center: The Physical and Developmental Environment of the High-Risk Infant
Alex Robertson, MD, East Carolina University Medical School, Chairperson
Carl Bose, MD, University of North Carolina, Chapel Hill William Engle, MD, Indiana University Medical Center Stanley N. Graven, MD, University of South Florida, Tampa, Director James W. Hall III, PhD, Vanderbilt University Warren Karp, PhD, DMD, Medical College of Georgia Patricia Martin, PhD, RN, Wright State University, Dayton M. Kathleen Philbin, RN, PhD, University of Texas - Houston Medical School Janet Stockard, MS, CCC-A, University of South Florida, Tampa Karen Thomas, PhD, RN, University of Washington
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