We carried out this study to evaluate the soundscape in our rodent housing facility and to document sources of background and transient sounds, loud sounds and noise variability in two rooms with different barrier caging systems. Our results suggest that despite the introduction of new mechanical equipment, the soundscape in our facility is not substantially different from those described in reports published 10–20 years ago1–3
. Furthermore, our results show that soundscapes can vary significantly among different animal rooms. Like Milligan and colleagues1–3
, we noted that noise varied over time during the work day, and noise fluctuations were reduced during non-work hours. Although it is difficult to make a direct comparison due to the different frequency weighting used in this study, overall sound pressure levels appear to be similar.
The soundscape in an animal facility is composed of background (baseline) noise arising from electrical and mechanical systems and of transient noises attributable to a variety of sources. In this study, sources that contributed to background noise but did not exceed 80 dB included changing stations, ventilated racks, lighting systems and building ventilation systems. Baseline noise is of concern if the sound level is sufficiently high to damage hearing, cause stress or impact research outcomes in other ways. Such noise would probably have the greatest effect on auditory research, because even moderate-level noise might affect auditory processing. For example, rats raised in moderate-level noise develop abnormal neural responses in the auditory system, including abnormal neural plasticity and abnormal processing of the frequency, time and intensity information that is important for perception of biologically relevant sounds11,12
. Thus, exposure to continuous moderate noise levels may result in effects that are not immediately detected by simple hearing screenings such as the Preyer reflex test or auditory brainstem response measurements. In addition, exposure to noise during gestation has been shown to affect maternal and fetal health in mice, perhaps through causing stress13–15
The present study shows that mice are exposed to frequent loud sounds, particularly during work hours. As in previous studies, most of the intermittent loud noises (>80 dB) we observed were attributable to human activities. Hearing in chinchillas can be damaged by irregular noise with intermittent sound peaks that vary in amplitude and distribution16
, a pattern of sound similar to that observed in the present study. Longer exposure to intense sounds can lead to permanent hearing loss in susceptible mouse strains such as C57BL/6 and BALB/c (refs. 17–26
). Susceptibility to noise-induced hearing loss varies across strains and ages; not all strains have been tested for susceptibility17–27
. Intense sounds can also induce audiogenic seizures in some strains of mice, notably DBA/J and C57BL/6 (refs. 28,29
). In humans and other species, noise can also result in sleep disturbances that can have multiple physiological and behavioral sequelae30
, and noise can induce a host of physiological responses independent of sleep effects4
In this study, the soundscape differed significantly between two rodent housing rooms that were located on the same corridor. This suggests that when animals in the same or related studies may be affected by noise, they should not be housed in different rooms unless the soundscapes are monitored and controlled. We do not, however, advocate creating a completely impoverished sound environment for several reasons. First, it is not possible to create a completely silent environment, as the animals themselves create noise through vocalizing, feeding and other behaviors. Second, a normal level of human interaction and animal health checks should be maintained to ensure the animals’ welfare. Third, an impoverished sound environment could have unintended effects on neural processing in mice. Rather than eliminate extraneous sounds from an animal’s living environment, laboratory staff members should control and monitor, to the extent possible, sounds originating from extraneous sources. Ideally, a housing room should be quiet enough for mice to hear one another vocalizing and should have minimal potentially harmful loud noise.
Caveats to measuring sound levels in the range of rodent hearing
To our knowledge, commercial sound level meters for measuring sound levels that span the entire range of mouse hearing and have 24-h data-logging capabilities are not currently available. Custom-designed systems are very expensive and require specific technical knowledge to operate. We chose to use a commercially available sound level meter with a frequency bandwidth that overlaps the range of mouse hearing so that similar measurements might be taken in other institutions that do not have access to sophisticated equipment. Although this limited the frequency bandwidth we could monitor, the range we did monitor encompassed the range of frequencies at which hearing is most sensitive in mice8–10
. Furthermore, our measurements and those published previously suggest that most sources of noise to which a laboratory animal may be exposed produce sound within the frequency range audible to mice1–3
A second caveat is that the presence of a human observer or recording equipment may have affected the behavior of investigators and animal care staff. Noise levels may actually be higher when no observer or monitoring equipment is present. Care should be taken to measure sounds as unobtrusively as possible to obtain the most accurate assessment of the soundscape.
Guidelines for safe exposure to noise
Current literature does not provide sufficient information to establish guidelines for safe exposure of mice to noise; it focuses mainly on mouse response to short-term, intense sounds17–27
. Safe levels of exposure to noise probably vary across mouse strains and studies. It seems prudent, however, to minimize loud and intermittent noises, which can reduce hearing ability, affect physiological responses and generally act as an uncontrolled study variable. Additional guidelines for safe exposure will probably become available in the future: the Acoustical Society of America has recently launched an effort to establish noise exposure standards for animals (Accredited Standards Committee S3, Subcommittee 1, Animal Bioacoustics).
Controlling vivarium noise may reduce the direct and indirect consequences of noise exposure. Our data suggest that controlling noise attributable to human activities will have the greatest effect on reducing overall animal room noise and noise variability. Efforts should be directed at reducing personnel traffic in animal rooms, locating noisy equipment and activities as far away from housing racks as possible and keeping animal room doors closed. To reduce activity inside animal rooms, biosafety cabinets and change stations should be located in separate procedure rooms when possible. If a change station must be located in an animal room, the distance between the change station and the rack should be maximized: in an anechoic environment (one that is treated to eliminate sound reflections and reverberations), sound level decreases by 6 dB for each doubling of distance from the source31
. This rule is probably not completely accurate for animal vivaria because they contain highly sound-reflective surfaces, but it does provide a guideline. To further reduce personnel noise, quiet working practices should be emphasized. One study showed that differing work styles can result in a difference of more than 15 dB in peak noise during cage changing32
. Particularly noisy activities such as vacuuming should not be conducted in animal rooms33
. Self-closing doors can reduce the incidence of doors left open by accident, and weighted door closure mechanisms can be installed to reduce door-slamming.
Regardless of noise control procedures, there will inevitably be more personnel activity during the day. If research results might be affected by sleep disruption owing to noise, reversing the light cycle (so that mice are awake during maximum noise disturbance, that is, during work hours) might be advisable, particularly in rooms with much personnel traffic. If the effects of noise disruptions during the dark (active) phase are a concern (for breeding success, for example), reversing the light cycle may not be appropriate.
For auditory research, control of background noise in addition to transient noise may be necessary. Although some noise attributable to various systems in the building is unavoidable, noise caused by ventilated racks can be reduced by placing ventilation motors outside the room or fitting the motors with covers. Most auditory research is carried out inside sound attenuation booths to control the effects of extraneous noise; however, sound exposure outside of experimental time may also affect the animals.
Finally, a system for sound surveillance should be considered so that loud noises and noise variability can be detected and minimized. With an ideal monitoring system, sound levels would be measured continuously, stored digitally and reviewed regularly. Such a system can be expensive, however, and may take up already limited space. As an alternative, periodic scheduled monitoring may also be effective in alerting animal care and veterinary staff when noise levels are not well controlled.