| Construction noise can negatively impact roosting bats, and mitigation needs to consider the frequency of noise generated and the hearing sensitivity of bat species at risk. We present two case studies for potential noise impacts to bats. At one project, a maternity colony of Eptesicus fuscus tolerated high decibel (dB) levels of low frequency sounds generated by chain saws (75-86 dB) and large graders (85-89 dB) within 100 feet of their roost. However, the colony abandoned their roost when workers used a high frequency (19-28 kHz) laser surveying instrument, inaudible to the human ear. At a second project, we recorded ultrasonic noise generated by an auger drill at distances of 10, 30, 60, and 90 feet. We graphed noise attenuation based on kHz values and used ambient noise levels to determine buffer zones (e.g., noise recorded at 60 feet attenuated to 22 dB, the ambient noise level for 40 kHz; therefore, the buffer zone would be 60 feet for bats with a peak sensitivity of 40 kHz). Because many bat species are more sensitive to higher frequency sounds, we suggest that careful attention be given to the high frequency noise generated by equipment when planning mitigation. |