Upgrading backup alarms to reduce encroachment on soundscapes in Denali National Park

Denali maintains a sizeable fleet of maintenance vehicles that are equipped with backup alarms. Across many years, park acoustical monitoring demonstrated that tonal backup alarms were audible at one to two miles distance from the hazard zone immediately behind the truck. Following a successful demonstration of backup alarms with broadband signals, 77 vehicles at Denali were upgraded. The aggregate cost of the devices was just under $11,000, and the Denali auto shop provided 25 h of staff time for the upgrade. CadnaA modeling was used to compare the park area in which the old and new alarms were audible, assuming that both broadcast the same level (107 dB, A-weighted). The broadband alarm was predicted to be audible in 2.9 km², compared with 6.9 km² for the tonal alarm. The adaptive level feature of the new alarms yielded additional benefits in quiet locales. After the upgrade, backup alarm noise became noticeably less along the Denali Road corridor, and these improvements motivated the park to incorporate language regarding backup alarms into construction contracts.