What ambient sound level threshold is used to determine the need for visual signal appliances?

The threshold used to determine the need for visual signal appliances in fire alarm systems is important for ensuring that alerts can be perceived by everyone in an environment, especially in conditions with high ambient noise levels. A sound level of 105 dBa is specified because it accounts for environments where noise can mask audible alarms, making it necessary to augment audio signals with visual signals for effective notification.

In noisy environments, it's crucial that everyone, including those with hearing impairments or those engaged in tasks that may distract them from auditory signals, can be alerted to emergencies. The choice of 105 dBa reflects the need to ensure that any fire alarm systems are effective in a majority of circumstances where excessive background noise may disrupt audible warnings. That's why visual alerts become essential at or above this sound level threshold; they provide a reliable means of notification regardless of individual circumstances or environmental factors.