When determining the threshold at which sound becomes dangerous for an individual's hearing, we refer to established guidelines and research findings discussed in the class lecture. Among the given options:
1. 50 db: This level is typical of a quiet office or a normal conversation and is generally considered safe for hearing.
2. 70 db: This level is equivalent to the noise of a vacuum cleaner or traffic noise inside a car. It is typically safe for extended exposure.
3. 80 db: This level corresponds to the noise of a busy street or a garbage disposal at a close distance. Prolonged exposure to this level might pose some risk, but it is not deemed immediately dangerous.
4. 120 db: This level is as loud as a siren or a thunderclap and is considered the threshold for pain. Exposure to sound levels at or above 120 decibels can cause immediate harm to hearing and is considered dangerous for an individual's auditory health.
Thus, according to the class lecture, sounds exceeding 120 db is the correct threshold at which noise becomes dangerous for an individual's hearing.
