Which strategy is ineffective for reducing potentially damaging sound pressure levels for plant personnel?

Changing the frequency of the sound is not an effective strategy for reducing potentially damaging sound pressure levels because while it might alter the perception of the sound, it does not necessarily reduce the overall sound energy or its impact. The harmful effects of noise are not eliminated simply by modifying its frequency; the sound pressure level at the ear can still be dangerous.

In contrast, building a barrier, modifying equipment operating conditions, and enclosing the noise source are effective strategies for mitigating noise. Barriers can block or absorb sound waves, reducing exposure. Modifying operating conditions, such as adjusting the speed or usage of machinery, can minimize vibrations and noise generation. Enclosing the noise source can trap sound within a specific area, preventing it from spreading and impacting personnel. These strategies are proactive in directly reducing the audible sound energy and, therefore, protecting plant personnel from potential hearing damage.