Understanding Signal Detection Theory

In the field of psychophysics, various theories explain how we perceive stimuli in relation to their physical characteristics. One prominent theory is the Signal Detection Theory, which considers not just the intensity of the stimulus, but also the psychological factors that affect detection. Consider a situation where a person is listening to a faint sound while simultaneously under the pressure of distraction from a loud conversation nearby. Which aspect of Signal Detection Theory would most likely affect the person's ability to discern the faint sound?