Final answer:
The answer is d) Spectral Composition. Loudness is determined not only by sound pressure level but also by the spectral composition of the noise stimulus, which refers to the distribution of energy across different frequencies in the sound wave.
Step-by-step explanation:
The answer to the question 'Noise stimuli reveals that loudness is not only determined by sound pressure level but also the ______.' is 'd) Spectral Composition.'
Loudness is not solely determined by sound pressure level, but also by the spectral composition of the noise stimulus. The spectral composition refers to the distribution of energy across different frequencies in the sound wave.
For example, two sounds with the same sound pressure level can have different loudnesses if they have different spectral compositions. A sound with a higher energy in the frequency range that the human ear is most sensitive to will generally be perceived as louder.