NCIDQ IDFX Practice Exam

A rating of 25 on the STC scale indicates what about normal speech?

1. It is completely inaudible

2. It can be clearly heard

3. It is barely understood

4. It is not intelligible

The correct answer is: It can be clearly heard

A rating of 25 on the Sound Transmission Class (STC) scale indicates that normal speech can be clearly heard through a partition. The STC rating is a method used to measure the sound insulation between spaces, and it reflects how well structures block sound. In practical terms, a rating of 25 suggests that conversational speech is discernible, without much difficulty. This level of sound insulation indicates that while there might be some blocking of noise, the transmission of speech is sufficient enough that words can be understood relatively well. Higher STC ratings would correspond to better sound isolation, leading to more muffled or inaudible sound, while lower ratings would indicate even greater sound penetration. In contrast, other options refer to varying degrees of sound clarity and intelligibility. For instance, inaudibility or lack of intelligibility would correspond to much lower STC ratings, where speech is absorbed or obscured beyond recognition. Therefore, a rating of 25 distinctly suggests that while some sound isolation is present, normal speech remains understandable.