Final answer:
The next lowest sound intensity level that is clearly less intense than 85 dB is 84 dB. This small decrease is considered the minimum detectable change in sound intensity level by the human ear.
Step-by-step explanation:
If your radio is producing an average sound intensity level of 85 dB, the next lowest sound intensity level that is clearly less intense is 84 dB. The sound intensity level scale is logarithmic, which means each 10 dB increase corresponds to a sound that is 10 times more intense.
Therefore a one dB decrease would indicate a sound that is less intense, but not by a large factor, making 84 dB the closest lower intensity level that is distinctly less intense. Sounds differ in intensity levels and their respective changes in dB levels might not correspond to an easily perceptible difference in loudness. However, a 1 dB change is generally considered the smallest difference in sound intensity level that a human ear can detect.