According to the World Health Organization (WHO), sound levels above which threshold are defined as noise pollution?

When a longitudinal sound wave propagates through air, what happens at a region of compression?

The loudness of sound depends on the amplitude of vibration such that loudness is:

Consider the following statements: 1. The loudness of a sound is determined basically by its frequency. 2. The frequency of the sound wave does not change when it travels from one medium to another. 3. Audible range of the average human ear extends from about 20 Hz to 20 kHz. How many of the statements given above are correct?