Science

Although Hz and dB are different units, the former is a unit for frequency and the latter a unit for sound intensity level, human ears would have different sensation of loudness to the same intensity of sound at different frequencies.

Human ears are particularly sensitive to sound of frequencies between about 1000 Hz to 3000 Hz. Thus if there are two sound sources of the same intensity (same dB levels), one at 50 Hz and the other at 1000 Hz, the 1000 Hz sound will be heard louder than the 50 Hz sound.

To take into account the difference in loudness heard by the human ear for the same dB level because of difference in frequency, the unit db(A) is commonly used in such aspect.
[Note: the db(A) actually uses frequency dependent [threshold of hearing] in the calculation of sound intensity level]

Hz is related to frequency, the number of oscillations within one second. dB is related to intensity, the amplitude of the wave. Therefore they have no relationship. Let's take sound wave as example. Hz is related to the pitch, i.e. "do re mi fa so la ti do" in music notes. dB is related to the loudness of the sound wave, i.e. the volume of the music.