LISTENING is not the same as HEARING.  Hearing is passive, and Listening is active.  While Hearing only involves the perception of sound (even if that perception is ignored), Listening involves actively considering what is being heard.
Human hearing is capable of frequency sensitivity from 20 hz to 20,000 hz (20khz).  Age and damage deteriorate hearing.

The Fletcher Munsen curve shows that we hear different frequency ranges differently at different volumes.  Some frequencies need to be louder to be heard clearly.  4K is easy, 8K needs to be louder, below 100 hz needs to be even louder.

A DECIBEL (dB) is smallest change in volume the average person can hear.

The human ear has evolved to maximize our hearing potential. The outer ear directs sound into the ear.  The Tympanic Membrane (Ear Drum) vibrates, which vibrates the small ear bones.  The Ear Drum vibrates freely due to the Eustacian Tube, which allows airflow.  The vibrations reach hairs (Cilia) in the Cochlea that fire to nerves.  When the hairs are damaged they will always fire (causing the constant perception of a ringing sound called TINITTUS).

Sound waves have energy (pressure) that can be physically felt. SPL stands for SOUND PRESSURE LEVEL, or the amount that sound is pushing.  It is measured in Decibels.

Decibel (dB) is a measure of sound level (pressure) used for calculating acoustic power and voltage.  A dB is the smallest increment of sound volume that a typical person can hear.

The threshold of perfect human hearing is 0 dB.  Of course as we get older and our hearing deteriorates due to age, loud sounds, illness, and many other factors we may need to hear sound at louder volumes to distinguish it.  For example, you can whisper to a child to be heard but may have to shout to an older person.  Since this decrease in sensitivity affects certain frequencies rather that all frequencies together an older person may hear midrage and bass with no trouble but have a hard time hearing the treble sounds that make up consonants such as S or Z.

Conversation is around 60 dB, loud thunder around 115 dB.  The sound of an airport runway may be around 120 dB, but if you are close to a jet engine the SPL may be more like 155 dB.  The average threshold of actual pain from sound pressure occurs at around 135 dB.

Sound volume meters can display the dB value of any sound.  Meters are shown in either VU or PEAK mode.  VU (volume unit) will show the average dB value and PEAK will show instant peaks and faster sound spikes called TRANSIENTS such as a fast drum hit.

Remember that hearing is passive and listening is active.  Many people can easily listening to sounds that are loud but are not able to distinguish softer sounds or even soft parts of loud sounds, such as the quiet instruments in a full orchestra or the amount of low frequencies in an acoustic guitar.  There are various exercises that can improve this ability, such as quietly listening to soft sounds (or even what you may initially think is silence) and trying to distinguish individual sound elements.

What are the quietest sounds you can hear in what you would call a “quiet room”?