The hearing threshold (HL) characterizes the lowest volume that a hearing person can perceive. It is an empirically determined characteristic curve of the hearing characteristic that represents the lowest perceptible sound pressure over the audible frequency range.
The reference value is 0 decibels( dB) at 1,000 Hz. The hearing threshold of 0 dB is at a sound power( Pa) of 20 µPa, which corresponds to `2*10^-5`N/m2. It is frequency-dependent and increases sharply for low and high-pitched sounds.
This means that low tones, if they are to be perceived at the same volume, require a much higher sound pressure. For example, a deep bass tone must be emitted at about 50 dB higher sound pressure to be perceived as loud as a 1 kHz tone. The threshold of hearing changes with loudness. For loud sounds, the entire characteristic curve rises toward higher levels, and for soft sounds, it drops and becomes more sensitive.