Are our ears more sensitive to low or high pitched sounds?
Short answer
Our ears are most sensitive to the mid-frequencies.  

Background
There are different ways of assessing sound level. The physical one simply determines the physical sound pressure level (SPL) in which all the frequencies weigh equally. This is referred to as C-weighting and is expressed in dB SPL.

Our ears, however, are most sensitive to intermediate frequencies, peaking at around 4 kHz (Fig. 1). The frequency weighting that takes this into account is dBA weighting. 

The reason why we are most sensitive to mid-frequencies can be sought in the fact that sounds with frequencies around 4 kHz are amplified in the middle ear canal through the process of resonance. In effect, they are perceived 10 to 20 dB louder than other audible frequencies. The pinna also affect resonance and hence the outer and middle ear together join forces to create humps and bumps in the equal-loudness curves.




Normal equal–loudness-level contours for pure tones. source: Duncan (1996)

Reference
- Duncan, High Performance Audio Power Amplifiers
For Music Performance and Reproduction (1996): 1-18