Infant hearing has been assessed by measuring and the results show?
1) audibility curves for a range of sound frequencies; they are at adult hearing levels by 6 mo of age
2) if they react to loud noises; they are not bothered by sounds until they are 1 yr old
3) audibility curves for a range of sound frequencies; they only reach adult hearing levels at 2 yrs of age
4) equal loudness functions; thresholds are very high until 4 years of age