Answer
$\approx 80 Hz-20000 Hz$.
Work Step by Step
See figure 12–6. At a sound level of 40 dB, the low-frequency threshold of hearing is $\approx 80 Hz$.
The “threshold of hearing” curve does not reach the 40-dB sound level on the right side of the chart.
Assume that the highest frequency a human can hear, about 20000 Hz, can be heard at a sound level of 40 dB. Your answers may vary.