Threshold hearing curve
shows that the sound intensity required to be heard is quite different for different frequencies. Most sensitive frequencies (≈ 3kHz) have lower dB values - less sound intensity is required to for the sound to be heard. Least sensitive frequencies have higher dB values - more sound intensity is required for the sound to be heard. Negative dB values indicate sound intensity is below I0
, the threshold intensity of hearing. The graph below shows how the curve varies with age: 20, 40 and 60 years.