The pitch of a note is described by its frequency, which is the number of pressure variations that occur per second, measured in Hertz (Hz). The audible frequency range for most people is approximately 20Hz to 20,000Hz, although the highest frequency that a person can hear is dependent on factors such as age and previous exposure to high noise levels.
For a given SPL, a high frequency sound will seem subjectively louder than one of a low frequency. This is because the human ear responds differently to a range of acoustic frequencies. Sound level meters can take account of this by applying an electrical filter or â€˜weighting curveâ€™ to the output of the microphone to match the sensitivity of the ear at different frequencies. Meter readings taken with this type of filter are referred to as A-weighted decibels or dB(A).