Because the ear is relatively insensitive to very low and very high frequencies, sound level meters have, by international agreement, weighted frequency responses. Hence different frequency weightings will produce, for the same sound source, different Lp readings.

Sound level meters may have any of the following frequency weightings.

• 'A' - Which best expresses the human ear's response to loudness, is the most commonly used frequency weighting.
• 'C' - This is often used to ascertain the acoustic emissions of machines, it has a much broader spectrum that that of the 'A' weighting curve.
• 'Z' - May vary between different instrument as it's characteristics are very dependant on the quality of the microphone and it's pre-amplifier. It is often used in conjunction with octave band filters. A typical linear response would be flat from 20Hz to 16kHz