SP395 SoundPro Audio Integrator
Form7492
Operation Manual
106
APPENDIX B –
ANSI Weighting Curves
SPL meters convert sound pressure variations into an electrical signal that changes in amplitude,
according to the sound pressure. The display shows the sound pressure level with a flat or linear
response to all audio frequencies. Our ears however, do not have the same sensitivity for all
frequencies. Instead, the sensitivity of our ears changes with sound level and frequency. In order
for SPL readings to more closely correspond with what we hear, electronic filters are added to
the SPL measurement circuitry. These filters, or weighting networks as they are commonly
called, filter the measurements to correspond to the various sensitivities of our ears.
Three standard ANSI (American National Standards Institute) weighting curves are used in SPL
meters. They are simply called A, B, and C weighting. The A-weighted curve approximates the
40 phons equal loudness curve, and is used when measuring SPLs of 20-55 dB. The B curve
closely follows the 70 phons curve, and is used when measuring SPL levels of about 55-85 dB.
The C curve, which closely follows the 100 phons curve, is used when measuring SPLs from
about 85-140 dB. Weighted SPL readings are often referenced by the corresponding weighting
curve; dB(A), dB(B) or dB(C).