A-weighted Digital Filter for 1/3rd Octave Band Analysis
Based in my comment, here are quite accurate coefficients for A-Weighting filter to be used with 33kHz (33000Hz) sampling only:
b = [0.759941332414235 -1.931718891800229 0.5144488202077094 2.040523943539552 -1.283344336756084 -0.5073918947145036 0.3880093863750892 0.03272320276137633 -0.01319156202714533]
a = [1 -2.840785425332851 1.688214038430988 1.889562806030876 -2.289753885038417 0.2591762588766977 0.4062228100411928 -0.122514417900642 0.009877893153322726]

Second order sections:
[1 -1.999817001037482 0.999817017779804 1 -0.3278552435476524 0.02858846174272314]
[1 -2.000182998962519 1.000183015708817 1 -1.992171272584749 0.9921865948281223]
[1 1.403903298176578 0.4701139795164294 1 1.327684185781194 0.4091629729586278]
[1 0.05416509551979914 -0.03692436132666149 1 -1.848443094981644 0.8511063988059516]

(0dB @ 1kHz and g = 0.759941332414235)