A-weighting : the A-weighting filter covers the full audio range - 20 Hz to 20 kHz and the shape is similar to the response of the human ear at the lower levels - see the Equal Loudness Contours entry.
A-weighted noise measurements are the most widely used ** and the Meter Class confirms the accuracy of the meter including the filters.
The preferred convention is to write LA = x dB, however dBA and dB(A) are often used, as are LAeq, LAmax, etc.
For more information on the dB (decibel) click here
** A weighted measurements only correlate well with the perceived Loudness at low sound levels, as originally intended, so many people object to the general use, often supported by regulations, for most noise measurements.
Low Frequency Noise, which disturbs many people, is suppressed by A weighted meters measurements. Currently thousands of wind turbines being built in rural areas are highlighting this low frequency anomaly.