When evaluating the sound pressure level in a space, the recorded levels in decibels (dB) are usually weighted with the A scale. This weighting is a function which is approximately representative of the frequency response of the human ear for low volume sounds, which is less sensitive to low frequency than to higher frequency sound. The sound pressure level weighted to the A scale is expressed as dBA SPL. Measurement equipment will often convert the dB SPL values to the A weighted SPL values, these measurements can be made with normal exponential responding meters in the Fast, Slow or Impulse settings or with integrating meters and the equivalent level or Leq and will result in SPL dBA values.

Figure 1: This curve is applied to the un-weighted frequencies and the resulting curve is called dBA. As humans we hear mid to high frequencies much better than we hear low frequencies.
For example if our microphone recorded a frequency spectrum with these values:

After applying the A-weighted curve, we approximate how our ears perceive sound, and our spectra would look like this:

Weighting each frequency band measured by our transducer (microphone) transforms the results into meaningful data. There are a number of other standard weighting curves, B and C for example. Each curve is used based on the amplitude level of the tested sound spectrum. The most commonly used scale is the A weighting scale.