Weighting filter

The frequency of review is a method of frequency-dependent adjustment of sound pressure levels in acoustics. Here, the measured variables are weighted by an evaluative filter, which takes the frequency response of the human ear and thus the measured values ​​conform to human hearing. The frequency weighting is a frequency-dependent deduction or surcharge on the determined level and is called sound pressure level. Today, only the A-weighting is used for practical reasons and partly in the unit dB (A) or dBA specified.

Rating curves

In order to take account of the fact that the human ear sounds with the same sound pressure in different pitches feels different loud, so-called frequency weighting curves are used as they are graphically illustrated in the adjacent figure. These filters are used with empirically fitted transfer functions.

Since the curvature of the curves of equal volume level, and thus the frequency response of hearing the sound pressure level is dependent, different weighting curves have been defined for different levels of sound pressure level:

  • A-weighting: corresponds to the curves of the same volume level at about 20-40 phon
  • B Rating: corresponds to the curves of the same volume level at about 50-70 phon
  • C- Rating: corresponds to the curves of the same volume level at about 80-90 phon
  • D Rating: corresponds to the curves of the same volume level at very high sound pressure levels

Rated levels are designated by the letter of the frequency weighting as an index of the measurand. For example, an A -weighted sound pressure level LpA or a sound power level is denoted by LWA in dB and - often in practice in dB ( A) - specified. This latter notation should be avoided, since it is as dB for dB ( A) any other unit.

The four weighting filters have the following transfer functions GA ... D (s), with the analytical description of the filter in the s- plane is:

With n ≈ 7.39705 × 109

With kB ≈ 5.99185 × 109

With kC ≈ 5.91797 × 109

With kD ≈ 91104.32

The specified constants n ... D are factors which normalize the transfer functions at the gain 1 (0 dB) at 1 kHz.


Especially in technical acoustics and the German legal system, the A-weighting is applied mainly. For the determination of the rating level in the field of noise protection, the A-weighting is used. As for a number of sound situations, the perceived loudness, the potential nuisance effect and the potential damage to your hearing by the evaluation curves but only incompletely rendered, in the noise assessment surcharges for certain noise situations are still common: eg surcharges for tonality ( "squeak " ), impulsiveness ( " hammering " ), etc. the resulting assessment level (sound pressure level weighting filter supplements) is used for checking whether a particular noise level due to legal guidelines is permissible or not.

An alternative to the use of rating curves is to determine the loudness according to DIN 45631 and ISO 532 B. Here, the perception of loudness of the hearing is better recorded.