Frequency weighting

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The frequency evaluation is a process for frequency-dependent adjustment of sound pressure levels in the acoustics . The measured variables are weighted by an evaluating filter that takes into account the frequency response of the human ear and thus adapts the measured values ​​to the human hearing perception. The frequency weighting is a frequency-dependent deduction or addition to the determined level and is called the weighted sound pressure level , which is specified as dB (X) or dB X. The symbol X stands for the specific evaluation filter used in the respective case. In practical applications, the A-weighting, expressed in dB (A), and for high sound pressure levels, a C-weighting in dB (C). The B and D ratings have been removed from the underlying standard for sound level meters IEC 61672: 2003 and are only of historical significance.

Evaluation curves

Since the human ear perceives tones with the same sound pressure at different pitches at different levels, so-called frequency weighting curves are used, as shown graphically in the following figure. These are filters with empirically adjusted transfer functions used.

Since the curvature of the curves of equal volume level and the frequency response of hearing by the sound pressure level is dependent on different evaluation curves have been defined for different levels of sound pressure level:

Weighting filters A, B, C, D and sensitivity of hearing
  • A-weighting : corresponds to the curves of the same volume level at approx. 20-40 phon
  • B-weighting : corresponds to the curves of the same volume level at approx. 50-70 phon
  • C-weighting : corresponds to the curves of the same volume level at approx. 80-90 phon
  • D-weighting : corresponds to the curves of the same volume level at very high sound pressures
  • G-weighting : used in the infrasound range from 8 Hz to around 40 Hz

Weighted levels are identified by the corresponding letter of the frequency weighting as the index of the measured variable. For example, an A-weighted sound pressure level is referred to as L pA or a sound power level as L WA and is given in dB - in practice often also in dB A or dB (A). The latter notation should be avoided, as dB (A) is not a unit other than dB.

The four weighting filters have the following transfer functions G A ... D ( s ), with the analytical description of the filters taking place in the s-level:

with k A ≈ 7.39705 × 10 9

with k B ≈ 5.99185 × 10 9

with k C ≈ 5.91797 × 10 9

with k D ≈ 91 104.32

The specified constants k A… D are factors which normalize the transfer functions to the gain factor 1 (0 dB) at 1 kHz .


New curves for the same volume level (Isophone) according to ISO 226: 2003

Especially in technical acoustics and in the German legal system, the A-rating is predominantly used. The A-rating and the C-rating are also used to determine the rating level in the field of noise protection (DIN EN ISO 9612). However, since the perceived volume, the possible annoyance and also the potential damage to the hearing are only incompletely reproduced by the evaluation curves for a number of sound situations, additions for certain noise situations are usual in the noise evaluation: e.g. B. According to TA Lärm, surcharges for tonality ("squeaking"), impulsiveness ("hammering") etc. The resulting assessment level (sound pressure level + evaluation filter + surcharges) is used to check whether a certain noise level is permissible due to legal guidelines or Not. These surcharges are only to be used for volume levels below 80 dB (as of 2015) and may not exceed 6 dB in total (see DIN 45645-2).

An alternative to using rating curves is to determine the loudness according to DIN 45631 or ISO 532 B . Here the volume perception of the hearing is better recorded.


Similar frequency weightings are used to assess the effect of vibrations on the human body. Thus, human vibration (whole-body vibration and hand-arm vibration), for example, for the evaluation according to the German Noise and Vibration Occupational Safety Regulation with the frequency responses of the respective different rated ISO standards. The most important are:

  • Wk evaluation for vertical whole-body vibrations in the z-direction (ISO 2631-1)
  • Wd evaluation for horizontal whole-body vibrations in the x or y direction (ISO 2631-1)
  • Wh evaluation for hand-arm vibrations (ISO 5349-1)

However, there are other characteristics according to ISO 2631-1, ISO 2631-2 or ISO 2631-4 for other purposes.


  • ISO / DIS 10845, Acoustics - Frequency weighting "A" for noise measurements . Berlin: Beuth Verlag, edition: 1995-06
  • Jürgen H. Maue, Heinz Hoffmann and Arndt von Lüpke: 0 decibels plus 0 decibels equals 3 decibels . Erich Schmidt Verlag, Berlin 2003, ISBN 3-503-07470-8 .

Individual evidence

  1. Low-frequency noises including infrasound from wind turbines and other sources , page 90. Website of the State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg , accessed on October 10, 2018
  2. Frequency weighting filter characteristics, (English) ( Memento from February 25, 2013 in the Internet Archive )
  3. see TA Lärm , 2.10 assessment level]

See also

Web links