ISO 532-1:2017 – Acoustics – Methods for Calculating Loudness – Part 1: Zwicker Method

ISO 532-1:2017 specifies two methods for estimating the loudness and loudness level of sounds as perceived by otologically normal persons under specific listening conditions. The first method is intended for stationary sounds and the second method for arbitrary non-stationary (time-varying) sounds, including stationary sounds as a special case.

The methods are based on the Zwicker algorithm. The method for stationary sounds is provided for reasons of continuity, therefore is in accordance with ISO 226:1987 instead of ISO 226:2003. Moreover, it also offers the use of measured one-third-octave-band levels as input. The more general method for arbitrary sounds calculates the specific loudness pattern based on measured time signals by applying a signal processing model that is directly related to physiological and psychological characteristics of the human hearing system. Loudness is calculated from the specific loudness pattern. It has been shown that this method provides a good match to the results of many loudness experiments using synthetic and technical sounds.

The method is an updated method B of ISO 532:1975 describing a modified treatment for low-frequency energy. The calculation is an algorithmic approach and not a graphical approach. The modified treatment for low-frequencies below 300Hz and this follows treatment as stipulated in DIN 45631:1991. The calculation of loudness of time-varying sounds is based on the calculation of the loudness of stationary sounds, thus employing spectral analysis using one-third-octave-band filters.
 
 

ISO 532-2:2017 – Acoustics – Methods for Calculating Loudness – Part 2: Moore-Glasberg Method

ISO 532-2:2017 specifies a method for calculating loudness and loudness level of stationary sounds based on spectral properties of sound. It is based on Moore-Glasberg loudness calculation algorithms. The calculation commences by converting a specified signal spectrum into a series of sinusoidal components representing that spectrum. This series is then transformed into a specific loudness pattern by applying four consecutive transformations, each of which is directly related to physiological and psychological characteristics of the human hearing system. Loudness is calculated from the specific loudness pattern.

The Moore-Glasberg method is limited to stationary sounds and can be applied to tones, broadband noises and complex sounds with sharp line spectral components. The method in this document differs from those in ISO 532:1975. Method A of ISO 532:1975 (Stevens loudness) was removed as this method was not often used and its predictions were not accurate for sounds with strong tonal components. The method prescribed in this document also improves the precision of calculated loudness in the low frequency range and allows for calculation of loudness under conditions where the sound differs at the two ears. It has been shown that this method provides a good match to the contours of equal loudness level as defined in
ISO 389-7:2005.
 
 

ISO 532-3:2022 – Acoustics – Methods for Calculating Loudness – Part 3: Moore-Glasberg-Schlittenlacher Method

ISO 532-3:2022 stipulates a method for calculating the loudness of time-varying sounds from the input signal, which may differ for the two ears. This calculation method is based on Moore-Glasberg-Schlittenlacher loudness calculation algorithms. The method allows calculation of two quantities:
 
(1) The short-term loudness, which is the momentary loudness of a short segment of a sound, such as a word in a speech sound or a single note in a piece of music;
(2) The long-term loudness, which is the loudness of a longer segment of sound, such as a whole sentence or a musical phrase.

For most everyday sounds, both the short-term loudness and the long-term loudness vary over time. The loudness of sounds with durations up to 2-3 s is well predicted from the maximum value of the long-term loudness reached during presentation of the sound. For long-duration stationary sounds, the long-term loudness based on the method described in this part of ISO 532 is very close to the loudness determined using the method described in ISO 532-2. Differences can occur for sounds with strong amplitude fluctuations, such as noises with narrow bandwidth; for such sounds the calculated loudness is more accurate for ISO 532-3 than for ISO 532-2.

The method of loudness calculation described in the standard can be applied to signals of any duration. However, it does not directly give an output corresponding to the overall loudness impression of a sound scene or soundscape over a period of minutes, hours, or days, which is called the “overall loudness” in this standard. The output of the method of loudness calculation described in this standard can be post-processed to estimate the overall loudness of a sound scene.