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Standard PN-B-02151-4:2015-06

The standard described here is the first official document of this kind, which is directly related to interior acoustics. Of course, this is not about places such as theatres' interiors or concert halls, which are approached in a separate way. We are talking about interiors of public utility places and buildings of collective residence. In such rooms, proper acoustics has a large impact on the well-being of people staying in them.

By adhering to the guidelines of the standard, it is possible to reduce noise in certain interiors as a result of reducing reverberant noise.

In rooms that are primarily used for verbal communication, the guidelines of the standard are expressed by T, i.e. reverberation time and STI, i.e. speech transmission rate. In the case of all other rooms, T and A are also used, i.e. acoustic absorption.

By adhering to the standard, the most important shortcomings that affect the acoustics in most interiors can be avoided. It is worth knowing, however, that even better results can be achieved through the use of other possible technical solutions.

T, or reverberation time

What is reverberation? This phenomenon consists in the gradual disappearance of sound energy after the source of sound has been switched off. This is due to the reflection both from the surfaces that surround the interior and from the surface of the objects that are in it.

The reverberation time is the time required for the sound pressure level in a room to drop by 60 dB after switching off the sound source. It depends primarily on the size of the room and its acoustic absorption. In a small interior with high acoustic absorption, the reverberation time will be relatively short. What matters here is also the geometry of the interior and how the objects and sound-absorbing and sound-distracting surfaces are placed in it. In one room we can record different reverberation time values for different frequency bands.


Speach Transmission Index (STI) is a parameter that can be used to express the degree of speech intelligibility. The higher its value, determined from zero to unity, the better the speech is understood. In practice, the selected room emits noise, the bandwidth of which is similar to the bandwidth of our speech. This band is additionally modulated by means of frequencies which also resemble those appearing in our speech. Also the sound level of the signal is the same as in our voice. The next step is to examine how large are the signal distortions in individual parts of such an interior.

And what does the STI value depend on? First of all, from the background noise level and reverberation time. The lower their values, the higher the STI values. If they are greater than 0.7, speech intelligibility is considered to be ideal.

A, i.e. the acoustic absorption of the room

The acoustic absorption of a room is an entirely hypothetical area of the surface that completely absorbs sound, where the reverberation time would be the same as in a particular interior, provided that only that surface would be an absorbing element in that room.

In the PN-B-02151-4:2015-06 standard described here, the smallest possible acoustic absorption of the interior is expressed by multiplying the area of its projection, i.e. A ≥ 0.6 x S, where S is the area of the room in m2.

The value of acoustic absorption of a given room is obtained by adding together the acoustic absorption of surfaces which limit the given interior, the acoustic absorption of air and objects which are located in it.

To calculate the acoustic absorption of a specific surface, we must multiply its surface by the sound absorption coefficient appropriate for its finish.

In the same room or material, different sound absorption values can be obtained for different frequency bands.