## 1 Introduction  
When staying in various types of interiors on a daily basis, we are always exposed to sound reception. Acoustic properties of the interiors have a large impact on the way people perceive sound [1]. The basic parameter used to describe interior acoustics is the reverberation time. Reverberation time has two definitions:  
**Definition 1**.: _Reverberation is a phenomenon of gradual decay of sound energy after the source is turned off._  
**Definition 2**.: _Reverberation time is the time measured from the moment the source is turned off, after which the sound pressure level in the room decreases by 60 dB [2]._  
There are many historical models of reverberation time estimation. They are still used today [3], and yet the estimation issue is of interest to many researchers [4; 5; 6], and research studies on new methods of reverberation time estimation are also carried out at present [7]. The reference pattern for measuring reverberation time in ordinary rooms is contained in ISO 3382-2: 2008 [8], which is used for 1/3 octave spectrum range from 100 Hz to 5000 Hz. The standard describes the measurement procedure using interrupted noise. It states that the test signal should be a broadband random noise, and the source should be able to produce a sound level sufficient to provide a decay curve at least 25 dB higher than the background noise. It should be noted that the measurement procedure does not distinguish between frequencies, i.e., it is the same for each band, and the reverberation times are calculated on the basis of the average of decays in a given band [9]. Low frequencies are not taken into account in this procedure, and for such frequencies, the sound decay is strongly non-linear due to the presence of more resonant frequencies in one band, whereof each has its own decay curve [10]. Such an approach entails high measurement uncertainties (the percentage relative standard deviations are in the order of 20-60%), and thus it provides inaccurate reverberation time values [11; 12; 13]. In cubature, or in highly reverberant rooms, humidity and temperature conditions are also a source of uncertainty [14]. According to [PERSON] [15], the accuracy of the measured reverberation time is determined by a combination of the acoustic system under test, the measurement procedure, signal processing and the evaluation of the decay curve. Alternatively, [PERSON] [16] found that the uncertainty was mainly related to the variance between the repeated decays. Although the reverberation time is a widely used parameter in the assessment of indoor acoustics, we should allow for the fact that it is defined assuming a diffused sound field. Therefore, well below the Schroeder frequency, its application as well as the adherence to standardized measurement procedures is questionable [17]. In recent years, the demand has increased for appropriate measurement procedures for acoustic quantities at low frequencies [18; 19; 20] and with complex geometries [21; 22; 23]. The review of the literature shows that the assessment of sound decay at low frequencies is still of interest to many scientists, and reverberation parameters affect many other acoustic quantities, such as sound absorption coefficient or sound reduction index. Some scientists claim that the reverberation parameters in low frequency range and for sound in a non-diffused field should be assessed using modal analysis [24; 25].  
In the present paper, less advanced methods were used, but certainly useful for simple measurements for interior designers. The objective of the paper is to demonstrate the impact of the averaging method of decay curves on the values of reverberation time and measurement uncertainties. The test measurement was carried out in the reverberation chamber of the laboratory of the Faculty of Civil Engineering of the Silesian University of Technology, and it was performed based on the standard EN-ISO 3382 [8]. To verify the mentioned method, the measurement of reverberation time in a room intended for teaching purposes was also presented.