## 1 Introduction  
Sound propagation through the air is attenuated by atmospheric absorption, mainly due to relaxation vibrations of the oxygen and nitrogen molecules. The air attenuation is negligible at low frequencies but increases rapidly with frequencies above 1 kHz. The molar concentration of water vapor influences the atmospheric absorption, acting as a catalyst for the relaxation of oxygen and nitrogen (Annex A) [1]. Thus, the absorption is lower in humid air than in dry air. In practice, the atmospheric conditions are characterized by the temperature and the relative humidity (RH) as a percentage. If the absolute humidity is kept constant, an increase in the temperature leads to decreasing RH, and vice versa. For example, the atmospheric absorption is approximately the same at (18 \({}^{\circ}\)C, 55% RH), (20 \({}^{\circ}\)C, 50% RH), or (22 \({}^{\circ}\)C, 45% RH). The measurement of reverberation time and other room acoustic parameters is described in the International Standard ISO 3382-1 [2]. All room acoustic measurements according to this standard are made in octave bands. The measurement results can vary with the air temperature and the relative humidity, and, consequently, the standard requires that these atmospheric data should be measured and included in the measurement report. It is realized that in cases with very different atmospheric conditions, the measured acoustic results may not be comparable. This is not a satisfactory situation, and [PERSON] suggested the following in 1966 [3]:  
\"In presenting reverberation-time data for large auditoriums, irrespective of the humidity and temperature at which measurements are made, correct the reverberation time so that it represents the reverberation time that would have been obtained if the measurements had been made at a relative humidity of 50% and a temperature of 20 \({}^{\circ}\)C, i.e., standard conditions\". The correction method presented by [PERSON] is relatively simple and uses the air attenuation coefficients at the center frequency of the octave bands below 2000 Hz and those of the one-third octave bands from 2000 Hz to 12,500 Hz. The reverberation time is converted to the decay rate in dB per second, and the correction for air absorption is made with the assumption of ideal exponential decays. The attenuation coefficients are those valid for pure tones at the center frequency of the applied frequency band, either octave or one-third octave. The method works well for measurements in one-third octave bands, but for field measurements in octave bands, the method is not accurate for long reverberation times and high frequencies, as will be explained later. There is no evidence of a general application of the method in practice. Since the bandwidth used for the measurements is important for the problem of air attenuation, it should be noted that octave bands have not always been preferred for room acoustic measurements. The first edition of ISO 3382 [4] from 1975 prescribes one-third octave bands from 125 Hz to 4000 Hz, i.e., the same as used for the measurement of the sound-absorbing properties of materials in a reverberation chamber. However, at least since the 1960s, it has been common practice to perform reverberation time measurements in concert halls in octave bands. Thus, in 1997, the second edition of ISO 3382 [5] was established, stating that reverberation time in concert halls should be measured in octave bands, while one-third octave bands can be used in other kinds of rooms. In a book by [PERSON], there is a reference to a simple approximate formula to estimate the attenuation coefficient directly from the frequency and the relative humidity [6] (Equation (1,45 a)). The formula can be expressed in terms of the energy attenuation coefficient m in reciprocal meters:  
\[m=2.85\left(\frac{f}{\mathrm{kHz}}\right)^{2}\left(\frac{\varphi}{\%}\right) ^{-1}\cdot 10^{-4}\left(\mathrm{m}^{-1}\right) \tag{1}\]  
where \(f\) is the frequency in kHz and \(\varphi\) is the relative humidity in %.