# An Estimation of Speech Privacy Class Based on ISO Parameter
## 3 Methodology  
### Speech Privacy Class (SPC)  
Some solutions for quantifying the state of speech privacy can be found in the literature, taking into account the \(SNR\) at the listening position. One acceptable solution for the research in this paper is outlined in the American standard ASTM E2638 [42]. The procedure presented within is based on publications by the [PERSON] group, who have extensively researched this topic [25; 27]. The standard introduces a parameter known as Speech Privacy Class (\(SPC\)), offering a quantitative measure of speech privacy within a room (the source room in Figure 1) from the perspective of a potential listener situated outside of it (the receiving room in Figure 1). The \(SPC\), defined in the standard as a measure of speech security and confidentiality in meeting rooms, can also be applied in other contexts, including residential buildings. According to the standard, \(SPC\) is determined as the sum of two factors: the non-normalized level difference (\(D\)) between rooms and the ambient noise level (\(L_{b}\)) at the listener's position:  
\[SPC=D+L_{b}. \tag{2}\]  
The first factor, the level difference (\(D\)), reflects the quality of sound insulation. Previous research on speech sound levels in meeting rooms and apartments has provided insight into the statistical properties of these sound signals. This information allows us to determine the \(SNR\) when listening to speech from an adjacent room by considering the level of the speech signal through the level difference \(D\). The second factor, the ambient noise level, defines the effect of masking due to sounds from the environment. A higher \(SPC\) value indicates a lower \(SNR\) at the listening position, implying greater privacy. The standard specifies that determining the \(SPC\) value requires that all relevant quantities are measured in 1/3-octave bands, including the calculation of isolation within these frequency ranges:  
\[D_{i}=L_{sour,i}-L_{reciv,i}, \tag{3}\]  
where \(D_{i}\) and \(L_{sour,i}\), are the level difference and sound level in the source room in the 1/3 octave band denoted by \(i\), respectively. \(L_{reciv,i}\) represents the sound level in the receiving room in the same frequency range. Both quantities are considered in the range relevant to speech, between 160 Hz and 5000 Hz, which constitutes a total of 16 bands. Calculating the \(SPC\) requires the calculation of the overall level difference, which is defined as the unweighted mean across all 16 considered 1/3 octave bands:  
\[D=\sum_{i=1}^{16}D_{i}/16. \tag{4}\]  
The overall noise level in the receiving room is calculated in the same way:  
\[L_{b}=\sum_{i=1}^{16}L_{b,i}/16. \tag{5}\]  
The anticipated states of speech privacy for different \(SPC\) values are described in [25] and in the appendix of the ASTM E2638 standard [42]. Table 1 provides these descriptions. The descriptions outline probable states of speech intelligibility and audibility. It becomes evident that privacy becomes relatively satisfactory only with an \(SPC\) value above 70 dB. Due to the dynamics within prolonged speech and within each sentence, the speech level can only be statistically observed through the long-term effective value. Therefore, \(SPC\) values from Table 1 can only be interpreted in terms of the most likely frequency of occurrence of more or less audible segments.  
Important conclusions regarding the required values of the sound reduction index in residential buildings can be drawn from the values shown in the table. For instance, at an ambient noise level of around 20 dB in the residential premises of an apartment, it is necessary to achieve a sound reduction index value of approximately 60 dB towards the neighbors for complete speech privacy. The minimum value of the sound reduction index of partitions between rooms, traditionally adopted for decades in the design of residential buildings and defined as such in the standard [49], falls significantly below the value needed to ensure privacy.