When listening to a conversation, the SPL of the voice should preferably exceed the SPL of the ambient noise. The difference between these two SPLs is called the signal-to-noise ratio (SNR). If the absorption area per speaking person is 25 m2, the SPL is 65 dB both for the ambient noise level and for the speech level at a distance of 1 meter. Thus SNR = 0 dB.
For normal-hearing people, it may be sufficient to have SNR = -3 dB, but in other cases this will be clearly insufficient, namely in case of elderly people with reduced hearing capability or in case of a conversation in a foreign language.
From the graphs in the above figure, we see that SNR = -3 dB corresponds to an ambient noise level of 71 dB. In order to obtain an SNR = -3, the absorption area should be 12,5 m2 per speaking person or approximately 3,5 m2 times the total number of persons. If this minimum of absorption cannot be established, the acoustics must be characterized as insufficient, and the total number of people in the facility should be reduced.