People increase their vocal output in noisy environments. This is known as the Lombard effect. The aim of the present study was to measure the effect as a function of the absorption coefficient. The noise source was generated by using other talkers in the room. A-weighted sound levels were measured in a 108?m3 test room. The number of talkers varied from one to four and the absorption coefficients from 0.12 to 0.64. A model was introduced based on the logarithmic sum of the level found in an anechoic room plus the increasing portion of noise levels up to 80?dB. Results show that the model fits the measurements when a maximum slope of 0.5?dB per 1.0?dB increase in background level is used. Hence Lombard slopes vary from 0.2?dB/dB at 50?dB background level to 0.5?dB/dB at 80?dB. In addition, both measurements and the model predict a decrease of 5.5?dB per doubling of absorbing area in a room when the number of talkers is constant. Sound pressure levels increase for a doubling of talkers from 3?dB for low densities to 6?dB for dense crowds. Finally, there was correspondence between the model estimation and previous measurements reported in the literature.