In 1911 Lombard described the effect that people tend to speak louder in noisy environments. Since then many investigators have tried to establish the increase in vocal output as a function of the ambient noise level, etc., but the influence of the amount of absorbing material has been investigated only occasionally in before-and-after studies when absorption was added to improve bad acoustics in rooms.

We had the opportunity to measure in a test room built for the new Conservatory of Amsterdam, Netherlands. The reverberation time could be changed between 1.36 and 0.21 s. Additional tests were done in an office and in the University’s anechoic chamber.

For low noise levels a constant SPL is often assumed in literature. For high noise levels the increase in vocal output is mainly given as 0.5 dB(A) per dB(A) increase in noise level. This was confirmed in our measurements, but to combine the effects over a range from 30 to 80 dB(A), a new curve was calculated based on the logarithmic sum of both extreme cases. With this curve the influence of absorption can be found as about 4 dB(A) per doubling of the absorbing surface in single talker situations up to a maximum of 8 dB(A) in very crowded rooms. For the increase per doubling of the number of talkers effects are found between 3 and 6 dB(A).