The most important problem with control room noise is that the noise disturbs communication between people and also makes it difficult for the operators to hear the various signals from both the control equipment and directly from the process. In order for speech to be easily understood, its level must be considerably greater than the background level in the room, as some individual speech sounds are much lower than the average level of the speech. If the average speech level is approximately the same as the background level in the control room, speech would still be mostly intelligible. If it lies at about 5 dB below the background level, it will only be partially intelligible even with concentrated listening.
The speech level of operators in a control room will vary widely depending on its strength, but a common value lies between 60 and 65 dBA at a distance of 1 metre. This means that for speech at a normal conversational level to be heard properly the background noise level should not exceed around 55 to 60 dBA. One measure of the interfering effect of noise on speech is the Speech Interference Level (SIL) which was defined by Beranek as the arithmetic mean of the sound pressure level in the octaves: 600 to 1200, 1200 to 2400, and 2400 to 4800 Hz (Beranek, 1954). In line with modern sound level metres, the SIL is now calculated as the arithmetic mean of the noise levels (in dBA) in the octave bands centred around 1 kHz, 2 kHz, and 4 kHz. This set of graphs is shown in Figure 7.8.