Heiss A.

If the engines of airliners are to be run for maintenance during night, some specific noise limits might not to be exceeded in the neighbourhood. Due to the relatively great distance to the relevant areas of sound immission and to the background noise a directly evaluated measurement usually would provide too high results related to the specific source and hence could produce an erroneous statement of the exceedance of noise limits. For this reason an adequate noise level separation principally should be taken into account. Fortunately for maintenance the engine usually is operated by constant power, i. e. constant emission level, which change from one time interval to the next. Additionally the information provided by a monitor microfone, installed in the engines vicinity, can be used. Taking this into account an acoustic model for the linear superposition of the source (engine) and the background noise in the immission area can be established, based on the equivalent continuous and percent exceedance sound pressure levels measured in real time as on the engine as on the immission site. This approach allows to determine separately the engine sound impact level, the background level and the attenuation of the engine sound to the immission site. For this principally two different states of engine operation with a minimum sound level distance of about 3 dB are neccessary. The uncertainty of the measurement in real time is recorded additionally. By these input data and an adequate algorithm and software also the confidence limits and the resolution limits of the immission level for the source (engine), the background level and the attenuation can be evaluated. The method has been tested on an airport. The distance between the testing and immission site was about 1300 m.