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  Simulation of pass-by noise
HEAD Press Release Herzogenrath (Germany), July 10, 2014
Simulation of pass-by noise
Alternative to long-distance measurements of passing vehicles
According to ISO 11819‑1 or ISO 362, pass-by noise is to be measured from a distance of 7.5 m (25 ft) to the middle of the lane. However, to assess the noise from a larger distance, these measurements cannot be applied directly. While an estimation of maximum or average sound pressure levels is possible with statistical methods, it is not sufficient for a psychoacoustic assessment of the noise. On the other hand, an actual measurement of pass-by noise from a larger distance is hardly practicable, particularly if psychoacoustic phenomena are to be examined in relation to the distance. In addition to the considerable measurement effort, dominant ambient noise, which cannot be controlled, leads to a bad signal-to-noise ratio, making measurement results difficult to reproduce due to their high dependency on weather conditions.
These disadvantages can be circumvented by simulating the sound propagation of moving noise sources on a vehicle to the desired location of the receiver. For this purpose, near-field signals of the relevant sources on the vehicle are recorded during a pass-by. Taking into account the Doppler effect and other propagation phenomena, it is then possible to calculate the sound propagation to a distant location. In addition to the sound signals, the simulation requires precise movement data of the vehicle as well as the ambient temperature and humidity values to calculate the atmospheric attenuation.
In the EU project CityHush (project no. FP7-233655), it has already been shown that this method effectively simulates valid far-field signals. The reproducibility and flexibility can be significantly improved over extensive far-field measurements. However, the overall effort remains high, since the noise sources must be fitted with microphones, and a virtual acoustic model of the vehicle is required for the simulation.
The measurement effort can be significantly reduced with the signal extrapolation method presented at this year’s DAGA. This method allows a microphone signal measured at close distance (typically 7.5 m / 25 ft) to be converted into a virtual microphone or artificial head signal from a larger distance. Here, a virtual near-field signal is initially calculated from the microphone signal, whereby the Doppler effect and other propagation phenomena are compensated for by taking the vehicle movement data into account. Based on this near-field signal, a far-field signal can then be simulated as described above.
The figure below shows a comparison between extrapolation and simulation for a distance of 100 m
(330 ft) from the source. The loudness and sharpness curves are virtually identical, and the spectrograms are very similar. The differences visible on the graphs are acoustically barely perceptible. Comparisons between the extrapolation method and real measurements showed that the character of the sound was represented very well. However, the maximum sound pressure levels differed significantly from those of the measurement. These differences can largely be explained by atmospheric influences (particularly wind) during the measurement.
The results show that for straightforward scenarios a simple sound propagation model (free-field measurements with atmospheric sound attenuation according to ISO 9613-1) is sufficient. Moreover, the method also offers enough flexibility to integrate more complex sound propagation models, allowing examinations of the acoustic influence of landscaping or buildings, for example.
With the presented extrapolation method, the microphone signal of a standard pass-by measurement can be converted into a virtual microphone or artificial head signal for any distance. This avoids unnecessary measurement effort and allows pass-by noise to be examined in relation to the distance. For example, this method proved that signals with prominent tonal components are significantly reduced in sound pressure level and loudness with increasing distance, whereas their tonality decreases only slightly.
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Source:
Simulation von Vorbeifahrtgeräuschen;
Aulis Telle; DAGA, Oldenburg, Deutschland; 2014;
  HEAD acoustics Publications : Virtual Reality.
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