As a train passes by a track side observer (or a microphone), first a rising sound pressure level as the train approaches/emerges is registered. When the train is straight ahead of the observer, the different carriages cause a fluctuating sound pressure level, due to the positions and strengths of the various sound sources. The sound goes down in level after the last carriage has passed-by. Sometimes, a ringing sound from the rail is the last which can be heard as the train recedes in the distant.
The noise generated is composed by many different sub-sources: rail, wheel, aerodynamic noise sources, etc. The total radiated sound impacts on nearby dwellings, causing a nuisance or even a hazard to the people living there. Regulations prohibit trains which are too noisy to traffic railway lines near dwellings. The speed has to be reduced, or high noise barriers built.
Sound View Instruments and KRRI (financed by Ministry of Land Infrastructure and Transport, Korea) have developed a software tool to simulate the train pass-by noise level, as a function of time, as a train passes by. The input parameters of the model are all the various noise sources (rail, wheel, aerodynamic noise sources, etc.), their location, speed, sound power, and directivity. Output is the sound pressure level at an arbitrary receiver position as a function of time. The receiver position may be located near the track, e.g. 7.5 m from track centre, or far away, e.g. where residential buildings may be located.
Noise source distribution map of noise radiation from the HEMU, resulting from a microphone array measurement (see High-Speed Train Noise).
Sound pressure level (8 m from the track) as a function of pass-by time resulting from the Virtual Train program.
The program may be used in various different ways:
To simulate train pass-by noise as picked up by a track-side microphone
New trains usually have to undergo a standardized noise pass-by test (type test), to prove they comply with noise regulations before they are allowed to traffic railway line systems in normal operation. The microphone may be positioned 7.5 m or 25 m from track center, the rail has to comply with a certain technical standard (TSI). With our program, this test may be simulated in the computer instead of, or before, carrying out the real test, which is a great advantage, since field tests are generally expensive to conduct. Also, it may not even be possible to arrange the standardized measurement conditions at all in the field required by the test standard. The European research project ACOUTRAIN addresses the same problem, although only for freight- and passenger trains — not high-speed trains — which is also included in our model.
To simulate the effect of noise control measures on train and/or track
By carrying out a series of simulations with different input parameters, the effect on the total noise of a specific noise control measure on a specific source (e.g. aero-acoustic noise from the pantograph) may be evaluated.
To simulate/predict noise from railway lines at nearby dwellings.
A simulation yields the sound pressure level as a function of time, outside an arbitrary flat window. Also, taking number of trains trafficking the railway line into account, yields the A-weighted equivalent sound pressure level. The maximum sound pressure level, is determined by the “worst” train passage. The calculated levels are to be compared to current limits and regulations.