Measurement of the sound produced by a TGV train (or any type of high
speed train) passing at high speed is an important area of research and
development. The power dissipated as noise goes up roughly with the cube
of speed, with the result that high speed trains are rather louder than
normal trains. Acoustic measurement techniques can pinpoint exactly where
the noise is coming from. This has several important applications:
- Environmental Impact Evaluation. Data is used to develop
governmental noise prediction schemes for evaluating noise emission
levels along high speed rail lines.
- Sound Barrier Design. Source strengths can be used to
evaluate the efficiency of sound barriers of various designs, which
are intended to reduce the noise reaching surrounding inhabited
- Train Design. Data identifies the principal sources of
noise, where improved aerodynamic design or sound shielding might
reduce the amount of sound radiated into the environment.
How is the Sound Measured?
The acoustic images were obtained with the SYNTACAN acoustic antenna
array of the TNO Institute for Applied
Physics (TNO-TPD) in Delft, The Netherlands.
The SYNTACAN is designed for highly directional sound measurements. The
complete system uses 36 microphones, forming a sparse array with a length
of nearly 80 meters. In the vertical position 24 microphones are used and
the height is 10 meters. In the picture, the SYNTACAN array is shown near
a high speed line in Belgium, measuring the sound made by a Thalys
trainset passing at 330 km/h, during a measurement campaign in 1996. The
SYNTACAN boom is held at a slight angle from vertical by a crane, to
correct for the cant of the track. A measurement van contains
computer equipment to log the data. In the foreground, two other
instruments are visible: a radar velocimeter, used to accurately measure
the speed of the train, and an artifical noise source used to calibrate
and align the acoustic antenna.
Using a two-dimensional Fourier analysis technique, the sound field is
decomposed into the frequency dependent contributions from different
directions, which can be associated with (partial) sound sources. The
SYNTACAN system allows measurements in the 1/1 octave bands of 125 up to
2000 Hz with a resolution of 1/12 octave. (An octave is defined exactly
the same way as in music, with 1/12 octave equal to one half tone; middle
A is 440 Hz.) Its spatial resolution in radians approximately equals the
ratio of wavelength to antenna length.
Acoustic Images of a TGV Atlantique
After being processed as described above, the data collected using the
microphone array can be displayed as an image of the train as it would
look if your eyes could "see" sound sources. The images don't look very
much like a TGV Atlantique trainset, but if you observe closely, several
interesting features can be picked out.
What you see is essentially several 2-dimensional side views of the
train, taken in different octave bands. The center frequency of each
picture is labelled above it. The images represent the measured sound
pressure levels at the position of the antenna, as a function of height
and lateral position along the train. Red is highest pressure (loudest),
while dark blue is lowest. The images are different for the various
octave bands, due to the varying importance and radiation characteristics
of the noise sources and physical phenomena that generate the noise.
High-pitched rolling noise clearly emanates from the wheels; look at
the picture in the 2000 Hz band. Most of the emission is concentrated at
track level (0 meters on the vertical axis), and you can distincly see the
individual bogies of the trailers. These show up as light blue spots at
the center of the image, at a spacing of 18.7 meters (the length of a TGV
Aerodynamic noise sources are in general found all along the train.
The power units (one at each end of the trainset) are clearly the loudest;
this results in the red areas on the left and right sides of each image.
You can also discern that the pantograph is raised on the right side power
unit (at the rear of the train, which is travelling from right to
Using this kind of data, it is possible to design quieter trains,
better sound barriers, and to better estimate the noise impact of high
Courtesy of Dr. A.C. Geerlings, TNO-TPD, firstname.lastname@example.org. Full
J.D. van der Toorn, H. Hendriks and T.C. van den Dool, Journal of Sound
and Vibration, 193 (1) 1996, pp 113-121, "Measuring TGV source strength
For more information, please visit the Acoustic Division
at the TNO Institute of Applied Physics.
Last modified: 7 September 1998