The rolling noise emission measurement trailer, also known as CPX trailer, is a measurement trailer that enables you to measure the acoustic properties of long to very long stretches of road surface. Between 1990 and 1997 we gained extensive experience with the development and building of a rolling noise emission measurement trailer (the 'ROEMER' for Rijkswaterstaat in Delft) and performing measurements with such a trailer.
Subsequently, in 2002 in collaboration with DGMR The Hague we developed and built our own two-wheeled measurement trailer. Both consultancy agencies have used this trailer in the past years for large number of road measurements in urban and suburban areas commissioned by road builders and councils. Furthermore in 2006 we built and delivered this type of measurement trailer to the Danish Road Directorate, part of the Danish Road Directorate. Click here for the leaflet of the Danish Road Directorate.
With pride we present our, under own management developed and built, worldwide unique, acoustically optimal, open rolling noise emission trailer.
In the world of CPX measurements there are quite a few tales going round on the supposed advantages of closed rolling noise emission measurement trailers (CPX trailers) over open CPX trailers, like the ones we use and manufacture. Moreover they state several disadvantages of open CPX trailers.
Manufacturers of closed CPX trailers state as main advantage of their trailer over an open trailer the reduction of noise of for example passing or on coming vehicles of neighboring carriage-ways. This noise reduction of the enclosure results in a signal to noise ratio which is better than 10 dB. It is indeed the case that an enclosure of the measurement equipment results in a reduction of the exposure of foreign noise within the measurement space. There are however several disadvantages of such an enclosure, that in our view do not outweigh the possible advantages.
- The noise prevention of the enclosure is always limited by the presence of a gap between the bottom of the enclosure and the road surface.
- The limited size of the common closed trailers only allows for absorption material of limited thickness, resulting in suboptimal total noise absorption of the measurement chamber.
The limited size of the common closed trailers can, at certain frequencies, cause resonance of the measurement space.
- The acoustic absorption is extremely subject to pollution through spattering rain water. Hereby the absorption characteristics change in time.
- Due to this suboptimal dimensions and absorption characteristics of the closed trailers the measurement results have to be corrected using a trailer correction factor. This factor has to be established using an artificial sound source, identical to a rim with tire. Even though this is still being denied, it is clear from various studies that the trailer correction factor is not only trailer dependent, but also road surface dependent. This road surface dependence can easily be explained by the fact that the relatively large road surface area forms part of the absorption surface within the measurement space. Whenever the trailer stands on a closed road surface the absorption coefficient of the road surface (the sixth surface of the measurement space) is close to zero. When standing on an open road surface the absorption of the road surface is added to the absorption of the measurement space, resulting in a reduction of the noise level at the microphones. This reduction due to the open road surface is not the reduction we want to measure, because one only wants to measure the reduction of noise production of the tire-road combination. Research has shown that the resulting measurement error is approximately 0.4 dB(A).
Another so called disadvantage of an open trailer would be the influence of the wind velocity at the microphones on measurement outcomes. And even though one can suppose that for example at a driving speed of 80 km/hour the wind velocity at the microphones is around 22 m/s and generates a certain noise, in practice this is not the case because the trailer seems to be sheltered by the towing vehicle from extreme driving wind. Furthermore it has been shown that possible noise production due to for example side wind is always located in the non-relevant low frequency spectrum.
As mentioned before both an open as well as a closed rolling noise emission trailer can experience the effects of interference during measurements; the open trailer slightly more than the closed models. For this reason we always listen in on the measurement signal of the most sensitive microphone and possible interferences are marked within the measurement to be discarded during processing of the measurements. When measuring in busy (urban) areas more often measurements are performed at night or in the weekends to avoid the presence of busy traffic. Whenever this is not possible but there is a high chance of interference we make use of an accompanying vehicle to keep other traffic at a distance.
The above mentioned does not necessarily mean that measuring with closed trailers gives unreliable results. The rolling noise emission measurement trailer with enclosure that was developed and manufactured by us in the nineties for the Dutch government (ROEMER) does not have the above mentioned drawbacks.
This one-wheel rolling noise emission measurement trailer has an extremely large measurement space, measuring inside approximately 2.2 m in width, 2.8m in length and 1.75m in height. The measurement space is fully equipped with acoustic absorption wedges of approximately 26 cm in height and a to approximately 7 cm in height reduced gap at the bottom of the trailer that is made noise absorbing over the 26 cm deep gap.
This type of trailer does not have the above mentioned drawbacks of a closed trailer; it does however has the disadvantage of its enormous size and weight. The latter is one of the reasons we now use an open trailer as depicted above.