Principle of CPX method
Within the investigations the close-proximity (CPX) method is applied. This method includes the measurement of the A-weighted sound pressure level generated by the interaction of the road surface and various reference tires over a specified road distance at two microphones (at least) in the vicinity of the reference tire. Simultaneously, the vehicle speed is recorded by a GPS sensor.
The measurement is carried out by using a special test vehicle or a towing vehicle with measuring trailer.
After dividing the test road into a run-in segment, a run-out segment and segments of 20 m length, the measurements are carried out at reference speeds of typically 40 km/h, 50 km/h, 80 km/h or 100 km/h.
By averaging the measured A-weighted sound pressure level of various microphones, road segments, test runs and reference tires, the so-called CPX index is determined for various vehicle categories that allows the evaluation and thus the comparison of road surfaces.
Within the investigations two internationally standardized reference tires are used for each test road and each reference speed.