Perception of tonalness of tyre/road noise and objective ... - Acoustics

another recording is measured leading to the same pattern noise strength. The experiment is
carried out by four subjects: one female and three male.
- Experiment 2 (Variation of the spectral shape of the first pitch harmonics): In a paired
comparison test Thurstonian scale values according to the pattern noise strength of 6 signals
with a systematically varied spectral shape of the first pitch harmonics are obtained. The
signals are based on one tyre/road noise recording. Besides the original spectral shape the
different spectral shapes are obtained by an amplification of one or two third around the
mean of the first pitch harmonics by 2 or 4 dB and a following attenuation of the level of the
first pitch harmonics to the original level. Since the differences between these signals is
small a paired comparison test is carried in order to have a great accuracy of the subjective
rating. By means of Thurstone's law of comparative judgment the subjective ratings are
interval scaled. Eight subjects take part in the experiment: two female and six male.
- Experiment 3 (Variation of the level and the spectral shape of the first pitch harmonics): In
the first two experiments the influence of the level of the first pitch harmonics and the
spectral shape on the subjective evaluation of the pattern noise strength are assessed isolated
from each other. In a third experiment the combined influence of both parameters on the
pattern noise strength is evaluated. A second paired comparison test leads to Thurstonian
scale values according to the pattern noise strength of 12 signals with a systematically varied
level and spectral shape of the first pitch harmonics. All signals are based on one tyre/road
noise recording. Here the level of the first pitch harmonics is the original level and an
amplification of the first pitch harmonics by 0.5 and 1 dB. The different spectral shapes are
the original shape and an amplification of one third around the mean of the first pitch
harmonics by 2 or 4 dB and an amplification of two third around the mean of the first pitch
harmonics by 4 dB. The first pitch harmonics than is attenuated in order to leave the level of
the first pitch harmonics constant for all different spectral shapes of the first pitch
harmonics. The experiment is carried out by ten subjects: three female and seven male.
The assessed signals are tyre/road noise recordings with an artificial head standing on the front
passenger seat while the car is coasting and modifications of these signals. The signals are about 6s
long and represent a coasting from 110 to 100 km/h.
The experiment is carried out in a soundproof chamber. The signals are played back via
headphones. All signals have the same dB(A)-level of 69 dB(A).
The subjects are untrained in evaluating tyre/road noise.
3 OBJECTIVE SIGNAL PARAMETERS
3.1 Level of the first pitch harmonics
From the averaged order spectrum of the left and right channel the level in the range of the first
pitch harmonics (approx. 50th to 85th order) is calculated.
The subjective ranking of the signals in the first experiment is significantly correlated with the level
of the first pitch harmonics. This can be seen in figure 1 for one subject (see also [1]). The
correlation coefficients of all subjects are between 0.85 and 0.92 and are therefore significant.
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