# Interior Sound Quality Prediction of Pure Electric Vehicles Based on Transfer Path Synthesis
## 4 Interior Sound Quality Synthesis  
On the basis of the measurement results of the transfer path functions and the working excitations of the different noise sources, the interior noise was simulated under each working condition. For this purpose, a sound-synthetic equivalent model of the PEV was developed. Figure 20 compares the measured and synthesized noise spectra in the PEV under a typical working condition (50 km/h). The synthesized and measured noise spectra of the PEV were very similar and followed the same overall change trends. Especially, the peaks at the key frequency points were completely matched. The amplitude differences between the spectral lines of the synthetic and actual in-vehicle noise signals can be explained as follows: (1) Insufficient accuracy of the transfer function measurements, and changes in the stiffness characteristics at the excitation source connection points after removing the electric powertrain and suspension systems; (2) errors in the excitation source data acquisition (despite the use of well-applied methods); and (3) uncertainties and errors in the whole test process introduced by the staff and test equipment.  
Figure 19: Excitation spectra on the front of the left rear suspension spring (in three directions from Connection Point 206).  
Figure 18: Excitation spectra on the front mount body (in three directions from Point 2).  
Since the transmission path of noise does not change, that is to say, the transfer function does not change with the working conditions, we only need to measure the sound source excitation under different working conditions and put it into the established sound synthesis model to predict the sound in vehicle. As shown in Figure 21, the noise-sound pressure levels in the synthesized vehicle interior at different operating speeds closely matched the measured results. This result indicates the high prediction accuracy of the established synthetic equivalent model. Particularly, the synthesized and actual noise spectra displayed very similar characteristics, guaranteeing the accuracy of the subsequent SQ analysis.  
The SQ evaluates the subjective feeling invoked by sound. Sound pressure is not the only criterion of SQ. Objective psychoacoustic parameters can systematically describe the different subjective feelings invoked by different sounds. The psychoacoustic parameters comprehensively consider the auditory characteristics of human ears and the physical characteristics of sound. In this study, the objective psychoacoustic parameters of the synthesized interior noise were calculated in a mathematical model implemented in MATLAB R2019b software. Figure 3 provides the calculation procedure, and the results are compared with the measured data in Figures 22-24.  
Figure 21: Comparison of synthetic and measured noise-sound pressure levels (SPL) at different operating speeds of the PEV.  
Figure 20: Comparison of synthetic and measured noise spectra in the vehicle operated at a constant speed (50 km/h).  
Under each working condition, the objective SQ parameters of the synthesized and measured interior noises were consistent, and the errors of all parameters were within 7%. This result consolidates the high prediction accuracy of the equivalent synthesis model of the interior noise in a PEV. The model accuracy meets the expected requirements. The  
Figure 24: Comparison of synthetic and measured noise roughness at different operating speeds of the PEV.  
Figure 23: Comparison of synthetic and measured noise sharpness at different operating speeds of the PEV.  
Figure 22: Comparison of synthetic and measured noise loudness at different operating speeds of the PEV.  
predicted objective psychoacoustic evaluation parameters almost equaled the measured values (with small errors). Therefore, they can replace the measured interior noise and can objectively express the related acoustic characteristics of PEV noise.