# Interior Sound Quality Prediction of Pure Electric Vehicles Based on Transfer Path Synthesis
## 2 Materials and Methods
### PEV Transfer Path Synthesis  
In this paper, virtual interior sound was synthesized in the PEV and simulated by the TPS method. Although a PEV has no engine, it contains additional motors, battery pack systems, power coupling devices, and other components that change and scatter the layout of the vehicle and increase the number of noise sources. Accordingly, the sound  
Figure 1: Schematic of the pure electric vehicle (PEV) noise transfer paths. synthetic model in the vehicle is highly complicated. Past experience has shown that, if all noise sources and paths were included in the model, the model became excessively large and requires too much data and workload, ultimately leading to failure of the mission. Therefore, in the modeling process, the model should be simplified to suit the actual situation. In the present model, we studied the contributions of the electric drive system, suspension, mount, and tire to the interior noise, neglecting the influence of wind noise. The whole experiment process was carried out in a semi-anechoic chamber. Therefore, the airborne noise came from the electric drive system and tires, and the structure-borne noises comes from the powertrain mount and suspension systems. The noise transfer function and source excitation are two basic elements of the interior sound synthesis. Their measurement processes are described below. (1) Transfer function measurements: The airborne noise transfer function is the frequency response function (FRF) from the noise source to the noise at the target point. The airborne noise transfer function is defined as  
\[H_{i,air}=P_{target}/P_{i,source}, \tag{2}\]  
where \(H_{i,air}\) is the transfer function from the \(i\)th noise source to the target point in the vehicle, \(P_{target}\) is the sound pressure of the noise at the target point in the vehicle, and \(P_{i,source}\) is the \(i\)th noise source excitation. The structure-borne noise transfer function measures the FRF in each degree of freedom at the coupling point of the structure to the response position. Note that the active and affected sides should be decoupled at each coupling point during the measurement. The structure-borne noise transfer defines the transfer function from the structure-borne excitation source to the target point inside the vehicle and is expressed as follows:  
\[H_{j,structure}=P_{target}/F_{j,source}, \tag{3}\]  
where \(H_{j,Structure}\) is the transfer function from the \(j\)th structure-borne noise source to the target point in the vehicle, \(P_{target}\) is the sound pressure level at the target point in the vehicle, and \(F_{j,source}\) is the exciting force of the \(j\)th structural noise source. (2) Noise source excitation measurements: The excitations of airborne noise sources are mainly measured as sound pressure levels or volume acceleration of the noise source, whereas the excitations of structure-borne noise sources are mainly measured as the working force input with different degrees of freedom at the coupling point of the structure. Noise source excitation measurements produce a large number of the experimental data, which directly affect the reliability of the final synthesis model and are requisite for constructing the sound synthesis model. The sound synthesis model of the PEV used the airborne noise source excitation (motor and tire) and structure-borne noise source excitation (suspension and mount) signals under actual working conditions. The interior airborne and structure-borne noises were simulated in combination with the airborne and structure-borne transfer functions, respectively, and superimposed to give the total interior noise. Figure 2 shows the interior sound synthesis model in the PEV. The noise sources transmit noise to the vehicle through the air and structure, but they can be transmitted in many transfer paths. The characteristics of each transfer path are not the same, so it is necessary to carry out effective analysis on each transfer path.