# Development of a New Sound Quality Parameter for Road Noise Perception Inside Vehicle Cabinet
## 2 Materials and Methods  
To achieve accurate measurements of road noise, particularly on rough surfaces, controlled testing procedures are crucial. Coast-down maneuvers are the preferred method, as they minimize engine noise interference. This involves reaching a designated speed of 130 km/h and then allowing the vehicle to decelerate naturally with the engine off until a speed of 30 km/h is attained. This standardized maneuver is replicated across all test vehicles on the same rough road surface to ensure consistent comparisons.  
Road noise measurement is a complex phenomenon influenced by several factors, primarily engine noise and wind noise, which becomes more prominent at higher speeds. To isolate and quantify the contribution of engine noise, coast-down tests were employed with the engine off for vehicles with internal combustion engines to remove any engine noise content.  
For this study, six different cars with different brands at various segments were selected to have a vast range of noise levels and to ease quantification during the subjective tests.  
Data acquisition throughout the measurements were performed using a PCB 378B02 microphone (PCB Piezotronics, Inc., Depew, NY, USA) positioned at the driver's left ear position, as seen in Figure 1. Siemens Simcenter SCADAS Mobile Data acquisition (Siemens, Leuven, Belgium) unit and Simcenter Testlab 2019.1 software were facilitated for the collection and analysis of the test data.  
The measurements were performed using PCB 378B02 \(\chi^{\prime\prime}\) free-field ICP microphones, positioned near the driver's ear locations to capture the sound environment experienced by the occupants. These microphones are ideal for this application due to their high sensitivity and broad-frequency response, making them suitable for accurately capturing a wide range of frequencies associated with road noise.  
A multi-channel array was not considered for this study because the objective was not to localize specific sound sources but to evaluate the overall sound pressure level (SPL) experienced in the cabin. Omnidirectional microphones were selected to ensure a uniform capture of sound from all directions, which aligns with the study's focus on overall sound quality rather than the spatial origin of the noise.  
Regarding near-field considerations, since the microphones were placed near the driver's ear but not directly adjacent to any specific noise sources, near-field effects were expected to be minimal. However, the setup was designed to avoid placing microphones too close to strong individual sources like the dashboard or windows, which could amplify  
Figure 1: Position of microphones in drivers’ seat.  
near-field effects and skew the results. By positioning the microphones at a sufficient distance from dominant sound sources, we mitigated near-field influences and focused on capturing the representative acoustic environment within the cabin. The data acquisition was performed in the range of 20 Hz to 25.6 kHz with 1.56 Hz resolution.  
To mitigate the influence of extraneous noise sources, the testing environment is carefully controlled. However, use of a semi-anechoic chamber was deliberately avoided to have real road excitation. Additionally, wind noise contribution was minimized by conducting all the tests under controlled conditions where wind speeds were below 3 km/h. The vehicles were thoroughly examined for any leakages, and it was ensured that the vehicles would not have any abnormal sealing or door settings. During the data collection, only the driver and the test engineer were allowed on the vehicles, so that the weight conditions kept unchanged between different vehicles. In this way, it was ensured that the gathered data accurately reflected true road noise characteristics.