This is in continuation of our previous blog about Tyre noise evaluation Part-I.
In earlier blog we have discussed about mechanism of noise generation and affecting parameters. In this blog we will mainly discuss about test methodology and quantifying parameters for tyre noise.
Mainly there are five types of test methodology used worldwide for tyre or road noise measurement, namely,
- Coast by Method
- Trailer Coast by Method
- Drum method
CPX- This method is extensively used in European countries and is based on near source measurement of sound pressure. This method is also explained in draft ISO 11819-2. Microphone positions is as shown in below picture.
In this method two microphones (M1 & M2) are mandatory which are 40cm apart and 10cm above ground. Other microphones are optional. Test speed are 50 and 80 KMPH.
OBSI- This method is generally used in USA for tyre/road noise. In this method sound intensity is measured using two intensity probes and its position is as shown in below picture.
Coast by Method: - In this method tyre noise is measured on vehicle in coast down condition on test track. Test track and microphone position are as shown below.
When front of vehicle reached AA’ line, vehicle should be in neutral gear and engine is switched off and vehicle should be coasting from AA’ to BB’ line. Maximum noise level and speed is recorded until rear part of vehicle reached BB’. Reference speed and its range depends on the class of test tyre (80Km/h for C1 and C2 and 70km/hr for C3). There should not be any large sound reflecting objects like fences, barriers, bridges or any building within 50m. Background noise should be at least 10dBA lesser than measured. Tyre should be loaded to 75±5% of its load index.
Trailer Coast-By Method: - Test methodology in this process are almost same only difference is that the test tyre is fitted in a special designed trailer and this trailer is towed on vehicle. Noise level to be recorded with and without trailer. Noise level without trailer should be at least 10dBA lower compare to with trailer condition. Test track and microphone position are as shown in below picture.
Vehicle with trailer should approach EE line with engine off and gear in neutral and clutch disengaged along the centile as shown in above pictures. Maximum sound level recorded by microphone should be reported when test tyre in between AA and BB line.
Drum Method: - This methodology in generally used in laboratory for evaluating tyre noise characteristics.
Modal Analysis: - Though modal analysis test does not give noise level, it plays a significant role in tyre noise. Natural frequency and mode shape are important parameters to understand noise generation mechanism. In our previous blog(Part -1) we have discussed impact and adhesion mechanism. Impact of road on tyre will excite the natural frequency of tyre which leads to vibroacoustic noise. Depending on natural frequency range tyre noise may amplify or attenuate. Typical mode shapes (circumferential and cross sectional) of a tyre are shown below.
Critical checks for outdoor testing
- There should not be any large sound reflecting objects like fences, barriers, bridges or any building within 50m.
- Wind speed at microphone height should not be more than the permissible limit.
- Test track temperature should not be less than 5 °C and air temperature more than 40°C.
- Calibration of all are checked pre and post-test as our internal test process.
- Test to be conducted at test track where background noise inside and outside the vehicle should be in the standard permissible limits.
- Tyre pressure to be per specification.
- Vehicle to be serviced before test and checked for any abnormal noise like squeak, rattle etc.
- If tyre under test is new, preconditioning is required.
Test Method developed at ASI-
ASI has developed test methods and process driven by industry requirement to understand tyre noise for customer as well as manufacturer. We combine near source measurement with interior noise measurement and coast down setup to understand the contribution of tyre noise at driver ear and for the passer-by. This combination of external- internal coast down and accelerated method helps in better understanding as well as improved development method for tyre.
Tests, scenarios and conditions to evaluate tyre noise.
To study the impact on tyre noise in various scenarios which occur during the normal course of tyre life following conditions and scenarios are generally studied: -
- Braking during cornering
- Coast down, etc
Quantifying Parameters and sound characteristics
Once measured, tyre noise data is used for various sound pressure level and psychoacoustic analysis to understand the sound characteristic and its impact.
These above parameters are used with standard Acoustic analysis methods like color-plot, order tracking , resonance study to quantify and understand the sound because of tyre.
In our endeavour to understand tyre more, we have developed methodologies to measure tyre tread and its impact on noise and we will be publishing that soon.