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Road Noise Testing

Road noise is one of the most annoying interior noises in any vehicle and this problem is even more accentuated with more and more electrical vehicles on the market, leaving road noise, aero acoustic noise and, at certain conditions, heating, ventilation and air conditioning (HVAC) noise to be the dominating noise sources. External tire noise is also more and more critical for fulfilling pass-by noise regulations.

The road surface excitation is the source, and the cabin noise is both structure-borne and airborne. Frequencies below ~300 Hz are mostly structure-borne and above this frequency more airborne.

The noise is caused by the tire tread and road surface interaction. The tire tread design and tire sidewall stiffness have a lot of influence on cavity resonances that cause tonal noise around 200-250 Hz. The vehicle verification is done by analysis of sound pressure level (SPL), third octave band spectra, narrow band spectra and Articulation Index (AI) at different vehicle speeds and welldefined road surface types for noise, vibration and harshness (NVH) testing, e.g. smooth, coarse, grooved and standardized surface for pass-by noise measurements. Transient noise and comfort while driving over bumps must also be evaluated.

Learn more about the challenges and how to select the right microphone

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Selected products

  • GRAS 146AE
    A rugged and versatile microphone thoroughly protected for maximum performance in conditions with shock, dirt and moisture

  • GRAS 147AX
    Makes ATF testing easy thanks to its small size and the possibility of mounting it even in the most difficult places

  • GRAS 42AG
    The ideal solution for calibration

ACOUSTIC TEST TYPES WITHIN ROAD NOISE TESTING

Vehicle interior noise testing

A vehicle test on a NVH test track typically includes two to four microphones inside the vehicle distributed between the driver's and passenger seats. External microphones with high-performance windshield or intensity probes with nose cones can be used for measurement of tire near-field noise. Impact noise is measured on a dedicated test track with impact bars.

These tests together with sound power measurements can also be performed on a NVH chassis dynamometer equipped with road surface or impact bars during development. A test rig for the wheel only, adapted for the test track or the NVH chassis dynamometer, can be used for acoustic source measurements or pure tire development.

Structural testing is used for determination of the vehicle chassis sensitivity. Excitation is done with an impact hammer or shaker as well as a force transducer, and the acoustic response is measured with a measurement microphone at ear position. These tests in combination with vehicle tests and spindle force measurements will explain, if improvements are needed for vibration isolation to reduce low frequency road noise and/or tire cavity tonal noise.

Acoustic transfer function (ATF) testing

Acoustic transfer functions are measured between several positions in front of and behind the tire to understand the noise insulation from the wheelhouse to the cabin. This is done either with the direct method or reciprocally. The direct method involves using a calibrated omnidirectional sound source located at the tire, while the response is measured with one or more measurement microphones at ear position. For reciprocal measurements, the sound source is at ear position and the response is measured at the tire.

These measurements must be performed at several positions to include the source as well as the response. The result from such tests can show, if the acoustic pack needs improvement or, if there is a sound leakage. Pass-by noise is measured with dedicated microphones at a pass-by test track where all test equipment and facilities are thoroughly specified in the applicable standards.