Automotive Application: Brake Noise Testing

Brake Noise Testing

Brake noise is a significant issue for car manufacturers, stemming from friction-induced vibrations that cause the brake system to radiate noise. This can lead to driver irritation, dissatisfaction, and increased warranty costs. Consequently, developing brake systems with minimal noise is a high priority in the automotive industry. The complexity of brake noise means research in this area is ongoing. The rise of electric vehicles presents new challenges, as the absence of engine noise makes brake noise more noticeable. While regenerative braking can reduce reliance on traditional brakes, these systems are still crucial.

The development process to mitigate brake noise typically involves several stages:

• Concept selection
• Design review and best practice checks
• Computer-aided engineering (CAE) noise predictions
• Component testing, including modal analysis and verification of modes for brake components (pads, rotor)
• CAE correlation
• System test for brake squeal
• Vehicle test (post-system test approval)

Common brake noise characteristics, particularly from disc brakes, include:

• Low frequency (100-1,000 Hz): Often broadband noise described as scraping, grinding, grunt, groan, moan, or judder.
• Medium to high frequency (1-5 kHz): Tonal noise caused by coupled modes between pads and rotor, leading to out-of-plane vibrations. Squeal occurs under specific conditions of brake pressure, rotor temperature, and environment.
• High frequency (above 5 kHz): Tonal noise, or squeak, resulting from exciting in-plane resonances of the brake rotor.

Acoustic Test Types Within Brake Noise Testing

Brake Dynamometer Squeal Noise Test

This standardized test (SAE J2521) is performed using an NVH brake dynamometer to ensure brakes are in optimal condition before vehicle testing. Brake squeal is defined as noise between 1-17 kHz.

The test procedure involves:

• Mounting the complete brake system in a fixture representing a vehicle corner (1/4 chassis with suspension).
• Testing thousands of brake conditions, varying wheel speed, brake pressure, rotor temperature, environmental conditions, and brake wear.
• Recording brake squeal with a measurement microphone.
• Using accelerometers to correlate vibrations with noise.
• Employing 3D laser vibrometers or acoustic cameras for root cause investigations.

This test is crucial for identifying and addressing brake squeal issues, allowing for modifications to brake pads, lining material, pad design, shims, mass loading, damping, or rotor/caliper modifications.

Wheelhouse Test

This is the final stage of testing, conducted through road tests under precisely defined conditions for extended periods. Noise is recorded inside the vehicle and in the wheelhouse, along with parameters like brake vibrations, pressure, temperature, driving conditions, deceleration, and subjective impressions.

The test requires extensive repetition across various vehicles and seasons, often involving long-term driving (e.g., 50,000 km) with multiple microphones, accelerometers, and detailed data logging to capture a wide range of brake noise results under diverse external conditions.

Challenges Common to Brake Noise Testing

The primary challenge is accurately reproducing the varied driving conditions that can trigger brake noise, such as city traffic or parking. The noise must be captured with high quality during these instances. Statistical information for noise between 1 kHz and 17 kHz is analyzed alongside subjective classifications of low-frequency noise. Microphones and accelerometers help identify the source (front or rear brakes), leading to further root cause analysis.

Robust microphones are essential for these tests, capable of withstanding harsh environmental conditions, including temperature variations, humidity, and brake dust. Tests often run automatically for extended periods (around 24 hours), making reliability and easy calibration verification critical.

Wheelhouse Test Environment

The wheelhouse test involves placing measurement microphones directly in the wheelhouse, a demanding environment exposed to dust, water, salt, and gravel. Microphones must be installed securely to withstand long-term driving and allow for reliable calibration. Care must be taken to avoid introducing rattling noises from microphone holders or cables and to ensure safe installation at the driver's ear position.

Selecting the Right Microphone

Brake Dynamometer Squeal Noise Test

The SAE J2521 standard recommends a 1½" free-field microphone with a windshield. The microphone should tolerate temperature fluctuations and brake dust. For noise source identification, intensity microphones or array microphones are used.

The 146AE 1½" CCP Free-field Microphone Set is suitable for dynamometer brake noise testing. Its rugged design protects against temperature variations and brake dust. The AM0069 Spherical Windscreen can be used in windy conditions. The AL0004 Small, Lightweight Microphone Tripod, combined with the AL0008 1½" Microphone Holder or RA0093 1½" 5-click Microphone Holder, facilitates microphone placement. The AL0005 Swivel Head is also required for the AL0008 holder. The 42AG Multifunction Sound Calibrator is used for daily sensitivity verification.

Wheelhouse Test

For in-vehicle measurements, a 1½" free-field or random-incidence microphone is recommended. For wheelhouse measurements, an extremely rugged microphone with good performance is needed to endure long-term testing in harsh conditions, including exposure to dust, water, salt, and gravel.

The 147EB CCP X-Rugged Microphone Set is designed for wheelhouse mounting, offering protection against water, dust, and oil. It requires watertight cables like the AA0121 5 m Waterproof BNC-BNC Cable. The 146AE microphone set can also be used as a random-incidence microphone with the RA0357 Random-incidence Corrector. The RA0504 GoPro Adapter aids in quick and easy microphone positioning, compatible with various tripods and clamps.

Easy calibration verification with the 42AG calibrator ensures minimal test time and repeatable results.

For sound intensity measurements, the 50GI-RP CCP Rugged Intensity Probe is recommended, useful for sound source localization in noisy or difficult-to-access areas. The 51AB Phase Calibrator (IEC 61043) is used for level and phase calibration of intensity probes.

Troubleshooting

For troubleshooting, measuring, or locating sound sources using techniques like beamforming or near-field acoustic holography (NAH), array microphones are effective. This includes cost-effective options like the 40PH and 40PL CCP Free-field Array Microphones, as well as array modules like the PR0002 Array Module for sound field analysis. The 42AG calibrator can also be used for array microphone calibration.

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About GRAS Sound & Vibration

GRAS is a global leader in the sound and vibration industry, developing and manufacturing state-of-the-art measurement microphones for applications where acoustic measuring accuracy and repeatability are critical. This includes industries such as aerospace, automotive, audiology, and consumer electronics. GRAS microphones are engineered for high quality, durability, and accuracy.

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