Acoustic Sensors for Automotive Testing Applications

Brake noise is a chassis-related noise and a big problem for car manufacturers. The noise is caused by friction-induced vibrations, which make the brake system radiate noise. This will in turn cause a lot of irritation and disturbance to the car owner and any person near the vehicle when it occurs. The brake noise can lead to poor results in surveys of customer satisfaction and high warranty costs. Therefore, the development of brake systems with minimum noise issues are highly prioritized in the automotive industry. Brake noise is a very complex problem and the research in this area is in continuous progress.

THE CHALLENGE

The major challenge for brake noise measurements, is to be able to reproduce the numerous driving conditions that can cause the noise. For example, brake squeal during city traffic, or creep groan during parking. When this happens, during any of the thousands of brake applies, the noise must be recorded at the best possible quality.

SELECTING THE RIGHT MICROPHONE

Brake dynamometer squeal noise test

The SAE J2521 standard recommends a ½” free-field microphone with windshield. The microphone should be able to handle temperature variations and brake dust. For noise source identification, use intensity microphones or array microphones configured according to the relevant equipment needs.

For test cell measurements, we recommend GRAS 146AE ½’’ CCP Free-field Microphone Set when testing brake noise on the dynamometer.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Wheelhouse test

The recommended microphone inside the vehicle is a ½“ free-field or random-incidence microphone as defined by company standards. For the wheelhouse measurements, there is a need for an extremely ruggedized measurement microphone with good performance. It should be able to endure long-term measurements at extremely tough conditions, different environmental conditions and exposure to dust, water, salt and gravel.

For wheelhouse measurements, we recommend GRAS 147EB CCP X-Rugged Microphone Set with its three layers of protection against water, dust and oil.

For in-vehicle brake noise measurements, GRAS 146AE ½’’ CCP Free-field Microphone Set is recommended, optionally with the RA0357 Random-incidence Corrector for 146AE.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN BRAKE NOISE TESTING

Powertrain noise, vibration and harshness (NVH) testing concerns the overall driver experience of the vehicle propulsion system, and this includes everything that makes the vehicle move. The noise level and sound quality are both very important since some parts of the sound provides important feedback to the driver while other parts are just annoying. The noise from all these sources, i.e. the internal combustion engine (ICE) with air induction, exhaust system, tail pipe, gear box, belt drive system, pumps and accessories like cooling systems and fuel pump is transmitted both airborne and structure-borne into the vehicle and contribute to the vehicle acoustic comfort. The powertrain noise also affects the external noise (the pass-by noise), which is regulated by legislation.

THE CHALLENGE

Challenges R&D typically deals with when working on powertrain NVH include:

• Concept selection for engine installation, engine content, layout and insulation

• Exhaust system installation and optimization

• Transmission design

• Concepts for auxiliary components

• Predictions of noise radiation

• Compromising between NVH and other important attributes like performance, fuel consumption and emission

SELECTING THE RIGHT MICROPHONE

Powertrain test

GRAS provides a wide variety of standardized measurement microphones that can be used in the many different stages of powertrain testing.

We recommend GRAS 147AX CCP Rugged Pressure Microphone for engine bay measurements during powertrain tests where a low profile microphone capable of handling high temperatures is needed. To test the powertrain under different load conditions both for far-field measurements in the dynamometer, road or drivetrain test rig and for interior measurements, we recommend GRAS 146AE ½’’ CCP Free-field Microphone Set as the best choice. When a random-incidence microphone is needed, the RA0357 Random-incidence Corrector for 146AE is recommended.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Sound intensity and sound source location

Array microphones like GRAS 40PH CCP Free-field Array Microphone and GRAS 40PL CCP Free-field Array Microphone, High Pressure, are cost-effective, free-field acoustic sensors for the analysis of sound fields. These types of microphones can be used in powertrain testing for measuring and locating sound sources using techniques like beamforming, near-field acoustic holography (NAH) and acoustic cameras.

We recommend GRAS 40PH CCP Free-field Array Microphone and GRAS 40PL CCP Free-field Array Microphone, High Pressure, for measuring and locating sound sources using techniques like beamforming, near-field acoustic holography (NAH) and acoustic cameras. GRAS 50GI-P CCP Rugged Intensity Probe can also be used for sound intensity measurements and source location, especially useful when testing in noisy environments and for areas difficult to access with microphone arrays.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, and GRAS 51AB Phase Intensity Calibrator are ideal choices.

Learn more about ACOUSTIC TEST TYPES WITHIN POWERTRAIN NVH TESTING

The exhaust noise from vehicles with internal combustion engines is very important. It must support a good engine sound quality while not being disturbing or violating noise legislation. The intake system and exhaust system are highly responsible for the engine noise character and sound pressure level (SPL) and must be tuned to meet the required performance. The engine sound quality should provide driver information about engine rpm as well as a suitable sound for the vehicle type, e.g. powerful and sporty during acceleration, but silent during constant speed driving. This also applies to the exterior sound where the sound character should match the vehicle brand identity.

THE CHALLENGE

The hot and dusty environment must be considered for measurement setup and equipment selection. Other important issues are exact positioning of microphones to be able to verify calibration before testing. The environmental conditions during the operational measurements are harsh with heat, dust, humidity and exhaust flow. The transducers are often moved to different positions so they must withstand handling and occasionally being dropped to the floor. It is also important to be able to position the microphones precisely and thus verify calibration before testing.

SELECTING THE RIGHT MICROPHONE

Sound transmission loss (measurements on dynamometer)

The operating measurements are done with ruggedized ½” pressure or free-field microphones as per the company standards. For extremely hot positions close to e.g. the exhaust catalyst or in the direct exhaust flow, use a probe microphone. Windshields are recommended both to minimize wind noise and to provide some microphone protection.

For test cell measurements, we recommend GRAS 146AE ½'' CCP Free-field Microphone Set when measuring in the near-field of the muffler or measuring tail pipe noise. When temperatures exceed 125 °C (257 °F), a probe microphone like GRAS 40SC CCP Probe Microphone can be used for exhaust noise measurements, even with the microphone placed in flow.

For calibration GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Sound transmission loss (test rig)

A sound transmission loss rig for exhaust systems typically uses four class 1 ¼” microphones.

For test cell measurements, we recommend GRAS 46BD-FV ¼" CCP Pressure Standard Microphone Set, Front Vented, as a reference microphone in the sound transmission loss rig for muffler testing.

For calibration GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Pass-by noise

The microphones for pass-by noise testing must fulfill the test procedures. Typically, a class 1, free-field microphone is used. Outdoor microphones with weather protection and internal heating are preferred for permanent installations.

For outdoor measurements, we recommend GRAS 146AE ½'' CCP Free-field Microphone Set as it is IP67-rated dust and water-proof and thus withstands harsh environmental conditions.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN EXHAUST NOISE TESTING

Wind noise is an aerodynamic noise related to the body design and turbulence from various components of the vehicle. Within automotive aeroacoustics,there are three main categories to study which affect the aerodynamic noise on a road vehicle: Vehicle form, open cavities and the body sealing - the latter being the ability to reduce wind noise and often related to the price range of the vehicle.

Important components related to the vehicle form and design which have a direct influence on wind noise testing results include A-pillars, side mirrors, windscreen wipers, antenna, underbody and glazing. Normally, wind noise is most disturbing at speeds above 100 km/h (approx. 60 miles/h), but is becoming increasingly important when the powertrain is reduced – especially on electrical vehicles (EVs). Careful design minimizes the problem, so early concept studies are important. Wind noise is, however, often an issue discovered late in the development process. It requires fast and efficient analysis since measures such as extended seal systems or sound packages are expensive.

THE CHALLENGE

For OEMs it is important that variation in production for critical design areas remains low. The tolerance chains need to be investigated and, if necessary, compensated for with good seal systems. This requires aeroacoustic testing.

Validation of CFD (Computer Fluid Dynamics) modeling is an important part of designing vehicles that move in airflow. However, airflow and turbulence cannot be fully simulated and predicted realistically – CFD methods do not provide all the answers needed in the design process. Due to the complexity of boundary layer pressure variations, simulations often require multiple aeroacoustic validations and correctional phases. 

Also, wind tunnel testing typically involves high costs linked to the preparation of the test item and operation of the wind tunnel. Hence, the testing must be performed efficiently to get the most out of the limited testing time. Mounting, demounting, remounting as well as verification of sensors become highly critical for ensuring reliable data easily, efficiently and within budgets.

For these reasons, high-quality measurements are a must.

SELECTING THE RIGHT MICROPHONE

GRAS provides a number of microphone solutions for testing full-scale models in wind tunnels and on-road.

We recommend our new suite of Ultra-Thin Precision (UTP) microphones or surface microphones for wind noise source investigations. GRAS 48LX-1 UTP Microphone, Medium Pressure, with a height of only 1 mm and GRAS 48LX-4 UTP Line Array, 4 Channel, Medium Pressure, or GRAS 48LX-8 UTP Line Array, 8 Channel, Medium Pressure, with a height of only 1.35 mm, makes these microphones ideal for measurements in boundary layers and turbulence.  The microphones are easily mounted, demounted and remounted and can even be mounted on curved surfaces.  

Surface microphones like GRAS 40LA ¼” CCP Precision Surface Microphone, High Pressure, or GRAS 40LS ¼” CCP Precision Surface Microphone may also be used for in-situ boundary layer testing. The total height of these microphones is 2.5 mm (approx. 0.1 inch).

All LX versions of the UTP microphones and GRAS 40LA/LS have integrated preamplifiers and TEDS, which makes installation and sensor identification easy.  

For in-situ verification in the field, we recommend GRAS 42AG Multifunction Sound Calibrator, Class 1, together with the GRAS RA4800 Adapter for Sensitivity Verification of Flush/Surface Microphones.

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Read more about our Measurement Microphones for Aeroacoustics and The World’s Thinnest Condenser Microphones.

Learn more about ACOUSTIC TEST TYPES WITHIN WIND NOISE TESTING

Buzz, squeak and rattle are all noises produced by parts installed in such a way that they create annoying sounds. Buzz, squeak and rattle is thus caused by the relative motion between installed parts, which make them interact, sometimes only at certain environmental conditions.

THE CHALLENGE

Design guidelines and computer-aided engineering (CAE) methods to predict buzz, squeak and rattle in combination with production tolerances are continuously improving, but squeak and rattle problems are often detected at a very late stage, even after production start, and can be the reason for high warranty costs. Good tools are important for fast problem identification and verification of measures like adding felt tape. Improved correlation of measurements to subjective valuation is also a continuous task for the engineers.

SELECTING THE RIGHT MICROPHONE

Silent shaker test

For optimum results, ½” measurement microphones should be used. The measurement in the NVH test cell with silent shaker requires low noise performance. The transducers must also withstand exposure to harsh environmental conditions such as strong vibrations, shock, drop, extreme temperatures and wet or dusty conditions.

For test cell measurements, we recommend GRAS 146AE ½’’ CCP Free-field Microphone Set as this microphone is capable of measuring signals down to 18 dB(A) and thus will record even very week buzz, squeak and rattle signals originating in the silent shaker test.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Test track

For in-vehicle measurements, we recommend GRAS 146AE ½’’ CCP Free-field Microphone Set, optionally with the RA0357 Random-incidence Corrector for 146AE.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN BUZZ, SQUEAK AND RATTLE NOISE TESTING

Vehicle interior noise, vibration and harshness (NVH) involves a number of different tests – all related to noise inside the vehicle originating from road, body, chassis and components of the vehicle.

The main focus areas are:

• TOTAL INTERIOR NOISE
  A way of testing the general experience of a vehicle’s acoustic comfort.
• ROAD NOISE
  Testing related to how road excitation contributes to the interior noise both structure-borne and airborne – depending on well-defined road surfaces and tires.
• THE SOUND PACKAGE
  An area that affects most of the NVH areas.
• COMPONENT NOISE
  Concerns all additional systems like the heating, ventilation and air conditioning (HVAC) system, power windows, etc. These are mostly electrical or driven by the engine belt, and sometimes hydraulic.
• BRAKE NOISE
  A chassis-related noise and a big problem for car manufacturers.
• COMPUTER MODEL VALIDATIONS
  Testing on prototype parts or vehicles

THE CHALLENGE

There are a number of general challenges within vehicle interior NVH:

• Testing time should be short since the access to prototypes is limited.

• Microphone positioning should be done fast and easy, and in a way to record repeatable results.

• The microphones should be placed so that they minimize structure-borne sound.

• Microphone holders and cables should not introduce any rattling noise.

• The installation should be safe for the test engineer to perform during vehicle testing.

• Calibration verification should be easy to perform.

SELECTING THE RIGHT MICROPHONE

Vehicle interior and component/system level noise measurements

GRAS provides solutions that can be used at the different project levels and under a wide variety of measurement conditions encountered in vehicle interior testing.

We recommend GRAS 146AE ½’’ CCP Free-field Microphone Set to test the properties of the acoustic materials used in the sound package or to measure component/system noise both in an anechoic, hemi-anechoic or reverberant chamber, optionally with the RA0357 Random-incidence Corrector for 146AE.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Vehicle interior sound source location

Array microphones like the 40PH and the 40PL CCP Free-field Array Microphones are cost-effective, free-field acoustic sensors for the analysis of sound fields. These types of microphones can be used in vehicle interior testing for measuring and locating noise sources using techniques like beamforming, near-field acoustic holography (NAH) and acoustic cameras.

We recommend GRAS 40PH CCP Free-field Array Microphone and GRAS 40PL CCP Free-field Array Microphone, High Pressure. GRAS 50GI-P CCP Rugged Intensity Probe can also be used for sound intensity measurements and source location, especially useful when testing in noisy environments and for areas difficult to access with microphone arrays.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, and 51AB Phase Intensity Calibrator are ideal choices.

Learn more about ACOUSTIC TEST TYPES WITHIN VEHICLE INTERIOR NVH TESTING

Road noise is one of the most annoying interior noises in any vehicle. This is even more accentuated in an electric vehicle, 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 CHALLENGE

Reliable equipment is critical since testing often is performed at a test track off-site. Rattle-free installation is extremely important to be able to evaluate high-frequency content that is not contributing to SPL, but to sound quality. The measurement microphone field type is important since road noise is often analyzed above 5 kHz, and time data is sometimes also used for later playback for subjective noise comparison.

SELECTING THE RIGHT MICROPHONE

Vehicle interior noise testing

Microphones used for vehicle interior noise testing should have a frequency range up to at least 11,22 kHz to allow for the analysis to include the 10 kHz third octave band. Free-field or random-incidence ½” class 1 measurement microphones are used at ear position, and six or more microphones are used for far-field sound power measurements. The choice depends on the test procedures. Pressure or random-incidence microphones with low noise performance are recommended for acoustic transfer function (ATF) measurements where frequencies above 3-4 kHz are of importance.

For in-vehicle measurements, we recommend GRAS 146AE ½” CCP Free-field Microphone Set, optionally with the RA0357 Random-incidence Corrector for 146AE.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Acoustic transfer function testing

For near-field measurements or troubleshooting of the tire, intensity probes and microphone arrays can be used and configured as per the analysis equipment needs. Measurements at the tire on road require nose cones or high-performance windshields. The possibility of easy verification of calibration will minimize test time and enable repeatable measurement results. The transducers are exposed to a lot of wear, often moved and even at worst case dropped to the floor. It is important to ensure that variation in environmental conditions do not influence the measurement performance.

For laboratory testing, we recommend GRAS 147AX CCP Rugged Pressure Microphone. which will make ATF testing easy thanks to its small size and the possibility of mounting it even in the most difficult places and challenging environments (high and low temperature, high humidity, dust, etc.). When space or mounting challenges are not a concern, GRAS 146AE ½’’ CCP Free-field Microphone Set can be used with RA0357 Random-incidence Corrector for 146AE if random-incidence response is needed for ATF testing. GRAS 46BD 1/4" CCP Pressure Standard Microphone Set or GRAS 46AO 1/2'' CCP Pressure Standard Microphone Set can be used if pressure microphones are needed.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN ROAD NOISE TESTING

Consumers are increasingly expecting high-quality sound systems and infotainment systems with voice commands. This drives the importance of ensuring good acoustics, free from reverberation and excess dampening in desired frequency ranges, to the fore.

In-cabin sound quality is frequently cited as a key aspect of how customers evaluate vehicles. But assessing audio quality in environments as complex as the cabin of a vehicle is a difficult task. The in-cabin sound field consists of a mix of varied damping surfaces and hard reflective surfaces, so the sound field varies from free field to random incidence (or undefined).

However, the multifield capabilities of 46BC combined with the measurement accuracy (particularly at higher frequencies) make it ideal for the job. Additionally, 46BC and the 46BL-1 pressure microphone, are ideally suited for measurements according to the AES In-car Acoustics Measurements recommendation.

Learn more about In-vehicle acoustics >

Environmental noise pollution must be minimized to ensure people’s health and comfort. Traffic noise is, therefore, regulated and without fulfilling the regional targets for pass-by noise, a vehicle model cannot get a type approval or be put on the market. International test methods have been developed to simulate as realistic driving conditions as possible and the noise level limits have been reduced all over the world. This, obviously, puts a lot of focus on vehicle development and robustness in production. The methods for measuring pass-by noise requires dedicated facilities like specialized test tracks, large indoor noise, vibration and harshness (NVH) chassis dynamometer test cells and high-performance test equipment to produce comparable measurement results. Before running verification tests, thorough investigations of the most contributing sources (engine and intake/exhaust system, tire noise) are performed to predict the total noise level.

THE CHALLENGE

Early tests and predictions become more and more important to be able to fulfill legislation within tightly planned vehicle development projects. Tire noise will also be more critical for future vehicles and, especially, for electric vehicles. The acoustic vehicle alerting systems (AVAS) in electric vehicles must not only fulfill target sound levels, but also sound quality design. Pass-by noise is an important part of the branding efforts of vehicles and should of course also be considered.

SELECTING THE RIGHT MICROPHONE

Outdoor testing

The 146AE ½” CCP Free-field Microphone Set, which is IP67 rated dust and waterproof, is the ideal microphone for pass-by noise measurements. Even when temporary rain will stop your outdoor measurements, it is not necessary to disassemble your measurement setup, if you use the 146AE, so that when the rain stops it is possible to quickly resume the measurement.

We recommend GRAS 146AE ½’’ CCP Free-field Microphone Set for outdoor testing.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Permanent outdoor testing

The microphones must fulfill the test procedures, typically for a class 1, free-field microphone. Outdoor microphones with weather protection and internal heating are preferred for permanent installations.

We recommend GRAS 41AC LEMO Outdoor Microphone with RemoteCheck for pass-by noise measurements in a permanent outdoor setup as this type of microphone is waterproof and can operate with very low maintenance under varying weather conditions and temperatures for very long periods of time, i.e. a year or longer.

Indoor testing

A microphone array consisting of the 146AE microphone sets can be mounted on each side of the vehicle suitable for making the pass-by driving simulation. The power-on LED indicator in the 146AE will help the test engineers to swiftly check that all the microphones are working correctly. In addition, the transducer electronic data sheet (TEDS) capabilities of these sensors will contribute to the fast setup of these multi-channel systems.

We recommend GRAS 146AE ½’’ CCP Free-field Microphone Set for indoor testing.

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN PASS-BY NOISE TESTING

Electrical propulsion is rapidly becoming more or less mandatory to include in every OEM’s vehicle range. Many naming conventions exist for the different vehicle technologies presently available. The main categories are the battery electric vehicle (BEV) and the hybrid electric vehicle (HEV) which also includes an internal combustion engine (ICE). The main difference between traditional powertrain testing and HEV/BEV testing is this addition of one or more electric motors, including inverter technology, battery cooling and the transmission design. The electric motor means new challenges compared to the combustion engine.

THE CHALLENGE

The main challenges are the low sound levels and tonal noise at much higher frequencies compared to traditional vehicles. The testing, therefore, requires high-quality equipment and careful measurement setup. Obviously, there is also less experience of customer reactions in this field.

SELECTING THE RIGHT MICROPHONE

Microphones used for electric and hybrid driveline testing must be robust and handle any electromagnetic fields. A small size is important for easy positioning when measuring acoustic transfer functions. Free-field or random-incidence microphones are most often used, and the choice depends on the test procedures. Low-noise performance and full audible frequency range is also required.

We recommend GRAS 146AE ½’’ CCP Free-field Microphone Set and GRAS 147AX CCP Rugged Pressure Microphone as both can be used to test signals down to 18 and 19 dB(A), respectively, and cover the full human audible frequency range. This means that both microphones will be able to measure the entire dynamic range of the characteristic tonal noise components from EVS. When a random-incidence microphone is needed, the 146AE can be used together with the RA0357 Random-incidence Corrector for 146AE to change its response from free-field to random-incidence (diffuse field).

For calibration, GRAS 42AG Multifunction Sound Calibrator, Class 1, is the ideal choice.

Learn more about ACOUSTIC TEST TYPES WITHIN ELECTRIC AND HYBRID VEHICLE TECHNOLOGY TESTING 

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