High-precision micro-measurements in NVH testing

1. Why do we need a specialized measuring microphone?

Recording microphones are typically designed to enhance the listening experience. The sound response can be adjusted to suit the user's perception.

Meanwhile, a measurement microphone needs to reproduce the audio signal consistently, minimizing changes to the amplitude and frequency spectrum of the input signal.

The three basic factors to consider are:

• - Frequency response matches the measurement range.
• - Stability during use.
• - Ability to calibrate and retrieve measurement results.

For measurements requiring comparisons across multiple trials or product samples, the stability and reproducibility of the measurement system are particularly important.

2. Operating principle

Most professional measuring microphones use a capacitive element. When sound pressure acts on the diaphragm, the distance and capacitance between the elements change, generating an electrical signal corresponding to the sound pressure.

Microphones are often paired with preamplifiers to deliver signals to data acquisition systems. In some configurations, the preamplifier is designed to reduce the effects of interference on the transmission line and ensure a stable signal within the measurement system.

3. Key technical specifications to consider

Frequency response

High-quality measurement microphones must have a flat frequency response across the entire measurement range, typically from 20 Hz to 20 kHz or wider depending on the application – ensuring no signal amplitude distortion at any frequency within the range of interest.

Sensitivity

Measured in mV/Pa, this determines the ability to detect low-volume sounds. High sensitivity is suitable for measuring in quiet environments such as soundproof chambers, while lower sensitivity is suitable for measuring high volumes without signal saturation.

dynamic range

The distance between the lowest and highest sound levels that the microphone can accurately measure without interference or signal distortion is crucial when testing involves both quiet and noisy phases within the same measurement.

Stability over time and temperature

The measuring microphone needs to maintain stable sensitivity across multiple measurements, without drifting due to ambient temperature or humidity – especially important when testing for extended periods in a test chamber.

Calibration and traceability capabilities

High-precision measuring microphones must have calibration certificates traceable to national or international measurement standards, ensuring that the measurement results are legally and technically valid for comparison or product certification.

4. Classifying microphones by application (NVH)

The above classification is for guidance only. The same type of microphone can be used for multiple measurements if the conditions and technical requirements are suitable.

4. Several GRAS measurement microphone models

GRAS offers a wide range of microphones for acoustic measurements, NVH (Noise, Vibration, and Harshness) testing, and product research.

Some models you can consider include:

4.1. GRAS 40AE – 1/2" Free-Field Microphone

The GRAS 40AE is one of GRAS's popular measurement microphones, utilizing prepolarized technology and designed according to IEC 61094-4. The microphone has a nominal sensitivity of 50 mV/Pa, a frequency range of 3.15 Hz–20 kHz, and is suitable for free-field measurements.

The 40AE is suitable for general acoustic measurements in the laboratory as well as field measurements, especially when high sensitivity and stable performance over extended periods are required.

Application:

• NVH Test
• Car noise measurement
• Acoustic measurement
• Measure sound levels
• Measuring machine noise
• Product research and development

4.2. GRAS 40AM – 1/2" Wideband Microphone

If the problem requires a wider frequency range, the GRAS 40AM is a noteworthy option.

The 40AM has a frequency range from 3.15 Hz to 31.5 kHz, a sensitivity of approximately 14.5 mV/Pa, and is capable of measuring sound levels up to approximately 163 dB. This microphone is designed by GRAS for laboratory measurements, especially when a wide frequency range and high SPL tolerance are required.

Application:

• Acoustic research
• NVH
• High-frequency sound analysis
• Component testing
• Acoustic research in the laboratory

4.3. GRAS 40AQ – 1/2" R Microphone for Random Incident Field Measurement

The GRAS 40AQ is designed for random incident field measurements, suitable for environments where sound reaches the microphone from multiple directions.

According to the documentation, this model has a frequency range of approximately 3.15 Hz–12.5 kHz and a sensitivity of 50 mV/Pa.

Application:

• Acoustic measurement in a room
• Measure echo time
• Evaluating the diffusion sound field
• Acoustic research

4.4. GRAS 40BE – 1/4" Microphone for High Sound Levels

The GRAS 40BE is a low-sensitivity 1/4" microphone designed for high-pressure and high-frequency sound level measurements.

This model has a frequency range of approximately 4 Hz–80 kHz and can measure sound levels up to approximately 168 dB. Its small size and low sensitivity make the microphone suitable for environments where conventional microphones might be limited by SPL levels.

Application:

• High-frequency sound measurement
• Measuring engines and machinery
• Aviation testing
• Measuring high negative pressure levels
• Acoustic research

4.5. GRAS 46AE – 1/2" CCP Free Field Microphone Set

The GRAS 46AE is a complete microphone system using a 40AE 1/2-inch cartridge combined with a CCP 26CA preamplifier.

According to the documentation, the system has a frequency range of 3.15 Hz–20 kHz, a dynamic range of approximately 17–138 dB, and complies with IEC 61094 WS2F and IEC 61672 Class 1. The system also supports TEDS when connected to a compatible data acquisition system.

This configuration is suitable for acoustic measurement systems that require stability and integration with data acquisition systems.

4.6. GRAS 40HL – Low Noise Microphone

The GRAS 40HL is designed for measuring very low-level audio signals.

According to the documentation, the microphone has a sensitivity of approximately 900 mV/Pa, a frequency range of 6 Hz–20 kHz, and a background noise level of approximately 6.5 dBA.

This series is suitable for measurements where the signal to be observed is close to the background sound level.

Application:

• Low noise measurement
• Acoustic research
• NVH
• Noise analysis in electric vehicle cabins.
• Sound system evaluation

4.7. GRAS 46BC – Multi-field microphone for automotive NVH (Non-Vehicle Radio Communication)

The GRAS 46BC is a 1/4" directional microphone designed for acoustic measurements in complex acoustic environments, particularly inside automotive cabins.

Inside the cabin, sound is affected by reflections from the glass, seats, dashboard, and various interior surfaces. Therefore, the characteristics of the microphone and the measurement location can significantly influence the results.

The GRAS 46BC is used for applications requiring control over the influence of complex acoustic environments and reduced measurement uncertainty.

Application:

• NVH Auto
• Sound measurement in the cabin.
• Electric vehicle sound testing
• Sound quality assessment
• Testing the entertainment system.
• Comparing the sound systems between different car models.

4.8. GRAS 46BL-1 – High-sensitivity 1/4" microphone

The GRAS 46BL-1 is a high-sensitivity 1/4" microphone suitable for low-level audio signal measurements and applications requiring a good signal-to-noise ratio.

According to the documentation, this line is also used in some microphone array systems for measuring in-vehicle sound.

Application:

• Measuring cabin noise in electric vehicles.
• Measuring sound levels in a car.
• High-end sound system
• NVH
• Measuring low-level audio signals

5. Applications of microphones in the NVH laboratory.

• - Measuring sound power according to ISO 3745: Measuring microphones are arranged in a standard hemispherical grid around the test object to determine the total sound power emitted.
• - Cabin noise measurement: Microphones are placed near the driver's/passenger's ears to measure the actual perceived sound pressure level.
• - Combined with an acoustic camera: The synchronized microphone array helps to precisely locate the sound source on the product surface.
• - Measuring aerodynamic noise at high speeds: Specialized wind-resistant microphones are used to eliminate spurious signals caused by airflow directly impacting the microphone diaphragm.

6. Common mistakes when using a measuring microphone

• Skipping calibration before each measurement: Microphone sensitivity can drift slightly over time; calibration with a standard sound source is necessary before each important measurement session.

• Do not use a wind shield when measuring outdoors or at high speeds: This causes false signal interference, completely distorting wind noise measurement results.
• Incorrect positioning compared to the measurement standard: Micro-positioning deviating from the specifications in ISO 3745 or related standards will render results incomparable or uncertifiable.
• Using a microphone with an inappropriate dynamic range: Measuring high volume with a microphone that is too sensitive leads to signal distortion, resulting in inaccurate frequency analysis.

7. GPower Vietnam – Measurement microphone solutions for cultural centers.

Choosing the right measurement microphone—from frequency response and sensitivity to traceable calibration capabilities—is fundamental to the reliability of any acoustic measurement. GPower Vietnam accompanies customers throughout this entire process:

• - Microphone selection advice: Identifying the right type of microphone for each measurement application - from sound power and cabin noise to noise source localization using microphone arrays.
• - Supplier consulting: Connecting with reputable micro-measuring brands within our international cooperation network.
• - Integrated solution consulting: Ensuring the microphone operates seamlessly with the existing DAQ system and analysis software.
• - Calibration, warranty, and maintenance: Regular calibration services ensure the microphone maintains its traceability accuracy over time.

Contact GPower Vietnam now for advice on the most suitable micro-measurement solution for your NVH lab - Hotline: 0936093289 | Email: info@gpower.com.vn .