The PCB Model 378A08 is a compact ¼in free-field microphone engineered to be the quietest in its class, with a low noise floor of 22dB(A). Tests demonstrate that it delivers improved accuracy when measuring high-frequency tonal noise, making it particularly well suited for EV NVH applications
As electric vehicles remove the masking effect of an internal combustion engine, the cabin’s true acoustic signature becomes far more noticeable. Drivers suddenly hear it all: tire and aerodynamic noise, the hum of auxiliary systems, the rumble of drivetrain components, and the high-frequency tonal ‘whines’ produced by electric motors and inverters. These tones occur at very low sound pressure levels, yet they significantly affect overall sound quality. The shift raises a fundamental challenge for NVH testing engineers: how do you measure extremely low sound pressure level (SPL) at high frequencies without the microphone itself introducing distortion?
Selecting the right microphone typically involves weighing competing performance factors. Measuring low SPL demands high sensitivity and a very low noise floor – conditions traditionally met only by a ½in microphone. However, a microphone of that size introduces substantial diffraction errors at high frequencies. Diffraction, or interference in a sound field, becomes problematic when the microphone diameter approaches the wavelength of sound – a condition that occurs increasingly as frequency rises. These errors depend on the characteristics of the sound field, the size and orientation of the microphone and the frequency of interest. Even small deviations from the ideal alignment can cause significant measurement inaccuracies.
To minimize diffraction-related errors, microphones are engineered for specific sound fields: pressure, random incidence (diffuse) and free field. Each design aims to maintain constant sensitivity or a flat frequency response when the microphone is aligned correctly. For free-field microphones, this means pointing directly at the sound source in an environment free of reflections. When the microphone is oriented away from 0° incidence, sensitivity at high frequencies is reduced. This is where the physics becomes limiting for traditional NVH tools.
A ½in free-field microphone such as PCB model 378B02 is a highly dependable acoustic measurement sensor, with low-power ICP (integrated electronics piezo-electric [IEPE]) operation, excellent stability, a low noise floor and 50mV/Pa sensitivity. Its larger size, however, makes it susceptible to diffraction and orientation errors at higher frequencies. In an effort to reduce diffraction, engineers may opt for a smaller ¼in free-field microphone such as the PCB model 378C01. However, its higher 42dB(A) SPL typical noise floor and lower 2mV/Pa sensitivity limit its ability to capture the low-level tonal noise that EVs reveal so clearly.
This long-standing compromise between accuracy and noise performance is exactly what PCB’s new model 378A08 was developed to overcome. With a noise floor of just 22dB(A), it is the quietest ¼in measurement microphone in the world. Its compact size dramatically reduces diffraction errors, while its high sensitivity enables accurate capture of low SPL at high frequencies. The microphone also retains all the advantages of ICP technology, including low-power two-wire connectivity and the environmental stability that PCB microphones are known for.
The benefits of this design become clear when examining how ¼in and ½in microphones behave in different acoustic conditions. In Figure 1, the deviation of microphone sensitivity from its calibrated value is shown for several orientations. This deviation represents measurement error. Both microphone sizes exhibit some attenuation in sensitivity as frequency increases, depending on orientation, but the reduction is far more pronounced for the ½in microphone. The ¼in microphone maintains much more consistent sensitivity, making it a more reliable choice when the sound field or microphone direction cannot be perfectly controlled.
The noise floor behavior shows a similar pattern. A microphone’s free-field noise specification is published for 0° incidence, but in real applications, microphones often operate at different angles or in mixed field types. Larger microphones experience an increase in noise floor under these conditions due to diffraction. As shown in Figure 2, the small diameter of the 378A08 allows it to maintain a low noise floor even when oriented away from 0°. Remarkably, at 120° incidence and frequencies above 13.5kHz, its noise is actually lower than that of a ½in microphone.
Together, these results position the 378A08 as a practical solution for applications that require both reliable high-frequency accuracy and exceptionally low noise, particularly in the low-level tonal environments now common in EV NVH testing.
