Rethinking Sound Power Measurements Outside the Lab
Sound power remains a key parameter for describing the acoustic output of machines, components, and systems. For engineers working in noise, vibration, and harshness (NVH), it’s not just a number; it’s a tool for validation, comparison, and quality control.
But achieving accurate, ISO-compliant sound power measurements outside of anechoic rooms remains a technical challenge. Traditional methods using pressure-pressure (P-P) probes are highly sensitive to factors related to the testing environment. That’s where a shift in measurement principle offers new possibilities.
A More Robust Alternative: P-U Probes
The use of pressure-particle velocity (P-U) probes in sound power determination was solidified by foundational work from Finn Jacobsen at DTU, particularly in his 2006 paper "A Note on the Calibration of Pressure-Velocity Sound Intensity Probes" (link). These probes directly capture sound intensity by simultaneously measuring pressure and particle velocity at a single point, eliminating the need for phase-matched microphones or assumptions about spatial gradients.
One key technical advantage lies in their figure-of-eight directivity pattern of the particle velocity sensors, which naturally attenuates diffuse and off-axis noise. This makes P-U probes far less sensitive to background noise and room reflections compared to their P-P counterparts.
A full explanation of the sensor principle and its calibration is available here.
Controlled Validation Under Realistic Conditions
In our own research, we compared P-U and P-P probes in two environments: an anechoic chamber and a typical untreated lab. A stationary sound source (a three-way loudspeaker) was used to compare measurement consistency.
- P-P probes showed large deviations in field indicators when exposed to reverberation and background noise.
- P-U probes held their accuracy, only showing minimal dependency on environment-related effects.
These findings underline a simple but important point: room quality is critical for P-P probes. For P-U probes, just increasing the measurement distance already helps ensure precision.
Industrial Application: AC Motor Case Study
A recent collaborative study in 2025 by Dana and Siemens took this technology a step further. The goal: determine the sound power of an AC motor undergoing speed sweeps in a non-anechoic lab where conventional scanning methods were not safe or repeatable due to a high-speed, high-voltage test environment.
Their solution was to deploy a half-cylinder array of 27 P-U probes, performing measurements based on ISO 9614-1. Key metrics such as the F4 non-uniformity index guided the probe distribution and surface discretization. Key findings:
- All ISO field indicators were met.
- Results were consistent with ISO 3744 measurements performed in a hemi-anechoic chamber.
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What It Means for NVH Teams
This method allows engineers to perform ISO-compliant sound power measurements in semi-reverberant or otherwise imperfect test setups, without compromising on safety or data quality. While the setup requires attention, most NVH software platforms support this technology. Compared to the cost of modifying or building a new test facility, or repeating questionable measurements, a properly configured P-U setup quickly pays off.
This technology is particularly beneficial:
- EV drivetrain acoustics
- Test benches with limited acoustic isolation
- Industrial product validation under time pressure
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