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User benefits
Before
particle velocity sensors became available, a finite difference approximation
of the pressure gradient using two phase matched pressure transducers (the so called
p-p method) estimated the particle velocity signal. For many reasons, p-p based
sound intensity measurements didn't get very popular in acoustics.
The p-p sound probes are relatively large, have limitations in both their lower and
upper bandwidth limit, and are susceptible to reflections (a high pressure –
intensity ratio).
- The minimum relevant dimension of a p-p probe (so two pressure transducers and required
spacing) is 5 cm. A p-u probe can have the size of a match.
- On the lower frequency limit, below 100Hz are difficult, depending upon the sound field.
Below 50Hz, measurements are practically impossible. The p-u probes can be used
down to 20Hz
- On the upper frequency, 6.3 kHz is the upper frequency limit of a p-p probe using one
spacer. It has been demonstrated that 10.0 kHz can be achieved using one spacer.
A p-u probe can be used up to 20kHz.
- In practice, this claimed frequency range of the p-p probe is narrowed down significantly by
the level of reflections prevailing in the environment, denoted as pressure -
intensity index.
A traditional p-p probe compared to a three dimensional pu probe (the USP)
With particle velocity being a direct measurable quantity, sound intensity measurement
will revive. Combining a small Microflown acoustic particle velocity sensor
with a miniaturized pressure transducer overcomes all the aforementioned
problems with p-p probes.
Particle velocity based sound probes are:
- Compact,
allowing
- Measurements
very close to a surface
- Measurements
on small objects
- In
(small) key hole cavities
- Broad
banded, allowing measurements from 1Hz up to 20kHz (at least), covering
entirely the audio range
- Are not
susceptible to reflections, so don't have pressure intensity index
problems
Applications of PU
intensity probes
- Source
determination in an confined space with reflections (like an aircraft
cabin or car interior)
- Important
step in reciprocal measurements
- Intensity
based holography
- End of
line control
Literature
The
working principle of a combined p-u probe allows measurements:
Prof. Finn
Jacobsen from DTU Denmark has published various papers on comparing p-p and p-u
probe measurement principles.
In his ICSV11 paper, prof. Jacobsen concludes that a properly calibrated p-u
probe is a reliable tool for sound intensity and sound power measurements if
reactive near fields are avoided.
In his NOVEM 2005 paper, prof. Jacobsen concludes that a properly calibrated
p-u probe is (unlike a p-p probe) not susceptible to high pressure – intensity
index.
Amongst others, it implies that with a p-u probe measurement can be made in an interior
cavity such as an aircraft cabin or car interior without the need to create
anechoic conditions first.
In his ICSV12 paper, prof. Jacobsen concludes that under adverse conditions
with background noise, a p-u probe is at least as good as a class 1 sound
intensity probe based upon a pair of phase matched pressure transducers.
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Applications 
