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The use of a PU mini probe array offers a quantum leap possibility in
car engine acoustics diagnostics!
An unprecedented high dynamic range can be obtained on extremely small
objects using sound intensity measurements rather than only pressure
transducers.
Real time and even post-processed visualisation of multi coherent noise
sources during an engine up is possible now in an engine bay.
There is no longer a need for complicated numerical software routines
reconstructing sound field parameters at the object itself.
It can be measured directly!
A PU mini probe array allows fast (hand held) measurements:
Demonstration
Apart from the PU mini probe array, there a various other near field and
far field measurement techniques available in the market place to
measure and visualise the sound field at the engine.
All these techniques are based upon using (an array) of single
omni-directional pressure transducers place at a certain distance from
the object.
These techniques require complicated numerical routines to calculate/reconstruct the sound field at the object and have (partially
implicitly) a number of acoustic limitations regarding:
Some methods can only be used in the acoustic
near field.
Some other methods can only be used further away from the object, in the
far field.
Microflown PU probes are really multi purpose, and can be used in the
far field, near field, and even in the very near field (where the normal
structural velocity equals the particle velocity).
Car engines acoustics cover the entire audio range fro 20Hz – 20kHz.
Engines are known for their lower frequencies sources in the range of
20Hz – 300Hz. Available systems in the market place can’t cover this
range.
For noise comfort purposes, the upper frequency limit should be 10 kHz
or even 20 kHz.
Available systems claim to work up to 10kHz. But in order to achieve the
claimed 10 kHz bandwidth, often a double number of sweeps is required,
e.g. to combine an STSF measurement and a beam forming measurement.
Microflown p-u probes measure in one sweep, saving at least 50% data
acquisition time covering and extending that frequency range.
A colourful visualisation of numerical results is not necessarily a
guarantee for true high dynamic range, that is often restricted to just
a few dB’s.
STSF types of measurements require either free field or mirror ground
conditions over the entire frequency range of relevance.
In practice, this implies measurements in an anechoic room that is as
such quite expensive (and often not available when really required!).
By definition, using sound intensity measurements instead of pressure
transducer signals, the susceptibility of the background noise is
practically eliminated.
Using p-u based sound intensity probes even helps more.
Pressure transducers as such are omni-directional. As a consequence,
they can’t discern between background noise and noise emitted from the
test object itself.
Sound intensity probes (be it p-p or p-u) measure a vector of the nett
sound intensity flows moving back and forth.
If background noise is present, intrinsically the use of acoustic
particle velocity sensors to capture data is much better than using
pressure transducers for a variety of reasons.
Close to an acoustic hard surface, due to the background noise, the
sound pressure can increase up to 6dB.
Simultaneously, close to a hard acoustic surface, the acoustic particle
velocity due to background noise decreases (up to – 20dB). Since a
particle velocity signal only measures one single vector, it measures
only 1/3rd of the background noise as it is measured by an
omni-directional pressure transducer.
Close to a sound emitting surface, the particle velocity level is high
and pressure level is low. At a further distance, the acoustic particle
velocity converts into pressure.
STSF requires a number of reference transducers (and thus data
acquisition channels) that is at least equal to the number of relevant
independent sound sources.
Microflown Technologies offers various options regarding:
-
Type of transducers
- Software
- Data acquisition hardware
The ½" probes are a rugged multi purpose tool. The
holes in their housings allow simply geometrical reconfiguration. The
packaging limits the upper frequency down to 10kHz and causes a 10dB
packaging gain.
The unpackaged p-u match
probes are extremely small, allowing measurements on very small objects
and/or in key hole cavities. The lack of packaging/housing allows a
frequency range up to 20 kHz.
As a consequence of the absent packaging/housing, the sensors are more
fragile, are less multi purpose, and there is no packaging gain.
Both real time and post-processing visualisation are possible, resulting
in two different hard – and software approaches. If 30 seconds delay for
post-processing is acceptable, a MATLAB based software is available for
real time visualisation.
The hardware is a 32 channel sound card.
For real time visualisation, a C++ based software is available,
developed by Akustik Technologie Goettingen, compatible with HEIM multi
channel data acquisition hardware (DISC 6). |
Microflown Sound Pressure & Particle Velocity Sensors