Microflown
Microflowns can be used for various applications in the form of arrays. Arrays of sensors are more than only a number of individual probes mounted together. Apart from the acquisition, an array application solution consists of a three basic elements:
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The method of positioning the sensors |
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The particular sensor used |
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The electrical connections |
The method of position the sensors
There are numerous methods to configure an array of sensors. The probes can be mounted in a grid so that handheld array is created, it is also possible to put them on a surface as is done in source path contribution methods. Some solutions are listed below:
The particular sensor used
The PU-mini, PU-match and the USP-mini are used in array configurations most often.
The electrical connections
Obviously, the number of cables and connectors increase if a lot of sensors are used.
The change of making mistakes, mixups and set up time increase with that. To minimize this,Microflown technologies designes therefore special preamplifiers, multipin conmnectors and BNC breakout boxes that go to the dedicated frontends (24channel, 96channel).
Microflown Applications
Microflown Technologies offers superior applications such as:
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fast and broad banded vehicle interior noise source path contribution analysis |
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( portable ) near field acoustic camera's |
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sound source triangulation techniques in the acoustic far field |
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in situ determination of acoustic properties of materials |
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non contact vibration measurements / modal analysis |
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acoustic surveillance of strategic objects |
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micropore leaktesting |
Sound source localisation using Acoustic Camera
The aim of the method is to be able to reconstruct and visualize sound pressure, particle velocity and sound intensity on a surface.
Measurement of sound intensity involves determining the sound pressure and the particle velocity at the same position simultaneously. The need for intensity measurement arises to achieve the noise control. Always it is necessary to assess the environment from the receiver’s viewpoint. It is required to reduce harshness in an automobile, reduce noise in a shop floor and reduce sound pollution in the environment. To achieve all these things, it is more important to locate the source of sound and take remedial measures.
When several sound sources are present, it is necessary to characterize these sources and transmission paths. It is necessary to identify Candidate Sources - for each receiver, Candidate Paths - for each source / receiver combination. Then Select Treatments. The best approach would be “always attack major systems first.” After knowing the cause it may be required to replace, redesign, reduce Size, reduce clearances, improve maintenance, reduce free-vibration time (damping), cut paths, using barriers (airborne) or isolators (structure borne), impede travel along paths, using absorption or mufflers (airborne).
The Microflown Acoustic Camera contains the complete system in order to visualize real time or film the particle velocity, sound pressure and sound intensity of a sound field. The PU array will acquire the particle velocity and the sound pressure data of the sound field. The camera is broad banded, real time and with a high dynamic range. The sound field is exactly measured, not calculated, so you have very reliable results.
The direct method, that is simply measuring the sound pressure and particle velocity close to the surface, can be used in diffuse sound fields and with correlated and non-correlated sound sources. So real life measurements in a car or plane, just as source finding on an engine are very well possible. Both the bandwidth (20Hz-20kHz) and the dynamic range (<40dB) are large.
With the Microflown PU-probe, PU-match or the USP it is simple to measure the sound intensity, as it is possible to measure particle velocity and the sound pressure simultaneously. These sensors can operate in (diffuse) sound fields where the traditional probe has problems. Smaller size allows measurements on places that the traditional probe can not reach and allows high resolution measurements on very small objects.
The Microflown based intensity probe is proven to perform much better in closed environments like the inside of a car, train or plane. The ability to measure sound intensity (and surface velocity) in a difficult sound field with a Microflown based intensity probe is the basis of the source path contribution technique.
Compared to planar holography (200Hz-2kHz) and beamforming arrays (2kHz-10kHz) the direct method has a large bandwidth. Bandwidth can be from frequencies far below 20Hz and above 20kHz. The direct method can be used in diffuse sound fields. The PU intensity method is not affected by a diffuse sound field.
A comparison of three methods is made in [1], see Fig. 8.1. It shows the result of a beamforming array (BFA), planar NAH (SONAH) and the direct method (intensity probe). As can be seen, the beamforming array works only at high frequencies, planar NAH works at somewhat lower frequencies but not higher frequencies and the direct method is performing well at all frequencies.
Helicopters interior noise testing
Noise levels recorded in the helicopter cabin are severely affected by the strength and vicinity of noise sources. Jet engines, the gearbox and the rotors can be considered as separated sources - whose spectral content is strongly tonal and rpm dependant - exciting simultaneously the cabin acoustic cavity. Measuring acoustic data on helicopters is a difficult matter because of the multiplicity of source, their high correlation (all sources are dependant on the rotational speed) and the high reflectivity of the acoustics field in cabin. Traditional testing techniques as Acoustic Intensity or Near Field Acoustic Holography fail correctly addressing the problem. The Direct Method proves able to deal with harsh environmental testing conditions while improving measurement accuracy and paves the way for a better understanding on how to reduce noise level in helicopter cabins.
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Fig. above shows two of the typical noise maps obtained with the new testing technique. The extremely valuable feature of the Microflown array is that the acquisition can be carried out in the wide spectra frequency range. There is no need to modify the probe, as it is the case for intensity probe using microphones (spacers dependency form the frequency content of the acquired signal). In the case under analysis only one test run was sufficient to make data all available for processing in the range 0Hz to 25kHz. The standard intensity probe would have required at least 3 different runs corresponding to the 3 different spacers. Furthermore, fee Microflown array works as a wide frequency intensity that allows showing at the same time both low frequency components (e. the main rotor BPF) and the high frequency components such as the turbines.
Panel Noise Contribution Analysis
An array of PU probes can be used to measure the noise in the interior of a car in days in stead of weeks. This tremendous time saving was due to the fact that the Microflown based sound intensity probe can measure inside a car without the need of a thick layer of damping material that is needed for traditional method. And contactless vibration methods at lower frequencies can be done at the same time and with the same equipment.
One Microflown PU array replaces a laser vibrometer or accelerometers at lower frequencies and a p-p sound intensity probe (plus foam) for high frequencies.
In situ reflection coefficient measurements
It is possible to measure the reflection coefficient of an acoustic damping material with the use of a ½” PU probe. The reflection coefficient is determined fast, in situ, on small surfaces and oblique. Until that time it was only possible to measure the normal reflection coefficient of a sample in a tube (a destructive measurement). The oblique values of the reflection coefficient can only be determined in a laboratory when a large sample (10m2) with a very time consuming measurement (4 hours).
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Vibroacoustics Studies inside the passenger car cabin
In cabin noise studies requires tools which can simultaneously measure vibration and Acoustic Noise. Historically, accelerometers were used for the vibration measurements and Intensity probes or other techniques were used for these kinds of studies. The limitations of them were time required to do measurements as the setup time were huge and also repeatability of the test conditions during many setups.
PU sensor measures both velocity (for low frequency Vibration) and Sound Intensity simultaneously at a given point. This reduces the testing time and gets more accurate results.
Non contact vibration measurements
One of the results of the inverse acoustics research was that it became clear that the particle velocity close to a vibrating object coincides with the vibrating of the object itself. The other two methods to determine the vibration of a Structure are: a laser and an accelerometer. The first one is very expensive, large and (therefore) difficult to use. The accelerometer is small and low cost but has to be attached to the vibrating object. This alters the structure under test and it is a lot of installation work. The Microflown is an alternative to these methods. It is a small sensor, a non contact method and it is easy to use.
Squeak and Rattle
Squeak and Rattle are some of the main concerns in the Automobile Industry. The issue with this problem is identifying the right area from which the noise is coming. This requires a tool which can pin point the component area which is causing the squeak or rattle. The unique directional sensing property of the PU sensor and the measurement of Particle velocity help the user to quickly identify the exact location from where the noise is originating.
Highly transient source Identification
Some of the problems in the passenger cars are like identifying the noise sources during door slam. It is a very transient phenomenon and you require tools which can work in reverberation environment. Microflown acoustic camera can be used even in such adverse conditions to identify the locations during the door slam and help engineers understand the areas of noise source better
Exhaust Noise testing
PU mini probes
A cylindrical configuration of PU mini probes allows the measurement of the normal sound intensity leaving an imaginary envelop beyond an engine exhaust. Dimensions and density of the mesh of probes probes can be customized to a certain set up.
USP mini probes
A cilindrical configuration of USP mini probes allows the measurement of the three dimensional sound field around an exhaust, providing directivity and phase information, allowing, amongst others, azimuthal reconstruction.
Dimensions and density of the mesh of probes can be customized.
3D Visualication of Sound Fields
Microflown Technologies offers 3D full bandwidth solutions up to 20kHz at least, solutions that are not susceptible to reflections. With an USP (3D Ultimate Sound Probe) determining the 3D sound intensity field of a sound source has become a relatively straightforward piece of work. This will become an ever increasing tool to (finally) validate existing theories and the output of numerical models.
Production line quality control, engines and gears
The Microflown proved to be an excellent tool to determine the quality of anything that vibrates (like an electric motor). Background noise (that is the main problem with other acoustic sensors) is cancelled by the Microflown for three reasons:
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close at the surface of the motor the background noise is reflected causing the particle velocity to reduce to zero (and the sound pressure to double), |
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the sound that is radiated by the motor is velocity based (and sound pressure is low) and |
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the Microflown measures only in one direction cancelling the other two dimensions of the background noise. All together a 30dB improvement over pressure measurements can be reached. |
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