FAQ

FAQ | General

Q1: What are the frequency limits of the Microflown?
Q2: Is the sensor susceptible to shock and environment?
Q3: How can the probes be calibrated? Per which period it is needed to recalibrate the probe?
Q4: What is the voltage output of the signal conditioner MFSC?
Q5: What is the self-noise of the sensor?
Q6: Can the Microflown sensors be used with other frontends?
Q7: Can the Microflown sensors be used with other acquisition suppliers that are available at the NVH market?
Q8: What is the maximal measurement range of Microflown probes?
Q9: What is the package gain of the PU regular and PU mini?
Q10: What is the temperature limit of the sensor?
Q11: What is the amplification difference between LOW-HIGH gain on the signal conditioner MFSC?
Q12: At what distance the sound Intensity and velocity can be measured for near field sound source localization?
Q13: What is the frequency range valid to measure sound intensity,  sound pressure and particle velocity with Microflown sensors?
Q14: What is the advantage to measure velocity or sound intensity for sound source localization?
Q15: What calculations are needed to display velocity or sound intensity?
Q16: What is the minimal distance to a reflective surface to measure correctly sound intensity?
Q17: How can be checked if the measured sound intensity is correct?
Q18: What should be the correct ratio between reactive/active sound intensity?
Q19: When are multiple probes required?

 

Q1: What are the frequency limits of the Microflown?  

For general acoustic applications the limit is 20-10KHz. For the frequency range 10-20KHz, special calibration can be requested. In some specific application the velocity sensor has shown capability to measure in a frequency range of 0.1Hz up to 10KHz. 



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Q2: Is the sensor susceptible to shock and environment?  

When the Microflown is used with the protection metal mesh cap is not more fragile than a normal professional microphone. The sensitivity variation for the velocity sensor can be expressed:

Temperature limit: 200°C
Sensitivity variation due to temperature: <0.02dB/K
Sensitivity variation due to humidity: (20-90%)0.2dB

Sensitivity variation due to pressure changes:

(1-0.82 bar) 0.5 dB

For the pressure sensor installed in PU and USP probes:

Temperature limit: 63°C
Sensitivity variation due to temperature: <0.02dB/K
Sensitivity variation due to humidity: (20-90%) 3.5dB 0.06dB/%RH
Sensitivity variation due to pressure changes: (1-0.82 bar) 0.5 dB


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Q3: How can the probes be calibrated? Per which period it is needed to recalibrate the probe? 

The calibration is really different from a standard microphone. We developed our own Piston on a Sphere calibrator to calibrate the Microflown. The recalibration is suggested every 2 years.



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Q4: What is the voltage output of the signal conditioner MFSC?   

The AC voltage output of the sound pressure is +/- 300mV. The particle velocity output is +/-900mV. 



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Q5: What is the self-noise of the sensor?   

The maximum self-noise of the sound pressure element of the PU regular and PU mini probe sensors is 10dB at 100Hz. For the particle velocity sensor of the PU regular and PU mini probes the maximum  is 10dB at 15kHz. For the PU match, USP and Scanning probe sensors it is 20dB at 15kHz.



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Q6: Can the Microflown sensors be used with other frontends?   
Yes, the sensor can be connected to any frontend available on the market with BNC Voltage inputs. 


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Q7: Can the Microflown sensors be used with other acquisition suppliers that are available at the NVH market?  

At the moment the Microflown calibration correction curve is included into the following software packages: Mueller-BBM PAK, LMS TestLab, Head Acoustics Artemis and Oros. For more info click here.



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Q8: What is the maximal measurement range of Microflown probes?   

The sound pressure microphone of PU regular, PU mini, PU match and USP probes has a limit at 110dB SPL. The particle velocity Microflown of the PU regular and PU mini has a limit of 125dB PVL. The limit of the velocity element (unpackaged) of the Scanning probes, USP probes and PU match is 135dB PVL. The velocity element of the high dB probe has a limit of 150 or 170dB PVL depending on the probe type.



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Q9: What is the package gain of the PU regular and PU mini?   

It is the increased sensitivity due to the mounting of the sensor in between a small opened cavity. Because of this reason the velocity sensor of PU regular and PU mini probes are 10dB more sensitive than the unpacked probes, but also have 10dB lower maximum level. Although it provides package gain the ½ inch diameter of the PU regular and PU mini becomes an obstacle at higher frequencies.



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Q10: What is the temperature limit of the sensor?   

For the Scanning probes (particle velocity only) the temprature limit is 200°C. The PU regular, PU mini, PU match and USP probes (sound pressure and particle velocity element) the limit due to the membrane sound pressure microphone is set to 63°C. In some cases if there is some spacing between the sensor and the hot surface, the temperature at the probe position can be considerably lower than the surface itself.



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Q11: What is the amplification difference between LOW-HIGH gain on the signal conditioner MFSC?   

There is 40dB amplification in “high gain” mode compared to the “low gain” condition. Depending on the noise level of the measurement the high or low gain should be adjusted.



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Q12: At what distance the sound Intensity and velocity can be measured for near field sound source localization?  

Generally speaking, intensity should be measured at 5-10 cm from the radiating surface, to avoid reactivity errors. For sound source location the particle velocity is a better indicator. Measuring 1-5 cm away of the radiating structure provides with the best results as it enhances the Signal to Noise ratio (SNR)



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Q13: What is the frequency range valid to measure sound intensity,  sound pressure and particle velocity with Microflown sensors?   

This depends on the distance from the surface and the measurement method. As a rule of thumb and considering only the sensor limitations:

Particle velocity: 0.1-10.000 Hz
Intensity: 400-10.000 Hz
Pressure: 20-10.000 Hz


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Q14: What is the advantage to measure velocity or sound intensity for sound source localization?    

The velocity in the near field is not affected by background noise and reflections, and as it is a vectorial magnitude it gives and indication of the sound source location. The sound intensity resembles the information of the amount of energy coming from a certain direction. PU intensity probes can measure in environments with high pressure-intensity indexes (due to background noise). From the sound intensity the sound power can be calculated, which (for closed surfaces) is not dependent on the measurement distance.



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Q15: What calculations are needed to display velocity or sound intensity?   

The velocity is directly measured. To display velocity the time signals or spectra can be shown. The intensity can be calculated from the real part of the cross-spectrum between pressure and velocity.



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Q16: What is the minimal distance to a reflective surface to measure correctly sound intensity?  

Depends on distance from the measurement surface, in the near field (2-3cm from the surface) the intensity is not affected by a reflective plate at 50cm distance. 



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Q17: How can be checked if the measured sound intensity is correct?

The PU probes are affected by the ratio between reactive/active intensity (J/I index). For the calibration performed with the POS system (standard Microflown calibration method) this index should lower than 5dB (which equals to a 72deg phase between pressure and velocity). The reactive intensity is the imaginary part of the cross-spectrum between pressure and velocity.



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Q18: What should be the correct ratio between reactive/active sound intensity?   

It should be lower than 5dB. At lower frequency in the near field measurement the reactivity is generally too high below 500Hz (depending on the sound source and environment). Sometimes it is possible to reduce this reactivity by increasing the distance from the measurement surface. For sound source localization you can use the velocity instead of the intensity at low frequency.



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Q19: When are multiple probes required?   

In non-stationary conditions or in case of transient noise multiple probes are required. The phase information is kept between all measurements points and these results can be looked back in the time domain. 



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