I have read some research papers [1, 2], where accelerometers and acoustic emission sensors are combined to assess the structural condition. As I understand, the combination is based on the consideration that these two types of sensors work primarily in different frequency ranges. AE sensors are usually for higher frequencies (in ultrasonic range), while accelerometers are typically for vibration tests in a relatively low-frequency range (up to several kHz). I have a question about that:
If we speak piezoelectric sensors for both types of sensors. Since they are both based on the piezoelectric principle with little difference (one measures acceleration and the other measures displacement), their performance should be comparable. Is it possible to compare the sensitivity of both sensors even though they have different units (one with V/g and the other with dB re 1V/bar)? The reason for this question is that I want to compare how the back ground noise affects both sensors differently. Will one of them generally have a higher signal-to-noise ratio under the same noise?
[1] Hyon Wook Ji, Hongcheol Lee, Inju Hwang, Supervised learning-based classification of acoustic emission and vibration signal for identifying condition change of district heating system, Measurement, Volume 220, 2023.
[2] Eline Vandecruys, Menno van de Velde, Edwin Reynders, Geert Lombaert, Els Verstrynge, Experimental study on acoustic emission sensing and vibration monitoring of corroding reinforced concrete beams, Engineering Structures, Volume 293, 2023.
If both sensors measure the same physical quantity (e.g. acceleration or velocity) it should be no problem to compare.
From the units you mention I deduce that the AE sensor in use detects a pressure in air (dB re 1V/bar). Using these kind of AE sensors makes it more difficult, because radiation from the vibrating structure to the air seems to be involved.
Maybe it is better to use an AE sensor that detects AEs with velocity (dB rel V/(m/s)) or acceleration, then you should be able to bring it to same physical quantity (by either integrating or differentiating acceleration or velocity respectively).
(Just one example by internet research, good up to 400 kHz measuring velocity: https://aendt.com/acoustic/products/Sensors/General_Purpose_Sensors/108.html)
Also displacement can be transformed into velocity or acceleration but each integration or differentiation step (in calculation) will also have an impact on SNR.
Stefan Zimmermann thank you for your idea. The AE sensors used are manufactured by Vallen and are typically applied to materials like steel or concrete to detect wave motion in solids. The following is a link to the product document.
https://www.vallen.de/de/sensoren/niederfrequenz-sensoren-30-100-khz/vs30-v-2/
As one can see, the sensitivity is indeed given in dB re 1V/μbar. So that is to say, that the measured physical quantity is pressure, not velocity or acceleration, which are measured by accelerometers. A direct comparison between these AE sensors and accelerometers may be difficult in this case, if I understand you correctly.
Thank you again for the clarification.
According to the datasheet information of Vallen, the sensors measure vibration in structures (not pressure in air). The calibration works with an ultrasonic pulse or sinewave and is given in dB rel V/µbar. Looking more into the details it seems they give the sensitivity in terms of the mechanical stress sigma=F/A (force per area Unit N/m²) of the piezo element, same unit as 'sound pressure', so that's a little confusing in the context of 'acoustic' emission. My recommendation would be to contact the vendor and ask, how this translates into a vibration related quantity like acceleration. In principle it should work somehow like this: If you know the mass of the sensor (or the mass moving on or coupled to the piezo transducer element) and the contact area between mass and the piezo element, then using F = m*a and sigma=F/A should give you the acceleration as a = F/m or a=sigma * A /m
But maybe that is the big difference between AE sensors and acceleration sensors. Acceleration sensors have a defined mass mounted on the piezo element with a defined contact surface, so their output voltage "translates" into acceleration, whereas acoustic emission sensors do not have a coupled mass on them, thus their output voltage is equivalent to the mechanical stress.
Stefan Zimmermann The design of AE sensors is indeed related to this discussion. According to my understanding (see e.g. Chapter 9 "Acoustic sensors" in the book "Sensors for Mechatronics" by Regtien), the piezoelectric material in an AE sensor is normally coupled with a matching layer, which is then connected to the measuring surface. So theoretically the working principle of F = m*a, and sigma = F/A to correlate pressure and acceleration would work if one knows the mass and contact area. I will verify this with the manufacturer.
Thanks a lot!
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