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Accelerometer mounting

One of the challenges in measuring vibration using accelerometer is how to mount the accelerometer on the surface of the object that is being measured. Choosing the proper mounting can affect both to the measurement results and practicality when we are conducting the measurement.

 

Accelerometer mounting affects the measurement results because it can shift the resonance frequency of the accelerometer. Accelerometers have a significant amplification factor at its resonance frequency. This implies that in conducting measurements using accelerometer, it is important to choose mounting techniques that does not shift the resonance frequency into our frequency of interest.

 

Generally, there are four ways to mount accelerometer which are:

  1. Stud mounting: this technique is done by bolting the accelerometer into the object. This option is often considered as the mounting technique that produces the best measurement result compared to other options. Stud mounting has a high resonance frequency that in most cases a lot higher than our frequency of interest. To increase the performance of stud mounting, coupling fluid such as oil, petroleum jelly or beeswax can be used.

The downside of this technique is that not all object has a possible location to be bolted at the surface. If this is the case, then we will need to modify the surface and might leave a hole on the object.

  1. Adhesive: there are few adhesives that are commonly used to mount accelerometers such as epoxy (usually chosen for permanent mounting), wax, glue, and double-sided tape. Use of adhesive has lower resonance frequency compared to stud mounting, but in majority of cases still high enough that it does not affect the measurement at the frequency of interest. Of course, this depends on the type of adhesive that is being used as well.

Usage of adhesive however, especially for temporary mounting, has its own problem which is it can leave stain on the surface of the object that we are measuring, as well as on the accelerometer itself.

Another option of mounting related with adhesive is to use adhesive mounting pad, which is a pad that can be mounted on the surface that we want to measure using adhesive, and then we can mount the accelerometer on the pad. This will allow us to move one accelerometer to few locations more easily. From practicality perspective, adhesive mounting pad has an advantage if we want to repeat the measurement. Also, by using adhesive mounting pad, we avoid direct contact of adhesive to the accelerometer so that it will not need cleaning.

  1. Magnet: For metal surfaces, one of the options that is easy and does not leave stain is by using magnetic mounting base on the accelerometer so that we can attach the accelerometer to metal. This is the reason magnetic base is one of the best options especially for short-term and temporary measurement on metal.

However, this mounting technique produces lower resonant frequency compared to the other two options that we have discussed above. If the frequency that we want to measure is high enough, say above 1 kHz, this mounting technique might influence the measurement results.

  1. Handheld: In some of the cases, the three options above are not possible to be chosen, and it leaves us with the last option which is holding the accelerometer by hand. In this kind of cases, a probe tip can be used so that we can put pressure by hand on the surface that we are measuring easier.

We will have to pay more attention to the frequency range that we are measuring if this mounting technique is used. Because this option will reduce our frequency range significantly, generally only in the range of 10 – 100 Hz. 

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Asia Noise News Building Accoustics Building Acoustics Environment Home Industrial Noise and Vibration Product News Noise-th Uncategorized Vibration Virtual Data Room

Helmholtz Resonator

Resonate absorbers are the most powerful of low-frequency absorption technologies. Pound for pound and square foot per square foot, resonant absorbers can not be matched for low-frequency absorption. They are sometimes called resonance absorbers. We are speaking about real low-frequency absorption which represents all frequencies below 100 Hz. Resonant absorbers are different than other absorbers. They work best in areas of high room sound pressure not high sound velocity areas like porous absorbers that handle middle and high frequencies.

Vibrations & Sound Pressure
A resonant absorber is a vibrational system that “runs” on sound pressure. As vibrational science will tell us a resonant absorber is a mass vibrating against a spring. The mass is the cabinet and front wall or diaphragm. The spring is the air inside the cavity of the resonant absorber. If you change the vibrating mass and stiffness of the spring, you can control and tune the resonant absorber to the resonant frequency of choice. The internal mass or cabinet depth determines design frequency. The spring or internal air and cavity are used for achieving the rate of absorption above the unit’s designed for resonant frequency. There are three types of resonant absorbers: Helmholtz and Diaphragmatic and Membrane.

Helmholtz / Membrane
A Helm resonator is a box or tube with an opening or slot at its mouth. Air enters the slot which has a calculated width, length, and depth. The slot is attached to a cabinet or cylinder of different widths and depths. A glass coke bottle is a good example of a Helmholtz resonator. It is a resonant absorber or as some would term a resonance absorber. The frequency or resonance is determined by the slot dimensions along with the cabinet or cylinder depth. Helms are frequency specific and narrow frequency band coverage. A membrane absorber works similar to a diaphragmatic. It has a membrane than vibrates in sympathy to sound pressure. This vibrating membrane is attached to a cabinet which has a certain depth and fills material. A diaphragmatic absorber works similar to a membrane with more performance per square foot.

 

Calculate Resonant frequency of Helmholtz Slot Absorber

Resonant Frequency Formula
fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))
fo = resonant frequency
r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec
If the gaps vary say 5mm, 10mm, 15mm, 20mm and the wall is angled as shown below, a broad band low mid resonator is created that still keeps the high frequencies alive.

Remember the cavity behind must be airtight!
By working out the different slat widths and slat gaps you can create a broadband low mid resonator at specific frequencies.

Credit : mh-Audio.nl , acousticfields

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