Generally, engine noise is measured by installing sound measuring devices in a standardized test environment, and then the engine noise level is obtained through certain corrections and calculations.
This process involves specific requirements regarding the test environment, equipment, calculation methods and adjustments. It is crucial that engine engineers understand these aspects rather than simply using a microphone for simple measurements.
Even so, what engine engineers measure is still engine noise, not bearing noise. We know that engine noise includes electromagnetic noise and mechanical noise, with a significant part of the latter being the noise generated by the cooling fan. These noises largely “mask” the noise from the bearings, acting as background interference.
Bearing standards also include test methods for bearing noise. However, bearing detection equipment generally used in engine factories involves installing the bearing in a test head, applying a load, and then converting the signal to numerical values or transmitting it to a high- speaker for testing.
In reality, this is a vibration tester for engine bearings, not a noise tester. Even though the tester hears the sound coming from the speaker, that sound is still a conversion of the vibration signal, not direct noise.
However, when the bearing is installed in the engine and the engine rotates, the load conditions that the bearing experiences are completely different from those on the test bench. This leads to different noise performance of the engine bearing.
Many manufacturers have encountered situations where bearings pass “test bench” inspection but still emit noise when installed in the engine. This confusion arises from the difference between test bench conditions and actual engine bearing conditions, as well as the difference between “vibration” and “noise” measurements.
Specialized bearing noise tests, such as engine noise tests, have their requirements for the environment, test equipment, test methods and calculation adjustments. It is a challenge for normal engine factories to do this.
For bearings, all other engine noises are interference from the test conditions or background noise. If the background noise of engine bearing noise can be corrected by calculation, then the engine structure itself poses a considerable bearing noise challenge. For example, the engine bearing end cover creates a certain level of noise obstruction.
Does this mean that on-site engine bearing noise cannot be assessed or assessed? Especially from a maintenance perspective, how should engine bearing noise be measured and listened to?
In fact, there are methods available.
Firstly, due to the high environmental requirements for noise measurement and its susceptibility to interference, the focus in practical failure analysis or equipment maintenance often shifts from noise to vibration. In routine maintenance of engine bearings, if engineers are concerned about possible bearing failures, vibration detection is a more direct and effective method than so-called noise measurement.
However, if engine manufacturers are more concerned about the overall “hearing feel” of the engine, dedicated noise testing will still be necessary. Regarding the measurement of auditory noise, there is an issue related to equal volume contours, which will be discussed in detail in a subsequent article. Let's first talk about the method of measuring the “noise” of potential failures.
Regarding on-site measurement of “noise” (actually vibration) from possible failures:
a) The most reliable method for vibration testing today is to use vibration sensors and corresponding analysis equipment. There are many manufacturers of this equipment.
Generally speaking, both time domain and frequency domain methods can be used for detailed analysis to identify possible causes of failure. (This involves more specialized knowledge of condition monitoring, which will be expanded on in subsequent articles, so it will be forgotten here.)
b) However, there are many on-site monitoring requirements that are not as detailed. For example, worker patrols. If there is no vibration monitoring equipment, how should it be handled?
In fact, the most common inspection method is the “stethoscope”. Even with a stethoscope, if the engineer does not understand its principle of use, the result often heard may not be the real one. As mentioned before, even with a “stethoscope”, we are still doing “vibration inspection” instead of “noise inspection”.
Therefore, when listening, one end of the stethoscope should only be pressed against the inspection part, and the other end should only be pressed against the outer ear bone. This way, there is a “rigid” connection from the inspection part to the ear bone, rather than an air connection. This eliminates interference from “noise” from other components.
What we’re really hearing here is “vibration,” not “noise.” One of the main reasons for doing this is to eliminate airborne interference noise.
Although this method is commonly used, the detail is whether to “press hard” or not. Just through this difference, it can often be seen on site whether an engineer understands the principle of this measurement method. Experienced engineers can often distinguish some abnormalities within the bearings from the “vibration” transmitted by the stethoscope.
Once you have mastered the correct method of “listening to the noise” on site, all subsequent engine bearing noise and vibration analyzes will become more targeted and timely. Many on-site engineers discuss various topics for a long time, only to find that in the end they are not talking about the same thing. Therefore, to avoid this situation, the small step of “listening” is actually very helpful.
