Have you ever walked in a plant and heard a sound from pumps or motors to be alternatively soft and loud? If you have, you experienced a phenomenon, which is called “beating”. The same phenomenon occurs in vibration. If two or more machines run close to each other, beating frequency is produced.
How does this happen? Let’s assume there are two machines, Machine A and Machine B. A measurement is taken on one of the bearings of Machine A. Machine B is located close to Machine A and running at the same time. Machine A and Machine B have different frequencies, as shown in top Figure. Since those two machines are located close to each other, the probe captures the vibration from both machines. What you see is beating frequency, shown in bottom Figure. During times when both waveform add constructively, the vibration increases (around 6.5s, 13s, etc); during times when both waveform add destructively, the vibration decreases (around 3,25s, 9.75s). In mathematical form, beating frequency can be express as,
fbeating = | f1 – f2 |
It is very important to be aware of this phenomenon since beating frequency is not the frequency that you are interested in. One of the methods to avoid beat frequency is to make sure there are no other machines running close to the machine that you want to measure.
