Using Order Tracking to Identify Speed-Related Noise in Variable-Speed Fan Tests

When measuring a fan or motor in an acoustics laboratory, a spectrum from a constant-speed interval makes it easy to inspect peaks. During run-up or run-down, however, the same rotation-related component may keep moving in frequency. Reducing the entire run to a single averaged spectrum can obscure its relationship to rotational speed. In this situation, it is useful to examine both a frequency-versus-speed map and the order-analysis results.

First identify the reference shaft

An order expresses a frequency component as a multiple of the rotational frequency of a reference shaft. For a reference shaft speed of n revolutions per minute, the rotational frequency is n/60 Hz, and the kth order corresponds to k × n/60 Hz. Siemens’ technical guidance specifically notes that changing the reference shaft changes the interpretation of orders, and that orders need not be integers.[1]

Consider a hypothetical fan with seven equally spaced blades. At 3000 revolutions per minute, the shaft frequency is 50 Hz and the blade-passing frequency is 350 Hz. At 4200 revolutions per minute, these become 70 Hz and 490 Hz, respectively. Relative to the fan shaft, blade passing corresponds to the seventh order in both cases. This example illustrates only the kinematic relationship; it does not imply that the seventh order must be the strongest component in the measured noise.

Why changing speed can broaden spectral peaks

If speed continues to change within a fixed-duration analysis block, the frequency of the related component also changes, potentially spreading its energy across a range of frequencies. Siemens’ comparison of fixed and synchronous sampling explains this broadening. Order analysis that organizes data by rotational angle can reduce this type of speed-induced broadening.[2]

When a broad peak appears, first check how much the speed changes within the analysis block before considering whether a new source of broadband noise is present. Relabeling the horizontal axis from Hz to “order” does not by itself perform angle-synchronous processing. Check the calculation method the software actually uses.

Check the speed channel too

Record the acoustic signal and reliable speed information synchronously, and document the reference shaft, pulses per revolution, detection settings, and processing method. If the speed comes from an external control system, also confirm that its update rate and time alignment are adequate for the analysis.

When checking the speed trace, examine the raw pulses as well. Missing pulses can produce incorrect instantaneous speed values, as illustrated in Siemens’ guidance on RPM measurement.[3] Do not simply smooth away spikes or sudden drops. First establish whether they reflect real changes in operation or a detection problem. Keep a processing record for any corrected data.

Use the plots to guide further verification

Start with a frequency-versus-speed map to inspect the overall changes, then examine the amplitude of the target order as a function of speed. When comparing two alternatives, align the operating conditions, analysis intervals, amplitude definitions, and extraction bandwidths. An extraction band that is too narrow may miss energy that has already spread; one that is too wide may include neighboring components.[2]

A bright line that moves with speed can provide a clue to rotation-related noise, but the line alone cannot establish that a particular part is damaged. To locate the cause, also consider the structure, other measurement locations, or controlled comparison tests that change a single condition. Features that remain at an approximately fixed frequency are also worth retaining and should not be removed from the report merely to emphasize orders.

Order tracking helps clarify which rotation-related components change and around which speeds. Documenting the reference shaft, the quality of the speed data, and the analysis settings allows the next round of measurements to investigate the same question consistently.

References

• [\[1\] Siemens, What's an Order?](https://community.sw.siemens.com/articles/en_US/Knowledge/what-s-an-order)
• [\[2\] Siemens, Order Tracking: Fixed Sampling versus Synchronous Sampling](https://community.sw.siemens.com/articles/en_US/Knowledge/order-tracking-fixed-sampling-versus-synchronous-sampling)
• [\[3\] Siemens, Measuring RPM: Missing Pulses](https://community.sw.siemens.com/articles/en_US/Knowledge/Measuring-RPM-Missing-Pulses)