Theory

A rotating machine radiates tones at the multiples of its shaft frequency: the blade-passing frequency of a compressor impeller with $Z_m$ main and $Z_s$ splitter blades is the order $Z_m + Z_s$ of the shaft speed, the main blades alone give the order $Z_m$ , and every tone has its harmonics. At a speed $n$ the order $k$ lies at $f = k\,n/60$ . The order analysis reads the transmission loss (or the insertion loss with the Results settings) of the solved sweep at these frequencies over a speed range: where an order's line crosses a dip of the curve, that tone passes the silencer with little attenuation.

The level between two solved frequencies is interpolated linearly; outside the solved range, and for the insertion loss above the outlets' first cut-on, there is no value. A dip is a speed range where the level is below the threshold. The resolution check looks at the dips of the curve itself: its width 3 dB above the minimum against the frequency step. With fewer than three solved points across a dip its depth is uncertain; "Refine frequency step and re-run" adds a band of finer frequencies around it to the sweep (the base grid stays) and re-runs the acoustics without the flow.

Equations

f_k(n) = \frac{k\,n}{60}, \qquad k \in \{1,\ Z_m,\ Z_m + Z_s,\ 2(Z_m + Z_s),\ \dots\}

(1)

where	meaning	unit
$f_k$	Frequency of order $k$ at speed $n$	Hz
$k$	Order (a number or an expression of $Z_m$ , $Z_s$ )	-
$n$	Shaft speed	1/min
$Z_m, Z_s$	Main and splitter blade counts	-

Default and rule

Default: 20 000
Rule: 20 000.
Source: A definition; the worked examples of the validation (a fuel-cell compressor and a regenerative blower) take their blade counts and speeds from the published cases.

Valid range 0 - 300 000

Post-processing

Changing the orders, the speeds or the threshold needs no solve; only the refinement re-runs the acoustics.

Theory

Equations

Default and rule

Valid range 0 - 300 000

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