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Have you seen this before? Prominence Ratio (PR) inconsistency

Experimenter
Experimenter

When calculating PR for a fan blade pass tone.  The calculated value from the legend is higher than from the cursor (illustrative data used):

PR_no_match.png

 

Why?

 

Calculated content settings:

 

calculated_content.png

 

 

Cursor settings:

 

pr_cursor.png

 

 

 

 

 

 

2 REPLIES

Re: Have you seen this before? Prominence Ratio (PR) inconsistency

Siemens Legend Siemens Legend
Siemens Legend

The value for Prominence Ratio (PR) in the calculated content of the legend is the maximum PR value across the entire spectrum.  This can be different than the PR value calculated around the cursor position.

 

The Prominence Ratio calculation is based on frequency bands, so even if the cursor is placed on the largest peak in the spectrum, the maximum PR of the spectrum might not correspond to the PR value shown by the cursor.

 

To understand why the values can be different, it is helpful to understand how Prominence Ratio is calculated. The Prominence Ratio calculation uses levels over three different frequency bands as shown in Figure 1.

PR_Basic.pngFigure 1: Prominence ratio is calculated by summing the frequency bands A and C, and comparing to the level of Band B.

When a cursor is placed on the spectrum, the bands are centered about it. The PR value in the calculated content of legend evaluates the PR value associated with every frequency in the spectrum and keeps the maximum.  Think of it as moving the cursor to each frequency and calculating the PR.

 

The Prominence Ratio is the difference between the level of band B and the sum of A and C. Prominence Ratio can be made higher by either increasing B, or decreasing A and C.

 

Consider the spectrum shown in Figure 2.  The value of Prominence Ratio is higher when the cursor is not aligned with the largest peak in the spectrum.

 

PR_left_right.pngFigure 2: Left – Prominence Ratio value with cursor on highest peak (left graph) is lower than value with cursor off of the peak (right graph).

The PR value of the left graph is lower than the right graph.  The calculated content of the legend would show the value on the right graph, while the cursor shows the value on the left graph.

 

The bands used in the calculation are always center around the cursor, as shown in the animation below. As seen below, the level in Band B remains constant while the cursor is moved, as the main tone is contained in it.  The masking bands A and C can change. 

pr_no_match6.gif

 

Figure 2 illustrates two reasons why the masking noise levels can become lower, and thus cause the Prominence Ratio higher when the cursor is not on the peak:

 

  • Narrowing of Band - The bands evaluated in the PR calculation become narrower at lower frequencies. For a constant level of sound, a narrower band will have a lower level.  The Masking Band A in Figure 2 becomes has a lower amplitude as the frequency is decreased. With a lower masking level, the Prominence Ratio becomes higher.

 

  • Background Sounds – Masking Band C becomes lower because the band no longer contains some high level content around 1900 Hz as the band changes width. This causes Band C to be lower, and thus increases the Prominence Ratio.

 

The bands used in the PR calculation vary in width in a similar manner to the Critical bands of the human ear.  They become wider at higher frequencies.  Unlike critical bands, which have fixed frequencies, these bands can be evaluated at any frequency.

 

For more information about Prominence Ratio, see TTNR and PR knowledge base article.

 

 

Re: Have you seen this before? Prominence Ratio (PR) inconsistency

Experimenter
Experimenter

Thank you.  Indeed, moving the cursor off of the peak did get the calculated content PR and cursor PR values to match

match.png