Octave Band

) plot to represent and plot a frequency response in frequency bands. An octave band plot corresponds to plotting the average or integrated value of, for example, the squared pressure over a given frequency band defined by the center frequency or midfrequency and the bandwidth. What the plot shows is a “white noise transfer function” of the system; that is, it assumes that the input is of the same nature as the output. The data input to an octave band plot is a frequency-domain solution; for example, it can be the acoustic pressure resulting from a Frequency Domain study or a parametric frequency sweep. To add this plot type, right-click a node and choose from the submenu. Right-click an node to add a Graph Marker subnode if desired.


	Go to Common Results Node Settings for links to information about these sections: Data and Title.


	This plot type is available with the Acoustics Module, MEMS Module, and Structural Mechanics Module.


	For examples that use an Octave Band plot, see Absorptive Muffler: Application Library path Acoustics_Module/Automotive/absorptive_muffler and The Brüel & Kjær 4134 Condenser Microphone: Application Library path Acoustics_Module/Electroacoustic_Transducers/bk_4134_microphone.

Selection

From the , select the entity level (default: ) for the evaluation of the octave band plot. Choose for a global evaluation. If you select line, boundary, or domain, the quantity will be averaged. You can use those entity levels to plot the response as the average over the outlet of a system, for example. Then, except for , use the selection tools to select geometric entities on that level for the evaluation.

y-Axis Data

In the field, enter an expression for the input to the octave band plot. Press Ctrl+space to select predefined expressions based on the physics of the model. If you want to add a description of the expression for the plot title, select the check box and enter a description in the text field below.

If available, for information about the settings in the list, see Expression Evaluated For.

From the list, choose (the default) for frequency-domain expressions, or choose for time-domain expressions. For , the (see below) can only be and . The spectrum of the time-domain signal is calculated through FFT, and those values are used. The freq variable is defined to be the corresponding frequencies to be used in weighting.

From the list, choose one of the following types:

•

(the default), to compute the octave plot treating the expression as an pressure prms. The input is the value p (it is generally a complex-valued variable). It is in turn used to calculate the rms pressure prms, which defines the level L:

In the field, enter a value of expression for a pressure reference pref. Click the (

) to pick a pressure reference from a list of predefined expressions.

•

(the default), to compute the octave plot treating the expression as an amplitude A. The input is the value A. It is in turn used to calculate the level L as L = A/Aref. In the field, enter a value of expression for an amplitude reference Aref. Click the (

) to pick an amplitude reference from a list of predefined expressions.

•

, to compute the octave plot treating the expression as a power P, which defines the level L:

In the field, enter a value of expression for a power reference Pref. Click (

) to pick a power reference from a list of predefined expressions.

•

, to compute the octave plot treating the expression as a transfer function H (generally, a complex-valued variable), which defines the level L:

In the field, enter a value of expression for a level reference Lref. Click (

) to pick a level reference from a list of predefined expressions.

Underneath those settings, the mathematical formula used for each expression type is displayed.

Outside the frequency range, the signal is assumed to be zero and no extrapolation is done.

Plot

You can choose one of the following quantities of the octave band from the list:

•

, to plot a continuous response

•

(the default), to plot the response using bands.

•

, to plot a bar plot with band averages.

If the is set to , you can instead choose one of the following quantities of the octave band from the list:

•

, to plot a continuous response spectrum

•

(the default), to plot the response using bands.

•

, to plot a bar plot with band averages.

For the two last options (regardless of the expression type), using bands or band average, you can also specify a band type from the list:

•

, to plot the response using octave bands.

•

, to plot the response using 1/3 octave bands.

•

, to plot the response using 1/6 octave bands.

Also, the check box is selected by default to use an integral that is evaluated as an average only based on data points inside the different octave bands.

You can use some predefined weighting or a user-defined weighting of the frequency data. The frequency weighting is used in acoustics to shape the response to match the characteristics of the human ear.


	The predefined weightings are defined in IEC 61672-1. See IEC 61672-1 Electroacoustics - Sound level meters - Part 1: Specifications for details.

From the list, select:

•

(the default), to use a zero weighting; that is, a flat weighting.

•

, to use a weighting that mimics the loudness perceived by the human ear.

•

, to use a C-weighting, which is an alternative standardized weighting that is in use within the acoustics community.

•

, to enter a user-defined value or expression for the weighting in the field. The expression defines the gain as a function of the frequency. The gain given in dB is then given as 20·log10(expression). Use the frequency variable freq for user-defined expressions.

The following COMSOL Multiphysics plot shows the different weighting types as gain (dB) versus frequency (Hz):

Figure 21-8: The plot shows Z-weighting (blue), A-weighting (light green), C-weighting (dark green), and a user-defined weighting (red). The 1/3 octave bands (light green) also show A-weighting.

Coloring and Style

When the plot style is set to continuous, you can specify settings for the line style and line markers. When the plot style is to show octave bands, select the type of visualization from the list:

•

Select (the default) to display filled bars. Then select the color for the bars from the list. Choose to select a custom color from the color palette that appears.

•

Select to display outlines of the bars. You can then specify settings for the line style and line markers for the outlines.

•

Select to display a graph connecting the centerpoints of the bands. You can then specify settings for the line style and line markers for the graph.

For the and options, see Common Results Node Settings.

Legends

Select the check box to display the plotted expressions to the right of the plot. In plots where each line represents a certain time value, eigenvalue, or parameter value, these values are also displayed.

When is selected from the list (the default), select or clear the , , , and check boxes to control what to include in the automatic legends (by default it includes the solution and description only). If is selected from the list, enter your own legend text into the table. If is selected, you can use the eval function to create an evaluated legend text in the field that include evaluated global expressions such as global parameters used in sweeps. For the numerical evaluation, you can control the precision in the field (default: 3).