The Octave Band Plot for Acoustics Simulation

The Octave Band plot provides you with an easy and flexible way to represent simulation results that are essential when modeling acoustics applications, including frequency responses, transfer functions, sensitivity curves, transmission losses, and insertion losses. Let’s learn a bit more about the Octave Band plot while highlighting its various options and settings.

Editor’s note: The original version of this post was published on January 21, 2016. It has since been updated to reflect new features and functionality available in the Acoustics Module.

The Importance of Octaves

When discussing an octave, we are referring to a frequency band in which the upper frequency is twice the lower frequency. Applying this concept results in bands of equal width on a logarithmic frequency axis.

Representing an acoustic response by binning the signal energy into octaves, or fractional octaves, is very common for acoustic and audio engineers. This visualization technique is closely related to specifications in standards and, thus, the way measurement equipment works (e.g., sound level meters). Physiologically, such representation is motivated by the fact that the human ear itself filters (via auditory filters) and perceives sound on a logarithmic frequency scale. In the same way, the human ear also has a logarithmic sensitivity to sound magnitude, hence the use of a decibel (dB) scale for sound pressure levels.

As we will describe next, the Octave Band plot includes more functionality than simply plotting acoustic responses in octave bands.

The Octave Band Plot, a Simple and Versatile Acoustics-Specific Plot

The Octave Band plot, available in the Acoustics Module add-on to COMSOL Multiphysics^®, includes built-in features specific to acoustics simulations that can be helpful for representing and analyzing frequency-domain data. The output of the plot is automatically given on the dB scale, with several formatting options. With regards to the data fed to the plot, you can use any global quantity, or readily pick it up at a point or as an average over a line, surface, or volume.

For the graphs, you have the option to format the results so that they render in either bands (octaves, 1/3-octaves, or 1/6-octaves) or as a continuous curve. The band format can represent the band power or band-average power spectral density (PSD), while the continuous curve represents PSD data. You can also easily add weighting to the response curve, with the choice of a Z-, A-, C-, or user-defined weighting. For the input to the plot, you can modify it to represent amplitude (e.g., the absolute value of a pressure), power (e.g., the power of incoming or outgoing modes at ports or an intensity integrated over a surface), or a general transfer function.

All of these options help to simplify postprocessing considerably. Furthermore, this plot type makes it much easier to compare results with measured data, which is often given in octave or 1/3-octave bands.

The screenshot below shows the user interface (UI) for the Octave Band plot. In the next section, we will explain its different options and settings in detail.

The UI for the Octave Band plot.

An Overview of the Different Options and Settings

Geometric Entity Level

The Geometric entity level drop-down menu enables you to select how the input data is picked up from the model. The data can be directly taken from a point in the model if, perhaps, a sensitivity is measured at a specific location. By selecting Edge, Boundary, or Domain, the input is automatically averaged. When averaging, the pressure or power is averaged before transformation to the dB scale. Such an approach is useful, for example, when calculating the response in an ear simulator. Here, the average pressure is easily picked up on the measurement microphone surface. Thus, it is not necessary to set up integration or average operators nor perform a surface average in an Evaluation Group.

The Geometric entity level selection can also be used to evaluate global quantities. You could, for example, evaluate the pressure in a point outside of the computational domain using the Exterior Field Calculation feature operator.

Expression Type

With the Expression type selection (in the y-Axis Data section), you can decide how the input data in the Octave Band plot is interpreted. There are three available options: Amplitude, Power, and Transfer function.

The default option is Amplitude. When this is selected, the input in the Expression field is treated as a complex amplitude, p, which is most often the pressure in acoustics applications. The entered value is then used to evaluate the level as

where the RMS pressure is calculated as p_\textrm{rms}^2 = 0.5 |p|^2. The reference pressure, p_\textrm{ref}, is entered into the Amplitude reference field and is assumed to be a RMS quantity. In most acoustics interfaces, the default value is phys.pref_SPL, which is defined in the sound-pressure-level settings at the physics interface level. The default value is typically 20 \mu Pa. As an example, when defining a transfer function, the reference amplitude could be set to the RMS amplitude, 1/\sqrt{2}\:\textrm{Pa}, of an incident plane wave that has a peak amplitude of 1\:\textrm{Pa}, and the input in the Expression field could be the average pressure measured over a surface.

The next option is Power. Here, the input to the plot entered into the Expression field is assumed to be a power, P. The value could be calculated, for example, from an integral of the acoustic intensity over a surface. The level is computed as

where the reference power, P_\text{ref}, is entered into the Power reference field. The default is phys.Pref_SWL, which is the reference sound power level of 10^{-12} \: \textrm{W}.

Transfer function is the final option. Any user-defined transfer function, H, and a reference level, L_\textrm{ref}, can be entered in this case. They are used to define the level

The input data is used to integrate the power within bands when the results are plotted as octave, 1/3-octave or 1/6-octave bands, depending on the style that you choose.

Plot Quantity and Weighting

Under the Plot selection, there are two drop-down menus that offer you different choices for formatting your data. These are the Quantity and the Weighting drop-down menus.

Using the Quantity drop-down menu, you can plot the frequency-domain data as either Continuous power spectral density, Band power, or Band average power spectral density. The choice of band power versus band-average PSD determines how the band power summation and averaging is performed. When either of the band options is selected, you can choose the Band type as Octave, 1/3 Octave, or 1/6 Octave. Finally, there is an option, Use in-band data only, that can be selected (the default) or deselected. When this option is selected, only data points lying inside a given band are used for the integration and interpolation of data. This option typically has an influence on results for data with low resolution (i.e., only a few points in each band).

Three different plot styles are highlighted in the figure below. The bars outlined in red indicate the 1/3-octave band power data, the bars outlined in green indicate the octave band power data, and the solid blue bars represent the 1/3-octave band-average power spectral density (PSD) data. (The formatting of the bars can be changed via the Coloring and Style section.) The plot is a modified version of the one from our Loudspeaker Driver tutorial model.

A graph depicting the different plot styles.

Lastly, using the Weighting selection, you can identify the weighting that should be applied to the data. The choices include:

• Z-weighted (flat): Zero weighting is applied (the default option).
• A-weighted: The IEC 61672-1 standard A-weighting is applied to the data. It is used to account for the perceived loudness of the human ear.
• C-weighted: The IEC 61672-1 standard C-weighting is applied to the data. This option can also account for the perceived loudness of the human ear, usually for very loud levels of noise.
• Expression: A user-defined value or expression is used for the weighting. In the Expression field, an expression that defines the gain as a function of the frequency, freq, is entered. The gain provided in dB is then given as 20·log10(expression).

The two curves in the figure below represent the A- and C- weights applied to a flat, 0-dB response. A user-defined linear weighting is given as 10^{-3}\cdot f and is represented in 1/3-octave bands (shown in red).

A plot showing the different weighting options.

Using an Octave Band Plot: An Absorptive Muffler Example

The Absorptive Muffler tutorial model (located in the Acoustics Module > Automotive folder within the Application Libraries) uses the Octave Band plot to depict the transmission loss of a muffler system. In this version of the model, the Transfer function option is selected for the Expression type. The input to the plot is the ratio of the total incident power to the total transmitted power. These elements are calculated by using the built-in variables for the power of incident and outgoing modes at the Port conditions.

For pure plane-wave propagation (below about 2500 Hz), there is another way to plot the transmission loss without using the port power variables. To do so, you set the Geometric entity level to Boundary and select the outlet boundary number 28. In the Expression field, enter p_in (the amplitude of the incident plane wave), and in the Amplitude reference field, enter sqrt(0.5)*acpr.p_t. Remember, it is assumed that the reference is a RMS value, hence, the sqrt(0.5) factor is needed (as seen in the screenshot below). Choosing the Continuous power spectral density plot style and clicking on the Plot button gives the results shown below for the case of a muffler with the liner.

Plot settings for the Octave Band plot for the absorptive muffler.

Transmission loss comparison plot for the absorptive muffler.

Explore Octave Band Plots with These Resources

In addition to the example referenced above, you can find many other tutorial models in the Application Libraries that utilize the Octave Band plot. We’ve listed a few of these tutorial models here, all of which are available for download from our Application Gallery.

• Loudspeaker Driver
• Lumped Loudspeaker Driver
• The Brüel & Kjær 4134 Condenser Microphone
• Loudspeaker Driver in a Vented Enclosure

Reference

1. IEC 61672-1 Electroacoustics — Sound level meters — Part 1: Specifications.