Comparing SPL Octave Weighting and RTA Measurements

Comparing SPL Octave Weighting and RTA Measurements

Have you ever calibrated an input for SPL measurement and then noticed the levels on the RTA plot are much lower than your live SPL reading in the top right corner? This is because by default, Smaart's SPL measurements are broadband. This means they account for the total energy from across the entire spectrum of the signal.

However, once this signal is run through a Fourier Transform and banded across the spectrum, its overall energy is divided between each band, proportional to the levels of frequencies present within each band.

A more in-depth explanation of how Banding works can be found here.

As a thought experiment, what if we wanted to compare SPL levels to the levels seen in an RTA plot? To do this, we would need to use a specific type of SPL weighting called "Octave" weighting. This can be selected in the SPL Config menu under "Custom Metrics." From here, you need to complete the following steps:

    1. Click the + sign button at the bottom left corner of the screen. This will create a new metric that can be customized as needed.

    2. Under the "Weight" column for this newly created metric, click "None." You will see a drop-down menu with different options.

    3. Select "Octave" for the weighting type. You'll notice that the previously greyed-out option to the right has changed from "Broadband" to "16 Hz."

    4. You can now click "16 Hz" to see a few different frequency options. Using Octave Weighting, Smaart will display the SPL value of just the specific octave- length "band" centered around the value selected.

    5. For the sake of this example, we'll use 1 kHz.

    6. Select "dB SPL Slow" under the dropdown in the far left side of the row. This will make it easier to track the SPL meter level alongside the level shown on the RTA plot.

    7. Next, navigate to "New Window" within the "Window" dropdown menu.

    8. Put this new window in RTA view. In the bottom right of the plot, you'll want to adjust the banding to "Octave."

    9. Navigate to the Graph Settings menu either by clicking the gear icon below the plot or by using the [Alt/Opt] + [S] hotkey.

    10. In the "RTA Display Settings" section of this menu, there is a checkbox labeled as "Plot Calibrated Level." Checking this will change the Y-axis of the RTA plot from its dB FS default to match your SPL metric.

    11. Next, simply run the spectrum measurement engine for the input you're using for SPL if it isn't already running.

    12. Hovering your cursor over the 1kHz band will display the live SPL values above the plot. Comparing these to the values in the SPL reader will show a near perfect match (depending on any differences in averaging/buffering between the two meters).

Octave Weighting can also be used to track the levels of specific frequency ranges together over an extended period of time. If we go back to the Custom Metrics tab of the Log Config menu, we can see our 1 kHz octave-banded meter from the previous example. Next, we're going to want to create an octave-weighted custom metric for each available option, 11 in total. The order of these doesn't necessarily matter, as they'll be plotted in order of level, not frequency.

Begin logging and enable each of these metrics in the top-right corner of the plot. As the logging continues, you will see a separate trace for the level of each octave band. To view the plot of a particular band, you can either cycle to it in the top plot or bring it to the front of the Z-order in the top-right drop-down menu.