Spectral correlation

Note: You need to have the Correlation module active in your portal to be able to perform correlation measurements.

Correlation can be a useful tool if you want to know how much of the sound of the source you are measuring contributes to the total sound level on your receiving location. For successful live correlation, you need to prepare a number of things:

- A soundmeter installed at the source location and a soundmeter installed at the receiver location. Class 1 and class 2 soundmeters can be mixed. In the case of a mix between class 1 and class 2, only correlation over the overlapping octave bands will be calculated.

- One sound measurement point, configured as source with the source soundmeter coupled.

- One sound measurement point, configured as receiver with the receiver soundmeter coupled.

For more information on how to create sound measurement points and how to perform your first measurements, see the following links:

First measurement: https://munisense.zendesk.com/hc/en-us/articles/212424805-First-Measurement

Sound Measurement Point: https://munisense.zendesk.com/hc/en-us/articles/211419869-Sound-Measurement-Point

Configuring a Sound Measurement Point as source or receiver

By default, every sound measurement point is a receiver. However, a measurement point can also be set to act as a source measurement point. With this setting it becomes possible to create pairs of source- and receiver measurement points for measuring correlation. To switch a sound measurement between source and receiver, edit the measurement point you wish to change.

Using the radio button you can change the type of this measurement point to a source. By default, a sound measurement point is a receiver so you don't have to change the setting.

Next to determining what kind of role the new measurement point will play in the new measurement setup, it is important to configure the location as correctly as possible. The correlation algorithm will use the exact distance between source and receiver in its calculations so be as precise as possible.

With the measurement point correctly configured and an active meter coupled, click the Add new receiver button to create a correlation job that will correlate the measurements of this measurement point with those of another.

Creating a correlation object

A correlation object is a very simple object that holds information about the two correlating measurement points and the sample period that needs to be used for correlation. This determines the amount of time which the algorithm uses to find similarities between the two sets of data. The default value for this is 180 seconds, or 3 minutes. This will yield accurate results in most situations but it can be adjusted if desired. Save the object after giving it a short description.

After creating one or more correlation jobs, information about them will be shown on the view page of both the source and the receiver sound measurement points. They will both show the correlating measurement points and the receiver measurement points will show the contributing sources in the form of the last two hours of correlation data. If a correlation job yields no data at all, it might not be correctly configured, there might not be sufficient correlation at all, or the sound levels at the source are too low.

Creating multiple correlation jobs at once

If you have multiple source and receiver sound measurement points, it saves a lot of time to configure them correctly as source or receiver and create the required correlation jobs in a single step.

Find the Correlation viewset in the menu to see your jobs and click the New (bulk) button.

The page explains itself at the top. It will create jobs between the selected source and receiver measurement points so only check what you need and leave out those you don't need. Of course it remains possible to add or delete individual jobs.

Finally, use the correlation overview to monitor the status of your jobs. When the indicator turns red, it will explain what's wrong and offer hints on how to solve the issue.

Viewing correlation between measurement points on the map

The Correlation Map is a map-based analysis page that focuses on all measurement points within the current measurement group, allowing you to click on any receiver at any point in time to see what sources are contributing to the measured sound level. A typical situation might look as follows:

This view is currently fixed on a receiver that has three significant sources contributing to the sound levels measured on the facade. Use the time selection widget to view a different period in time in the graph. The statistics will show for the displayed period, the percentage of contribution of each source as well as the average sound level and the percentage of time it was over the limit.

Every different color in the graph shows the added sound contribution of a single source. If the total height of the column minus the grey part, which is the background noise level, exceeds the red threshold line it has exceeded the maximum sound level for the current event. This threshold can be changed at any time using the gear icon at the top left corner of the screen.

How to create a Correlation Export file of a created Correlation Job?

There are some important things that are required to be selected when a new correlation export job needs to be created. You want to download the correlated data from the SoundMeasurementPoint of a Correlation Job, not the correlation object itself. Therefore, take the Receiver Measurement Point of the created Correlation Job that you want to be exported (and not Correlation).

• Go to the Export Page in the portal and click on “+ new export job” to create a new export file.

• Fill in a description first (the name of the export file) in the General field (for example, Correlation Results of the created Correlation Job).

• Continue to the Targets field and select the targets of it.

• Select the group, the DomainObject Type and the Object that need to be exported. In this case, select the SoundMeasurementPoint of the Receiver of the Correlation Job as DomainObject Type. And select a Single Select and the Object itself. For example, SoundMeasurementPoint "Bewoners- Dutch GP".

• Continue to the Export Actions field.

• Select Export Results (CSV).

• Fill in the required details of the data to be selected in the field Export Results (CSV) below in the new field that appears.

• Select your Start Timestamp and End Timestamp.

• Importantly, make sure to select a Sample Period of 0 Seconds.

• Select the following properties to be exported: the LAeq,T (source), LCeq,T (source) and LZeq,T (source) because these properties show the correlated sound of the source on the receiver location in A, C and Z flavors.

• Save the export job by clicking on “Save” to create the export job and click on "Export data" on the following screen to start the export job.

• Now the ZIP file can be downloaded.