Project Acoustics Unreal Bake Tutorial

This document describes the process of submitting an acoustics bake using the Unreal editor extension.

There are five steps to doing a bake:

  1. Create or tag your player navigation mesh
  2. Tag acoustics geometry
  3. Assign acoustic materials properties to geometry
  4. Preview probe placement
  5. Bake

Open the Project Acoustics editor mode

Import the Project Acoustics plugin package to your project. For help with this, see the Unreal Integration topic. Once the plugin is integrated, open the Acoustics UI by clicking the new Acoustics Mode icon.

Screenshot of Unreal Editor Acoustics Mode option

Tag actors for acoustics

The objects tab is the first tab that gets displayed when you open the Acoustics Mode. Use this tab to tag actors in your level, which adds the AcousticsGeometry or AcousticsNavigation tags to the actors.

Select one or more objects in the World Outliner, or use the Bulk Selection section to help select all objects of a specific category. Once objects are selected, use the Tagging section to apply the desired tag to the selected objects.

If something has neither AcousticsGeometry nor AcousticsNavigation tag, it will be ignored in the simulation. Only static meshes, nav meshes and landscapes are supported. If you tag anything else, it will be ignored.

Objects Tab Reference

Screenshot of Acoustics Objects tab in Unreal

  1. The tab selection buttons (Objects tab selected). Use these buttons to walk through the various steps of doing an acoustics bake, from top to bottom.
  2. A brief description of what you need to do using this page.
  3. Available selectors for actors in the level.
  4. Clicking Select will select all objects in the level that match at least one of the checked actor types.
  5. Clicking Deselect all will clear the current selection. This is the same as hitting the escape key.
  6. Use these radio buttons to choose whether to apply the Geometry or Navigation tag to the selected actors.
  7. Clicking Tag will add the selected tag to all currently selected actors.
  8. Clicking Untag will remove the selected tag from all currently selected actors.
  9. Clicking Select Tagged will clear the current selection, and select all actors with the currently selected tag.
  10. These stats show how many actors are tagged with each tag type.

Tag acoustics occlusion and reflection geometry

Open the Objects tab of the Acoustics window. Mark any objects as Acoustics Geometry if they should occlude, reflect, or absorb sound. Acoustics geometry can include things like ground, walls, roofs, windows & window glass, rugs, and large furniture. You can use any arbitrary level of complexity for these objects. Because the scene is voxelized before simulation, highly detailed meshes, such as trees with many small leaves, aren't more costly to bake than simplified objects.

Don't include things that shouldn't affect the acoustics, such as invisible collision meshes or skyboxes.

An object's transform at the time of the probe calculation (via the Probes tab, below) is fixed in the bake results. If any marked objects in the scene are moved later, probe calculation and baking need to be redone. Do not include any moveable objects in acoustics bakes. If your scene contains any dynamic geometry, it is best to exclude that geometry from the bake (i.e. don't include doors).

Create or tag a navigation mesh

A navigation mesh is used to place probe points for simulation. You can use Unreal's Nav Mesh Bounds Volume, or you can specify your own navigation mesh. You must tag at least one object as Acoustics Navigation. If you use Unreal's Navigation mesh, make sure you have it built first.

Select acoustic materials

Once your objects are tagged, click the Materials button to go to the Materials Tab. This tab will be used to specify material properties for each material in the level. Before any actors are tagged, it will be blank.

The acoustic materials control the amount of sound energy reflected back from each surface. The default acoustic material has absorption similar to concrete. Project Acoustics suggests materials based on the scene material name.

The reverberation time of a given material in a room is inversely related to its absorption coefficient, with most materials having absorption values in the 0.01 to 0.20 range. Materials with absorption coefficients above this range are very absorbent. For example, if a room sounds too reverberant, change the acoustic material of the walls, floor, or ceiling to something of higher absorptivity. The acoustic material assignment applies to all actors that use that scene material.

Graph showing negative correlation of reverberation time with absorption coefficient

Materials Tab Reference

Screenshot of Acoustics Materials tab in Unreal

  1. The Materials tab button, used to bring up this page.
  2. A brief description of what you need to do using this page.
  3. The list of materials used in the level, taken from the actors tagged as AcousticsGeometry. Clicking on a material here will select all objects in the scene that use that material.
  4. Shows the acoustic material that the scene material has been assigned to. Click a dropdown to reassign a scene material to a different acoustic material.
  5. Shows the acoustic absorption coefficient of the material selected in the previous column. A value of zero means perfectly reflective (no absorption), while a value of 1 means perfectly absorptive (no reflection). Changing this value will update the Acoustics Material (step #4) to Custom.

If you make changes to the materials in your scene, you will need to switch tabs in the Project Acoustics plugin to see those changes reflected in the Materials tab.

Calculate and review listener probe locations

After assigning the materials, switch to the Probes tab.

Probes Tab Reference

Screenshot of Acoustics Probes tab in Unreal

  1. The Probes tab button used to bring up this page
  2. A brief description of what you need to do using this page
  3. Use these options to control the simulation resolution. Coarse is faster, but fine is more accurate. For more details, see the FAQ.
  4. Choose the location where the acoustics data files should be placed using this field. Click the button with "..." to use a folder picker. For more information about data files, see Data Files below.
  5. The data files for this scene will be named using the prefix provided here. The default is "[Level Name]_AcousticsData".
  6. Click the Calculate button to voxelize the scene and calculate the probe point locations. This is done locally on your machine, and must be done prior to doing a bake. After the probes have been calculated, the controls above will be disabled, and this button will change to say Clear. Click the Clear button to erase the calculations and enable the controls so that you can recalculate using new settings.

Probes must be placed through the automated process provided in the Probes tab. You may optionally augment the automatically placed probes with additional, Pinned Probes. In a simple example, let's say we want a single probe higher in the corner of the room, without wanting to add a fake navigation mesh there. We can add an Acoustics Pinned Probe actor and place it where we want, then re-do the probe calculation.

Pinned Probe Example

What the Calculate button calculates

The Calculate button takes all the data you have provided so far (geometry, navigation, materials, and the coarse/fine setting) and goes through several steps:

  1. It takes the geometry from the scene meshes and calculates a voxel volume. The voxel volume is a 3-dimensional volume that encloses your entire scene, and is made up of small cubic voxels. The size of the voxels is determined by the simulation frequency, which is set by the Simulation Resolution setting. Each voxel is marked as being either open air or containing scene geometry. If a voxel contains geometry then the voxel is tagged with the absorption coefficient of the material assigned to that geometry.
  2. It then uses the navigation data to calculate acoustically interesting locations where the player might go. It tries to find a reasonably small set of these locations that includes smaller areas such as doorways and hallways, and then to rooms, to open spaces. For small scenes this typically is fewer than 100 locations, while large scenes may have up to one thousand.
  3. For each of the final listener locations it computes, it determines a number of parameters such as how open is the space, the size of the room it is in, etc.
  4. The results of these calculations are stored in files at the location you specify (See Data Files below)

Depending on the size of your scene and the speed of your machine, these calculations can take several minutes.

Once these calculations are complete, you can preview both the voxel data and the probe point locations to help ensure that the bake will give you good results. Things like a bad navigation mesh or missing/extra geometry will typically be quickly visible in the preview so you can correct it.

Debug display

After probe calculation is completed, a new actor will appear in the World Outliner called AcousticsDebugRenderer. Checking the Render Probes and Render Voxels checkboxes will enable the debug display inside the editor viewport.

Screenshot showing Acoustics Debug Renderer actor in Unreal Editor

If you don't see any voxels or probes overlaid on your level, make sure real-time rendering is enabled in the viewport.

Screenshot of real-time rendering option in Unreal


Voxels are shown in the scene window as green cubes around participating geometry. Voxels that contain only air are not shown. There is a large green box around your entire scene that denotes the complete voxel volume that will be used in the simulation. Move around your scene and verify that the acoustically-occluding geometry has voxels. Also, check that non-acoustics objects such as collision meshes haven't been voxelized. The scene camera has to be within about 5 meters of the object for the voxels to show.

If you compare the voxels created with coarse resolution vs fine resolution, you will see that the coarse voxels are twice as large.

Screenshot of Acoustics voxels preview in Unreal editor

Probe points

Probe points are synonymous with possible player (listener) locations. When baking, the simulation calculates the acoustics connecting all possible source locations to each probe point. At runtime, the listener location is interpolated among nearby probe points.

It's important to check that probe points exist anywhere the player is expected to travel in the scene. Probe points are placed on the navigation mesh by the Project Acoustics engine and can't be moved or edited, so ensure the navigation mesh covers all possible player locations by inspecting the probe points.

Screenshot of Acoustics probes preview in Unreal

Bake your level using Azure Batch

You can bake your scene with a compute cluster in the cloud using the Azure Batch service. The Project Acoustics Unreal plugin connects directly to Azure Batch to instantiate, manage, and tear down an Azure Batch cluster for each bake. On the Bake tab, enter your Azure credentials, select a cluster machine type and size, and click Bake.

Bake Tab Reference

Screenshot of Acoustics Bake tab in Unreal

  1. The Bake Tab button used to bring up this page.
  2. A brief description of what to do on this page.
  3. Fields to enter your Azure Credentials once your Azure account has been created. For more information, see Create an Azure Batch Account.
  4. Launch Azure portal to manage your subscriptions, monitor usage and view billing information etc.
  5. Azure batch compute node type to use for the calculation. The node type must be supported by your Azure data center location. If not sure, leave at Standard_F8s_v2.
  6. Number of nodes to use for this calculation. The number you enter here affects the time to complete the bake and is limited by your Azure Batch core allocation. The default allocation only allows for two 8 core nodes or one 16 core node, but can be expanded. For more information on core allocation constraints, see Create an Azure Batch Account.
  7. Select this checkbox to configure your compute pool to use low-priority nodes. Low-priority compute nodes have much lower cost but they may not always be available or may be preempted at any time.
  8. The amount of elapsed time it is expected to take for your job to run in the cloud. This does not include node startup time. Once the job starts running, this is about how long it should be before you get back the results. Note that this is only an estimate.
  9. The total amount of computing time needed to run the simulations. This is the total amount of node compute time that will be used in Azure. See Estimating bake cost below for more information on using this value.
  10. Click the Bake button to submit the bake to the cloud. While a job is running, this shows Cancel Job instead. If there are any errors on this tab, or if the workflow on the Probes tab has not been completed, this button will be disabled.
  11. The probe count for your scene as calculated on the Probes tab. The number of probes determines the number of simulations that need to be run in the cloud. You cannot specify more nodes than there are probes.
  12. This message tells you the current status of the job, or if there are any errors in this tab, what those errors are.

You can always get complete information about active jobs, compute pools, and storage at the Azure portal.

While a job is running the Bake button changes to Cancel Job. Use this button to cancel the job in progress. Canceling a job cannot be undone, no results will be available, and you will still be charged for any Azure compute time used prior to cancellation.

Once you've started a bake, you can close Unreal. Depending on the project, node type, and number of nodes, a cloud bake can take several hours. The bake job status will be updated when you reload the project and open the Acoustics window. If the job has completed, the output file will be downloaded.

The Azure credentials are stored securely on your local machine and associated with your Unreal project. They are used solely to establish a secure connection to Azure.

Estimating Azure bake cost

To estimate what a given bake will cost, take the value shown for Estimated Compute Cost, which is a duration, and multiply that by the hourly cost in your local currency of the VM Node Type you selected. The result will not include the node time needed to get the nodes up and running. For example, if the compute node you selected has a cost of $0.40/hr, and the Estimated Compute Cost is 3 hours and 57 minutes. The estimated cost to run the job will be $0.40 * ~4 hours = ~$1.60. The actual cost will vary based on the actual compute time used. You can find the hourly node cost on the Azure Batch Pricing page (select Compute optimized or High performance compute for the category).

Reviewing the bake results

After the bake completes, check that the voxels and probe points are in their expected locations by running the runtime plugin.

Data files

There are four data files created by this plugin at various points. Only one of them is needed at runtime and is placed in your project's Content/Acoustics folder, which is automatically added to your project's packaging path. The other three are inside the Acoustics Data folder and are not packaged.

  • [Project]/Config/ProjectAcoustics.cfg: This file stores the data you enter in fields in the Acoustics Mode UI. The location and name of this file can't be changed. There are other values stored in this file that affect the bake, but they are for advanced users and should not be changed.
  • [Project]/Content/Acoustics/[LevelName]_AcousticsData.ace: This file is what is created during the bake simulation, and contains the lookup data used by the runtime to render the acoustics of your scene. The location and name of this file can be changed using the fields on the Probes Tab. If you want to rename this file after it has been created, delete the UAsset from your Unreal project, rename the file outside of Unreal in File Explorer, and then re-import this file into Unreal to produce a new UAsset. Renaming the UAsset by itself will not work.
  • [Project]/Plugins/ProjectAcoustics/AcousticsData/[LevelName]_AcousticsData.vox: This file stores the voxelized acoustics geometry and the material properties. Computed using the Calculate button on the Probes Tab. The location and name of this file can be changed using the fields on the Probes Tab.
  • [Project]/Plugins/ProjectAcoustics/AcousticsData/[LevelName]_AcousticsData_config.xml: This file stores parameters computed using the Calculate button on the Probes Tab. The location and name of this file can be changed using the fields on the Probes Tab.

Take care not to delete the *.ace file downloaded from Azure. This file isn't recoverable except by rebaking the scene.

Next steps