Table of Contents

• Transmission Loss Overview
• Geometric Transmission Loss (GTL)
• Room Transmission Loss (RTL)
• Usage Scenarios for RTL and GTL
  • Flexible Configuration of GTL
  • Example of an Emitter in Nested Rooms
  • RTL behavior for an emitter inside a nested room
  • GTL vs RTL in Direct Sound Paths
  • Transmission Loss of Room Tone
• Summary

Transmission Loss Overview

Transmission Loss refers to the proportion of sound energy that is lost when sound passes through an obstacle. It describes the portion of sound that is absorbed or attenuated during propagation when encountering a barrier. Different materials and structural compositions affect how sound is transmitted. Unlike diffraction or reflection, transmission loss specifically refers to energy attenuation through an object.

In games, Transmission Loss is often used to simulate the attenuation of sound as it passes through walls or partitions, helping players more realistically perceive spatial separation and distance.

In Wwise Spatial Audio, transmission loss is used to simulate energy loss as sound travels through geometry or between rooms. By adjusting the transmission loss value, you can control how much volume attenuation and filtering is applied when sound propagates between different objects. The transmission loss value ranges from 0 (no loss) to 1 (completely blocked).
The transmission loss value is converted into an audio effect using the Transmission curves defined in the Attenuation ShareSet. These curves can be custom-defined or reference the project-wide curves set in the Project Settings.

In Unreal, transmission loss can be configured through AkRoom, AkSurfaceReflectorSet, and AkGeometry. However, their implementation and effect differ slightly in practice. Transmission loss can generally be categorized into two types: Room Transmission Loss and Geometric Transmission Loss.

Geometric Transmission Loss (GTL)

Geometric Transmission Loss refers to sound energy loss as it passes through physical geometry, such as walls, windows, or other structural elements. Each surface can be assigned its own transmission loss value (from 0.0 to 1.0), allowing for realistic modeling of how different materials block or transmit sound.

In Wwise 2023.1, when a sound path intersects multiple geometric surfaces, the system uses the maximum transmission loss value among those surfaces. This ensures that the surface with the greatest blocking effect dominates the final attenuation applied to the sound.

In Wwise 2024.1 and later, when a sound ray intersects multiple geometry surfaces, the total transmission loss can be computed in one of several ways, depending on the selected AkTransmissionOperation:

• Add: Transmission loss values from each surface are summed until reaching 100%.
• Multiply: The inverse of each transmission loss is multiplied (i.e., (1–TL1) × (1–TL2)…), then inverted. This makes additional surfaces contribute progressively less.
• Max (default): Only the surface with the highest transmission loss is used.

This operation is applied per raycast from listener to emitter, and can be customized at the geometry instance level using the API. If a geometry is marked as solid, then each pair of entrance/exit surfaces will be treated together, and their transmission loss will be calculated using the selected operation.

These values are then mapped onto audio effects (volume attenuation, low-pass/high-pass filters) using the Transmission Curves defined in the Attenuation ShareSet.

AkSurfaceReflectorSet

AkGeometry

Room Transmission Loss (RTL)

Room Transmission Loss is typically used for simulating sound propagation in two situations: 

• When playing room tones across rooms.
• When a listener or emitter is in a room that doesn't have geometry.

Each Room is assigned a transmission loss value, representing the Room's ability to absorb or reduce sound energy when sound travels through or out of that Room. When multiple Rooms are involved, Wwise uses the maximum RTL among them to determine the final loss.

AkRoom

Usage Scenarios for RTL and GTL

Taking Unreal as an example, when using the AkSpatialAudioVolume (hereafter referred to as SAV) to configure spatial acoustic properties for an area in the scene, it consists of AkSurfaceReflectorSetComponent, AkRoomComponent, and AkLateReverbComponent. These components include configuration options for both Geometric Transmission Loss (GTL) and Room Transmission Loss (RTL).

Flexible Configuration of GTL

By enabling Edit Surface in the Surface Properties, you can switch to Brush Editing mode. This mode hides all Actors in the level and allows you to modify the properties of specific surfaces. You can select one or multiple surfaces on the Volume individually to configure different GTL values.

 

Example of an Emitter in Nested Rooms

GTL behavior for an emitter inside a nested room

Wwise uses acoustic ray tracing to calculate sound propagation paths from the emitter to the listener. If there are geometric obstacles between the emitter and the listener, the transmission loss values of those surfaces are taken into account based on the Transmission Operation defined in the project settings.

In the following examples, each room is defined using AkSpatialAudioVolume with AkSurfaceReflectorSet enabled on all surfaces, and each surface is assigned a same GTL value per room.

Keep in mind that Room Priority determines which room an object is logically in, even if it’s physically overlapping with others.

All surfaces in each volume are assumed to have the same GTL value in these diagrams for clarity.

Scenario 1: Higher GTL in inner room

• Volume 01 (outer): GTL = 0.3
• Volume 02 (inner): GTL = 0.6
• Emitter is in Volume 02, Listener is in Volume 01  
• Rooms with GTL are shown with white backgrounds and borders, where all surfaces use the same GTL value.

The ray from the emitter crosses one surface: the boundary between Volume 02 and Volume 01. This surface is assigned a GTL of 0.6, so the final TL is 60%.

Scenario 2: Higher GTL in outer room

• Volume 01 (outer): GTL = 0.6
• Volume 02 (inner): GTL = 0.3
• Emitter is in Volume 02, Listener is in Volume 01
• Rooms with GTL are shown with white backgrounds and borders, where all surfaces use the same GTL value.

 

In this case, even though the outer volume has a higher GTL overall, the sound path still crosses only the same single surface — the boundary between Volume 02 and Volume 01, which now has a GTL of 0.3. Therefore, the resulting TL is 30%.

This demonstrates that GTL is determined only by the geometry surface intersected by the sound path — not by the room hierarchy. Enclosing rooms with high GTL values have no effect unless their surfaces are directly intersected.

Scenario 3: Nested Volume setup with different Transmission Operations

• Volume 01: GTL = 0.5
• Volume 02: GTL = 0.4
• Volume 03: GTL = 0.3
• Sound travels from Volume 03 (innermost) – Volume 01 (outermost), passing through all three.
• All volumes are solid, meaning the sound ray enters and exits each volume's geometry surfaces.
• Rooms with GTL are shown with white backgrounds and borders, where all surfaces use the same GTL value.

This demonstrates how different Transmission Operations affect the final result when a sound path intersects multiple geometry surfaces.

Prior to Wwise 2024.1, the default GTL operation was Max. Starting in 2024.1, the default can be changed via AkTransmissionOperation, giving designers more control over how layered materials behave –  e.g., Multiply offers a smoother buildup of TL for semi-transparent media like glass or fabric.

Scenario 4: Effect of Solid vs Non-Solid Geometry on Transmission Loss

This scenario illustrates how the Solid setting affects transmission loss when sound travels through nested volumes with the Multiply Transmission Operation. The Solid setting can be enabled in the AkSurfaceReflectorSet.

The Emitter is placed near the left edge of Volume 03, and the Listener is positioned outside all three volumes, on the right.

All volumes (Volume 01, 02, 03) have geometry and GTL values:

• Volume 01: GTL = 0.5 (non-solid)
• Volume 02: GTL = 0.4 (non-solid)
• Volume 03: GTL = 0.3 (varies: solid or non-solid)
• Volume 01 and Volume 02 remain non-solid in both examples to compare the effect of Volume 03’s setting.

Left Image – Volume 03 is Non-Solid: Each surface applies transmission loss independently. The ray intersects two surfaces of Volume 03 (enter and exit), so TL is applied twice for Volume 03.

Right Image – Volume 03 is Solid: TL is applied once for Volume 03 when entering or exiting the geometry.

Solid geometry applies TL once per volume boundary (enter/exit), regardless of how many surfaces are crossed.

Non-solid geometry applies TL at each surface, making rays that cross multiple walls accumulate more attenuation.

The difference becomes apparent when geometry is entered and exited within a sound path, as shown in this layout.

The Solid setting has no effect when Transmission Operation is set to Max.

RTL behavior for an emitter inside a nested room

In contrast to Geometric Transmission Loss, Room Transmission Loss does not rely on surface geometry. Instead, it assigns a transmission loss value to the entire room. When a sound travels from one room to another, Wwise takes the maximum transmission loss value among the rooms along the path — regardless of how the volumes are physically nested or intersected.

In the following examples, no Geometry or GTL is configured — only Room Transmission Loss values are used.

Keep in mind that Room Priority determines which room an object is logically in, even if it’s physically overlapping with others.

Scenario 1: Higher RTL in inner room

• Volume 01 (outer): RTL = 0.3
• Volume 02 (inner): RTL = 0.6
• Emitter is in Volume 02, Listener is in Volume 01
• Rooms with only RTL are shown with solid color backgrounds; the shade represents the relative RTL level.

Wwise selects the higher value between the two rooms (0.3 vs 0.6), which is the inner room's 60%.

Scenario 2: Higher RTL in outer room

• Volume 01 (outer): RTL = 0.6
• Volume 02 (inner): RTL = 0.3
• Emitter is in Volume 02, Listener is in Volume 01
• Rooms with only RTL are shown with solid color backgrounds; the shade represents the relative RTL level.

Again, the system chooses the maximum value, which is now the outer room's 60%.

Unlike GTL, RTL applies a room-level transmission value rather than being calculated from intersected surfaces.

Scenario 3: Emitter inside inner room, listener in outermost room

• Volume 01 (outer): RTL = 0.5
• Volume 02 (middle): RTL = 0.4
• Volume 03 (inner): RTL = 0.3
• Sound travels from Volume 03 (inner) – Volume 01 (outer)
• Rooms with only RTL are shown with solid color backgrounds; the shade represents the relative RTL level.

This shows that, regardless of how many rooms are nested or how deep the emitter is, the system only uses the room with the highest RTL value.

This behavior can be understood as follows: when the outer room's SAV has Transmission Loss configured only in the Room settings, the room can be regarded as a space with a specific "medium density." The density of this medium (the transmission loss value) determines how easily sound propagates within that room. Therefore, sound propagation in the inner room may be affected by the outer room's RTL.

GTL vs RTL in Direct Sound Paths

When an emitter is inside a room that has both Room Transmission Loss (RTL) and Geometry Transmission Loss (GTL) configured, only the GTL value is used for the direct path. The room’s RTL is ignored in this case. This prioritizes geometric transmission loss over the room-wide RTL, effectively discarding the idea of “medium density” when GTL is defined.

However, in nested room scenarios, if the outer room has only RTL configured and the inner rooms use GTL, the outer room’s RTL may still participate in the final transmission loss calculation if it provides a higher value. The following examples illustrate this:

• Rooms with only RTL are shown with solid color backgrounds; the shade represents the relative RTL level.
• Rooms with GTL are shown with white backgrounds and borders, where all surfaces use the same GTL value.

This demonstrates that GTL overrides RTL when present, but in multi-room traversal, the highest participating TL—whether GTL or RTL—may dominate depending on configuration.

Transmission Loss of Room Tone

When Room Tone is enabled and the listener is a different room than the one emitting the Room Tone, transmission loss is always determined by the configured Room Transmission Loss (RTL), regardless of whether Geometry Transmission Loss (GTL) is enabled. This means that even if geometry is present, the sound propagation will not be affected by material or surface transmission loss, and is instead fully simulated based on the room’s overall RTL. 

In scenarios involving Reverb Zones (where one room is nested inside another and assigned as a child of the outer room), the RTL of the inner Reverb Zone influences the Room Tone propagation, even when geometry is configured. This makes RTL a key factor in indirect sound simulation, particularly in nested room or Reverb Zone structures.

When the outer room has a lower Room Priority than the inner room:

If the inner room is a regular Room (not a Reverb Zone): The system does not consider the nesting relationship when calculating transmission loss. 

• When the listener is inside the outer room, the transmission loss is the maximum RTL between the Room Tone source room and the listener’s room. 
• When the listener is outside the outer room, the transmission loss is based solely on the RTL of the inner room. 

If a Reverb Zone is nested inside another room and has that room assigned as its parent:

• When the listener is in the outer room, transmission loss is determined by the RTL of the inner room.
• When the listener is outside the outer room, transmission loss follows the standard rule: it is the maximum RTL between the source and listener rooms.

When the outer room has a higher Room Priority than the inner room:

If the inner room is a regular Room (not a Reverb Zone):

The listener is considered to be in the outer room, so the Room Tone from the inner room is continually affected by transmission loss and takes the maximum RTL between the two rooms. 

• When the listener is outside the outer room, transmission loss is again based solely on the RTL of the inner room, with no contribution from the outer room’s RTL.

If a Reverb Zone is nested inside another room and has that room assigned as its parent:

Even though the listener is logically located in the outer room, the TL uses the inner room’s RTL. 

This can be understood as the Room Tone propagating through an acoustically dense inner space, whose characteristics influence the result. 

By establishing a parent-child relationship, Wwise treats the two rooms as an acoustically nested structure, allowing the inner room’s transmission properties to affect the final outcome. 

• When the listener is outside the outer room, the transmission loss is the maximum RTL between the two rooms.

Note: Using lower priority for inner Rooms is usually incorrect, as the listener won’t enter them. But in some creative cases (e.g., fog zones), it might be used intentionally. Still, a Reverb Zone is typically the better tool for this purpose.

Summary

Room Transmission Loss (RTL) and Geometric Transmission Loss (GTL) each serve distinct purposes:

• RTL simulates overall sound attenuation for rooms without geometry. It also exclusively determines transmission loss for Room Tone, regardless of whether GTL is enabled.
• GTL offers finer control by assigning transmission loss to individual surfaces. The final loss along the direct sound path depends on how rays intersect these surfaces, combined using the project’s configured Transmission Operation (Add, Multiply, or Max), and whether the geometry is marked as Solid or Non-Solid.

In nested room scenarios:

• If all rooms have GTL, the transmission loss is calculated entirely from surface hits along the path.
• If only some rooms have geometry, surfaces contribute where geometry exists, and RTL may still apply in non-geometry segments.
• If all rooms only use RTL, the system uses the greater RTL value between the emitter and listener rooms.
• If the outermost room has the highest RTL and no GTL, it may dominate the result, even when inner rooms use GTL.
• Combining RTL and GTL allows more flexible and realistic spatial behavior.

In nested Reverb Zones, assigning a parent room establishes an acoustic hierarchy. When the listener is outside the nested structure, transmission loss is determined by the maximum RTL of both the parent and child rooms.

Special thanks to Thalie, Thomas, Kris and Guillaume for their valuable feedback and suggestions during the drafting of this article. Your support and expertise helped ensure the content on Transmission Loss is presented more accurately and clearly.