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What is Dolby Atmos?
Dolby Atmos is an audio format that extends traditional multichannel sound to include a height plane and freely positionable audio objects. In addition to fixed channel groups, individual sound elements can be assigned positional information. The playback device uses this information to calculate how the sound should be output through the available speakers.
The term can now be found on many TVs, soundbars, headphones, and even smartphones. However, the way Dolby Atmos is reproduced on these devices varies significantly. Whether the spatial effect is noticeable—and to what extent—depends on the speakers, signal processing, and the listening environment.
What is Dolby Atmos?
Older surround sound formats, such as Dolby Digital, use fixed channels. In a 5.1 system, for example, there are six of them. During mixing, each sound is assigned to one of these channels. Dolby Atmos builds on this principle and expands upon it in two ways.
Channel Groups and Objects Side by Side
Dolby describes his format as a combination of two components. The foundation consists of fixed channel groups, also known as “beds” in technical jargon. These typically exist in a 7.1.2 configuration, where everything that does not need to move—such as background ambiance or music—is placed.
This is supplemented by audio objects. These are individual sound elements that can be placed anywhere in the space, independent of the channel groups. They carry position data in the form of coordinates, including information about the object’s size. An airplane flying across the screen is a typical example of such an object.
During playback, a so-called renderer handles the process. It adapts the playback to the existing speaker configuration. An Atmos audio track can therefore be played back on systems with different setups, although the spatial effect varies significantly depending on the number and arrangement of the speakers.
The plane above one's head
The second difference is the height. Traditional surround sound distributes the sound to the front, back, and sides—that is, all around the listener. Dolby Atmos adds additional channels above the listener’s head, creating a third layer.
You can tell this by the three-digit numbers. In a 5.1.2 system, the five represents the surround speakers, the one represents the subwoofer, and the two represent the height channels. In a 7.1.4 system, the numbers are seven, one, and four, respectively.
| Characteristic | Classic 5.1 or 7.1 surround sound | Dolby Atmos |
|---|---|---|
| Basic Principle | Audio is assigned to fixed channels | Combination of channel beds and audio objects |
| Positioning | linked to the channel structure | Objects can be placed in space using position metadata |
| Elevation level | Not supported for standard 5.1 or 7.1 systems | Additional level possible |
| Playback | Mixed for defined channel assignments | The renderer adapts objects to the current display |
Dolby Atmos does not replace traditional channels; rather, it supplements them with audio objects and positional information, which the playback device processes according to the selected speaker configuration.
Playback in the living room
There are three different options for the high-level plane. They differ primarily in terms of technical complexity and how precisely the sound is perceived above the listening position. How well the effect works depends heavily on the specific room and ceiling conditions.
- Ceiling speakers are mounted in or on the ceiling and project sound directly downward. While this provides the most precise sound localization, it requires an AV receiver and wiring in the ceiling.
- Upward-firing speakers reflect sound off the ceiling back toward the listening position. They are available as standalone speakers or built into soundbars.
- Virtual sound processing is used in soundbars that do not have dedicated tweeters. Depending on the design, this involves the coordinated operation of multiple speakers and psychoacoustic techniques.
The Limits of Reflection
Upward-firing speakers work best when the ceiling is as smooth as possible, sufficiently reflective, and at an appropriate height. Very high, highly absorbent, or sloped ceilings can diminish the high-frequency response because the sound reflections do not reach the listening position as precisely.
With headphones, the principle works fundamentally differently. In this case, spatial sound is calculated binaurally for both ears. This simulates acoustic cues of direction, allowing the sound to be perceived as coming from different directions, even though it physically originates only from the headphones.
Significance of the Labeling
The Dolby Atmos label indicates that a device supports the format. How pronounced the spatial effect—and especially the overhead effect—is in practice depends heavily on the system setup, the speakers, the signal processing, and the room.
The Path to the TV
For an Atmos audio track to work, several factors must align. It’s not enough for the TV to recognize the format. The source and the connection to the audio device are also important.
Source and Audio Track
The content itself must be available in Dolby Atmos. This is the case for many streaming services and 4K content, but by no means for all others. Older movies and regular TV shows are generally not available in this format. Before setting up the playback chain, you should therefore check what audio format the desired content is actually in.
It's important to note that the information on the packaging often refers to the original version. A German audio track may be available in a different format, even if it says "Dolby Atmos" on the packaging.
Transfer to an audio device
HDMI is the standard way to connect a TV to a soundbar or AV receiver. Dolby Atmos, for example, can be transmitted in compressed form via Dolby Digital Plus. However, with compatible devices, this is also possible via HDMI ARC. For high-bitrate and lossless formats such as Dolby TrueHD with Dolby Atmos, HDMI eARC is the appropriate connection.
The optical output is not suitable for this purpose. It was designed for stereo and compressed multichannel audio and does not have sufficient capacity for the formats mentioned here. Therefore, anyone setting up an Atmos system should connect the TV and audio device via HDMI.
Significance for Dialogues
The question remains: How does this format improve speech intelligibility? The answer is twofold. Technically, object-based audio theoretically opens up some possibilities. In practice, however, a lot depends on how a movie or TV series was produced.
Options for Object-Based Transmission
Object-based audio essentially provides the technical capability to treat individual components of a mix separately. If speech is present as a separate component and the playback system supports corresponding personalization, the dialogue level can be adjusted independently of music and effects.
Limitations of the Format
This does not mean that Dolby Atmos automatically provides an adjustable dialogue track. Whether dialogue can be adjusted separately depends on how the content was produced and transmitted, as well as the features provided by the playback device.
The relationship between speech and other sounds also remains crucial to intelligibility. This relationship is determined by the mix. The playback format does not change this. A film with quietly mixed dialogue will remain quietly mixed even in Dolby Atmos.
Depending on the device, Dolby Atmos can be virtually reproduced through built-in TV speakers. However, the spatial effect differs significantly from that of a system with separate surround and tweeter speakers.
Frequently asked questions
Dolby Atmos is an immersive audio format. It combines fixed channel groups—known as "beds"—with individual audio objects that can be freely positioned in space using positional data. It also includes a height plane, allowing sound to come from above. A renderer in the playback device distributes the output to the available speakers.
The first number refers to the speakers surrounding the listening position, the second to the subwoofer, and the third to the overhead channels. So, in a 5.1.2 setup, there are five speakers, one subwoofer, and two channels for overhead sound. The third number indicates the overhead channels, as is typical in immersive setups using Dolby Atmos.
eARC is not strictly necessary. Dolby Atmos, for example, can also be transmitted in compressed form via Dolby Digital Plus, which is possible via HDMI ARC on compatible devices. HDMI eARC becomes particularly important when transmitting high-bitrate or lossless Atmos audio tracks, such as Dolby TrueHD. It is also crucial that both the TV and the audio device support the respective format.
This is because the label initially only indicates that a device supports the format. Some devices reproduce the sound through their own tweeters, while others create the high-frequency effect through virtual signal processing. With headphones, this is done binaurally—that is, through a calculation designed for both ears.
If there is any influence at all, it is indirect. Object-based audio generally offers the possibility of treating dialogue separately, provided it is present as a distinct component and the playback device supports it. However, how loud the dialogue is relative to the rest of the audio is determined by the film’s mix, not by the playback format.