Dolby Atmos, Guides, Sound technology

The Difference Between Dolby Binural and Apple Spatial Audio

Posted on by Kasper Randler

The interest in immersive sound has exploded, mainly thanks to new technologies like Dolby Atmos. As a result, many terms are emerging that can sometimes cause confusion – for example, Dolby AC-4 ISM (often referred to as “binaural,” which is slightly misleading) and Apple Spatial Audio (usually shortened to Spatial Audio, which is also somewhat misleading). In this guide, we clarify what these technologies actually mean – in relation to traditional stereo sound – how they work and how they are connected in the production chain, from studio to listening.

The Difference Between Dolby Binaural and Apple Spatial Audio

Traditional Stereo Sound

To start from the most understandable context possible, we begin with the basics: stereo sound. It is the technology that has been used for decades in music production and film. Stereo sound consists of two channels (left and right) that together create a wide sound field. It is a static format, which means that all sound is tied to channels – unlike the Dolby Atmos format, where the audio file contains information about where the sound should be perceived as coming from, making the format adaptive, in contrast to the static stereo format. Instruments and sounds are instead given the illusion of position in stereo using panning – if a guitar is panned to the right, it is perceived as being to the right of the listener, and a synth to the left if it is panned to the left.

The advantages of stereo sound are that it is a simple and universal format that is compatible with all playback devices and streaming services.

Stereo sound is limited to the horizontal plane and lacks the reproduction of height and depth, which binaural sound can add. Compared to binaural sound, some may find stereo unnatural or flat in headphones (the sound is experienced inside your head rather than around you as a listener). However, few people experience this, as we have all become so accustomed to stereo sound. The fact remains, though, that stereo as a format is not nearly as adaptable as something like Dolby Binaural. 

Dolby Atmos – Object-Based 3D Sound

To understand the differences between “Dolby Binaural” and “Apple Spatial Audio,” we first need to understand what Dolby Atmos is. Dolby Atmos is an object-based mixing format where sound is placed individually in a virtual three-dimensional space instead of traditional fixed channels. An Atmos mix can handle up to 128 separate audio channels, or sound objects, which are dynamically scaled to the listener’s playback equipment – whether you have a small headphone setup or a full-scale home theater system.

Atmos is now widely used in music production because it allows producers to create a truly immersive sound experience. If you’re curious to learn more about the format, you can either check out our article on what Dolby Atmos is, or read our guide on Dolby Atmos mixing!

From Studio to Streaming Service – ADM and Stereo

When we at KMR Studios produce, mix, and master music in Dolby Atmos, the end result is two files:

  • An ADM file (Audio Definition Model), which contains the actual Atmos mix with up to 128 separate sound objects and channels. The file looks like a WAV file on the computer, but is much more complex than a regular stereo file. It is from this file that AC-4 IMS and Apple Spatial Audio versions are created to optimize playback depending on the playback device – we’ll return to that later in the article. 

  • A traditional stereo file, also in WAV format, which is used as a fallback for devices or services that do not support Atmos. This file is static and does not adapt to the current playback equipment.

These files are then sent to our distributor, who in turn delivers them to streaming platforms such as Apple Music, Tidal, and Amazon Music.

KMR Studios - Studio A - Revolutionerar musikprojekt

A Short Note on the Word Binaural

Binaural means “with two ears” and was already used in the 19th century to describe how we humans localize sound. The term gained practical significance in the 20th century when researchers first began exploring the possibility of recording (and playing back) lifelike, three-dimensional sound. This was done by recording sound with microphones placed in artificial ears – so-called dummy-head recordings – to create a realistic listening experience in headphones.

Today, the term is also used for playback, not just recording. When we talk about binaural rendering, it’s about how multichannel audio (e.g., an Atmos mix/ADM file) is converted into two channels that still sound three-dimensional – just as if you were in the middle of the soundscape with only a pair of headphones.

Dolby Binaural (AC-4 IMS) – Atmos in Headphones via Streaming Services

What is often referred to as Dolby Binaural is in fact the technology Dolby AC-4 Immersive Stereo (IMS). AC-4 IMS was developed by Dolby Laboratories with the purpose of efficiently streaming Atmos content binaurally – that is, through two audio channels – via streaming services. This format renders a binaural experience directly from the ADM file, allowing the listener to perceive sound all around them – from all directions and angles – using regular headphones.

AC-4 IMS is a very efficient audio codec that can deliver both object-based and channel-based audio to users with low bandwidth, without significantly compromising sound quality. An AC-4 IMS file contains metadata from the Atmos mix that is used to create a realistic binaural version of the track. Streaming platforms like Tidal and Amazon Music currently use AC-4 IMS to provide their users with an optimal and authentic Atmos experience in headphones.

There are several profiles of the AC-4 format, where AC-4 IMS is a specific one within the AC-4 family. The format is optimized for headphones – but the AC-4 format itself can carry multiple versions of the same audio mix in a single package:

  • A multichannel Atmos stream for speaker systems (e.g., 5.1.2 or 7.1.4)

  • A binaural render for headphones (AC-4 IMS)

  • A stereo fallback for simpler devices

The playback device automatically selects the version that fits best, depending on its capabilities. This is why AC-4 is popular in, for example, TV broadcasts (DVB, ATSC 3.0) and on some streaming services.

It’s important to note that Dolby AC-4 is currently not supported directly by Apple devices (iPhone, iPad, Mac). This means Apple users cannot access Dolby Binaural unless the streaming service delivers a pre-rendered binaural file, as is the case with the Tidal app (iPhones and iPads only). In that case, the version cannot adapt to the device’s speaker configuration or headphone type. However, the spaciousness compared to regular stereo is still more defined. 

Apple Spatial Audio – Apple’s Own Solution

Apple Spatial Audio, usually referred to simply as “Spatial Audio,” is Apple’s own playback format, created to reproduce a three-dimensional audio experience in headphones. The format is based on Dolby Atmos, but has several unique characteristics that distinguish it from Dolby’s AC-4.

Instead of using “AC-4 IMS”, Apple uses its own “binaural renderer” that runs in real-time on your iPhone, iPad, Mac or Apple TV. Using the ADM file, a multichannel Dolby Digital Plus stream with Joint Object Coding (DD+ JOC) is created and then placed in a virtual 7.1.4 speaker system inside your device – in other words, the sound is divided into multiple objects that are placed around you in a simulated speaker setup, so you can experience immersive sound even with regular headphones/speakers. Apple’s Spatial Audio engine calculates how the sound should be heard in headphones. On our website, we have written a detailed article for those curious about how this works.

This enables several advanced features:

  • Dynamic head tracking – the sound field adjusts as you turn your head, creating a stationary audio world where, for example, the vocals stay centered even as you move.

  • Personalized HRTF – starting with iOS 16, you can scan your ears with the TrueDepth camera, allowing Apple to tailor the sound field to your unique anatomy.

  • Seamless integration into the Apple ecosystem – the speakers in an iPhone, iPad, Mac, or MacBook support the format. As do most of Apple’s headphones.

It’s important to understand that Spatial Audio is not its own mixing format, but a playback solution. Apple’s device performs the binaural conversion in real-time from a DD+ JOC version of the ADM file – unlike Dolby AC-4 IMS which creates its rendering/version directly from the ADM file. This may sound a bit messy, and like it’s the same thing. But the deeper you dive, and the more familiar you become with the format, the clearer the differences will become.