Better sound and smaller climate footprint with the right audio format

If you have audio files on your website in the MP3 format, it's worth checking whether that still is the optimal format for your content.

More and more people are “reading with their ears” – listening to podcasts, audiobooks and news on their mobile phones. And after video, audio is the content that requires the highest bandwidth.

If you spend a little time compressing audio files smartly, you can both reduce your website’s carbon footprint, give the listener a better user experience and help people use older mobile devices for a longer time.

If you have MP3 audio files on your website, there are better alternatives to MP3 if your files are voice-only (i.e. not music). In this article we will introduce you to AAC & Opus, which are alternatives to MP3 and which, in addition to having the same sound quality, also provide a better user experience and half the climate footprint. We have a clear favourite – read on to see which – and why.

As an example of audio files in a format other than MP3, we have used the sound from a project with a virtual guided tour around Augustenborg. Here we experimented with newer audio formats, and our conclusion is that we can halve the file size without any audible loss of quality with the Advanced Audio Coding or AAC file format, which is our favourite by the way.

Sound clips compressed in four different qualities

Below are examples of an audio file compressed in four different formats. The first is a classic MP3 file at 96 kbit, which is sufficient for speech. Next are a few variants in AAC and Opus. Can you hear the difference?

Audio MP3 · 96 kbit
Audio HE-AACv2 · 64 kbit
Audio Opus · 40 kbit
Audio HE-AACv2 · 40 kbit

File sizes at different formats and bitrates

Our starting point was a 1.4 MB MP3 file at 320 kbit. This is an unnecessarily high bitrate for speech that is expected to be played on a phone while walking around town. The large file size also meant long loading times, especially in areas with only 3G coverage.

By reducing the bitrate to 96 kbit, we achieve a more manageable file size and still maintain good sound quality, even when using MP3. But if we switch to AAC or Opus, we can lower the bitrate even further.

File formatFile size:
MP3 · 96 kbit395 KB
HE-AACv2 · 64 kbit264 KB
Opus · 40 kbit175 KB
HE-AACv2 · 40 kbit165 KB
File size of audio files with comparable sound quality.

Same sound quality, half the file size

With Opus and AAC formats, you can lower the bit rate and halve the file size. With HE-AACv2 we hit 41% of the file size of our MP3 version.

MP3 · 96 kbit


HE-AACv2 · 64 kbit


Opus · 40 kbit


HE-AACv2 · 40 kbit


Is it fair to compare a 96 kbit MP3 file with a 40 kbit AAC file? It depends on the nature of the audio files. In this project, none of the three test subjects could really hear the difference.

AAC – our recommended audio format

When experimenting with different audio formats, we need different tools for converting and playing audio files. HE-AAC v2 was ISO-standardised back in 2006, and we could immediately use the format in the applications we usually use.

This, combined with the ability to significantly reduce file size, makes AAC our preferred choice when using audio on our websites.

Broad support for HE-AAC across browsers

Chrome, Safari and Firefox all have good support for AAC. Edge, Opera and Samsung Internet are all Chromium-based browsers, which means they also enjoy the same support.

Audio - CanIUse AAC Codec
Audio – CanIUse AAC Codec

The ability to visit our websites with slightly older equipment that may no longer be updated is important. Fortunately, AAC works flawlessly in Safari, including in tests on our 2014 iPad Air.

You can read more about the AAC and HE-AACv2 formats in the following Wikipedia articles:

Advanced Audio Coding (AAC)

Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieved higher sound quality than 20th century MP3 encoders at the same bit rate.

Wikipedia – Advanced Audio Coding

High-Efficiency Advanced Audio Coding (HE-AAC v2)

High-Efficiency Advanced Audio Coding (HE-AAC) is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio. The usage profile HE-AAC v2 couples SBR with Parametric Stereo to further enhance the compression efficiency of stereo signals. HE-AAC is used in digital radio standards like HD Radio and DAB+

Wikipedia – High-Efficiency Advanced Audio Coding

Opus – an interesting audio format with a catch

The Opus format is said to have slightly better sound quality than HE-AACv2 at the same bitrate. The online media Zetland chose to switch to the Opus format in 2021, achieving a 35 percent reduction in file size compared to MP3 and correspondingly lower climate footprint.

We send audio files through the internet to your phone for you to listen to our articles.
That’ s not free. Neither in dollars and cents nor in CO2 emissions. Every megabyte that needs to be stored and transported through the network requires power, and that power needs to be produced.

Zetland – How a tip from a member turned our audio files 35 percent greener.

Opus requires special treatment in Safari

The downside of Opus is that playing the files of those using Safari requires something special. In order for the Opus format to play in Safari, it must be wrapped in the Apple-developed Core Audio Format and it only works on iOS 11 or later. This makes it just a tad more tricky for us to work with.

Audio - CanIUse Opus Codec
Audio – CanIUse Opus Codec

We didn’t immediately succeed in finding a Windows tool that supports the Core Audio Format, and since our ears can’t really hear the difference between HE-AACv2 and Opus anyway, we’ve opted out of Opus for now. But maybe you can hear the difference? You can, at least on a PC, listen to both variants in the examples above.

Wikipedia on Opus

Opus is a lossy audio coding format developed by the Xiph.Org Foundation and standardized by the Internet Engineering Task Force, designed to efficiently code speech and general audio in a single format, while remaining low-latency enough for real-time interactive communication and low-complexity enough for low-end embedded processors.

Wikipedia – Opus audio format

Applications and HTML elements used

We like open source software and standardized HTML5 elements. For this project we used Audacity for Windows for audio editing, as well as the HTML element <audio> which you can use whatever tool your website is developed in.

Audacity – free open source audio software for PC and Mac

Audacity is a free, easy-to-use, multi-track audio editor and recorder for Windows, macOS, GNU/Linux and other operating systems. You can use Audacity to … Edit WAV, AIFF, FLAC, MP2, MP3, Ogg Vorbis sound files. AC3, M4A/M4R (AAC), WMA, Opus and other formats supported using optional libraries.


Audio – an HTML element that works everywhere

The <audio> HTML element is used to embed sound content in documents. It may contain one or more audio sources, represented using the src attribute or the <source> element: the browser will choose the most suitable one. It can also be the destination for streamed media, using a MediaStream.

MDN Web Docs – The Embed Audio element

Podcast production? Then listen up.

Thanks to Anette Hallstrøm for invaluable help with the language in this article. If you’re thinking about podcasting, reach out to Anette.

Anette Hallstrøm

Podcastproducer Anette Hallstrøm

Anette is a podcast producer, has a past as a host in, among others, Danish Radio. She now lives by producing and editing podcasts in her company Hallkom

Anette has created podcasts for Bayer, Gjensidige Forsikring, Building Green and is a regular podcast producer for Bæredygtig Business, GEUS, PowerLunch and many others.

One comment

  1. Great article! Some comments:

    Support for AAC does not necessarily imply support for HE-AACv2, but in practice it’s an okay assumption — even Linux supports both via gstreamer-libav. Using the m4a/mp4 container is recommended to cover firefox.

    Finding a legal HE-AACv2 encoder can be hard. The Apple encoder is great and can be invoked by a number of applications on macOS; it’s also available on Windows iTunes, and if you are an outlaw, via “QAAC”. FDK AAC, with can be used legally even for commercial streams, also does HE-AACv2, but you may have to build from source code because the license is not compatible with GPL. Nero AAC is old and you need to negotiate a commercial license — not sure if they still provide one.

    ffmpeg can be used on Windows and supports generating CAF. However, Apple platforms not only require Opus in CAF, they require CBR opus in CAF — this reduces the coding efficiency of Opus. There’s also a javascript polyfill, `@web-media/ogg-polyfill` to do the conversion in browser, but for now it (1) has the same CBR issue (2) cannot be used in a streaming fashion.

    At least almost everyone has iOS 11+ now. Almost.

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