An Introduction to Ambisonics

2017/03/01

You may have heard about it. Field recordists are experimenting with it. Microphones manufacturers are embracing it. Gaming studios are pushing it. Social media companies are discovering it. What is it?

It’s the Ambisonic field recording format. How does it work? What is it? What do sound pros have to say about it?

This month will feature a brief series exploring the basics of the format. It will also feature interviews with some notable field recordists who are working with Ambisonic field recordings now. They’ll describe their experiences, share their tips, and their thoughts of the future of this format. To wrap up, I’ll share a final article with resources they’ve mentioned.

Let’s get started.

What is Ambisonics?

Soundfield Microphone Tetrahedral Microphone Array

Ambisonics is a special technique for gathering and using audio recordings. Ambisonic sound effects are captured by rare, uniquely designed microphones that house not one but four sub-cardioid microphones pointed in different directions. This unusual arrangement of microphone capsules is known as a tetrahedral array.

Together these four channels capture audio not only on the typical horizontal plane, but also above and below the microphone, too. In other words, these microphones record audio from every direction surrounding the microphone. So, Ambisonic recordings use hardware and software to gather a full 360 degree representation of sound.

Flexible Field Recordings

Raw Ambisonic recordings – known as B-Format – must be translated or decoded to be used properly. What’s interesting about it is that these four channels don’t translate to a fixed representation of sound. Instead, the decoding software can interpret and reshape the sound recording in multiple ways. What does this mean?

Well, a decoder can take one Ambisonic recording and “output” the audio as mono, stereo, quad, 5.1, and much more – it’s your choice. That’s not all. If you like, a decoder can fashion the recording into a “virtual” hypercariod, figure-of-eight, or omnidirectional microphone pattern. So, you could take an Ambisonic recording and decode it to stereo today. Then, you could output the same recording to 5.1 tomorrow. It’s incredibly flexible.

Wait, if this format is so versatile, why isn’t everyone recording with it? Well, it's been observed that listeners must be in a precise sweet spot to take advantage of the sound. That also makes it possible for listeners to hear unpleasant phase issues if they move. Also, with four channels of info, it has the potential to produce massive sound files from what are sophisticated microphones and recorders. The need for decoding requires specialized software or plug-ins, many of which are just maturing now.

Other Flavours of Ambisonics

When people refer to Ambisonic formats, most of the time they mean the B-Format, 4-channel type (W, X, Y, and Z channels). Some microphones also output Ambisonic info from the capsules directly. The signal from these A-Format microphones must be further processed to be used as the W, X, Y, and Z channels.

What’s more, there are also other Ambisonic versions that add more channels. These are referred to as higher-order Ambisonic . The increased amount of channels contribute to more “spatial resolution.” They’re more accurate this way, too. They are recorded in different formats, which, of course, must be decoded to be used, too.

How to Use Ambisonic Recordings

How can you use Ambisonic sound effects? Well, since they are so flexible, they could be used for conventional stereo field recording. Since they capture all sound surrounding the microphone, these recordings shine when they are used to represent a full 360 degree or 3D (three-dimensional) sphere of sound.

Because of this, Ambisonic recordings work well in theatre settings as surround sound for both film and stage. Ambisonics have also become popular with the rise of Virtual Reality and Augmented Reality in video games and apps, and so on. That allows users to strap on a visor and headphones, then experience a full sphere of sound as they turn their heads. Google, Samsung, Facebook, and others are probing the format to use on hardware such as the DayDream View, HTC Vibe, and the Oculus Rift. It has also been known to be used in Auro 3D, Dolby Atmos, Facebook 360, and others.

Oculus Rift Development Kit 2

Pros and Cons of Ambisonic Recordings

Advantages of Ambisonic recordings:

  • Flexible. One recording can be translated into dozens of channel arrangements, or even virtual microphones.
  • Evocative. A complete 360 representation of audio.
  • Purity. Ambisonic field recordings do not experience phase issues.
  • Future-proof. The raw Ambisonic format can be extracted into various formats at later dates.

Disadvantages of Ambisonic recordings:

  • Shines only in a specific listening location.
  • Requires decoding to be used.
  • Uses expensive equipment.
  • Produces large sound files.
  • Image shift issues created during recording have strong negative effect on Ambisonic recordings. That means if an Ambisonic mic is shifted, bumped, or turned on a stand while recording and the image shifts, it's a much larger problem to fix (compared to stereo recordings).

A Side Effect

An interesting side effect of Ambisonic field recordings is that they in part transfer microphone positioning choices from the field recordist to the sound editor. What does this mean?

Well, have you ever listened to an ambience and wished the field recordist had angled the microphone just a bit to the left? Wonder if an atmosphere would sound differently if the nose of a mic was pointed upward? With an Ambisonic decoder, an editor can position the perspective themselves.

This doesn’t mean Ambisonic field recordists can recklessly set up an Ambisonic microphone anywhere. It does not absolve them from positioning a microphone properly, of course. However, Ambisonic field recordings in some part liberate sound editors from the consequences of a single field recordist’s choices. Whether this is a benefit is up to you.

Explorers of Ambisonics

While the format has been around for decades, many are only hearing of it now. I am one of these people. So, I reached out to my accomplished colleagues to share with us their thoughts and experiences with the format so we can all learn. They’ll be featured in the following posts.

My thanks to all the sound pros who have participated in this series.

Many thanks to John Leonard for his invaluable contributions to this introductory article.

Read More

Abmisonics is a detailed, deep subject. This post touched on just the basics. Learn more:





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