Cool tech you've never heard of: Wave Field Synthesis

Around 20 years ago I went to University, studying electrical engineering with a specialisation in telecommunications engineering. I was particularly interested in everything that had to do with audio. Signal processing, sound generation, audio compression etc. were endlessly fascinating to me, so I spent a lot of time at the Chair of Multimedia Communications where they worked on all kinds of sound and audio related topics. And this is the coolest thing I encountered there:

It's exactly what it looks like: a circular frame made from aluminium profiles, 3 metres in diameter, raised to roughly ear level, on which several dozen loudspeakers are mounted (48 to be precise), pointing inwards. And when you stood inside this ring and they turned the system on, and you closed your eyes, something magical happened. You knew you were in a small room with grey curtains all around, but all of a sudden you could hear a violin playing in the far distance. You could hear footsteps approaching you and walk right past you. You could hear a door closing much further away than the door of the actual room was, then someone whispering in your ear, as if they were standing right next to you. You could hear two people having a conversation in front of you in a loud and busy auditorium. And all of these things sounded completely realistic.

This is called a wave field synthesis (WFS) array or system. In order to understand what it does, let's quickly look at how sound works first.

Sound waves and wave fronts

Imagine you're in a big room, maybe a ballroom, and a few metres away from you is a person playing a violin. The vibrations of the violin's strings create sound waves which spread out in all directions and travel through the air, much like ripples of waves in water travel outwards when something falls in, forming what's called a wave front. Part of this wave front is reaching your ears, other parts are hitting the walls and are reflected from there, reaching your ears too but from different directions and being slightly delayed compared to the original wave front (because they have to travel a longer way, first to the walls and then to your ears). Based on where the wave front is coming from and where the reflections (echos) of the sound are coming from, you can tell where the violinist is in relation to you, whether he is close to you or far away, how big the room is that you two are in and many other things. Your brain can get a lot of information about the environment you are in just based on the sound waves which are reaching your ears.

What happens when you record this violin player with a microphone and then play the recording back through a speaker that's placed right in front of you? Does it sound like the sound is coming from where the violinist originally stood? No. It sounds like the sound is coming from the speaker in front of you. Because it is.

But what if you could find a way to exactly reproduce the wave front, including the echos, that hit your ears when you were there in the actual room when the violinist was playing? Then it would sound exactly the same as it did at that time, because the sound waves reaching your ears would be the same.

This is what the wave field synthesis system does. It aims to reproduce the wave front of a sound source anywhere, independently from where the actual speakers are located. In other words, with a WFS system you can freely place sound sources at any point in space. ¹

Here's how it works.

Wave field synthesis

A large number of speakers are placed very close together. Each speaker on its own is just one small sound source and when it's playing in isolation, you will hear that the sound is coming from this one speaker. But if all the speakers are playing together and are reproducing the sound at just the right moment, then the sound waves from all these individual speakers will merge together forming one big wave front hitting your ears, instead of many individual sound waves from individual speakers. This combination of waves from small wave sources into one big wave front is called the Huygens-Fresnel principle.

Have a look at this graphic, this hopefully makes it a bit clearer:

In this scenario, you're standing all the way to the right. The violinist is playing, producing soundwaves which are moving towards you as a wave front. If you replace the violinist with an array of speakers (shown in the middle) and have them all play back the recording of the violin with precise timing, this will result in a wave front which looks and sounds just like that of the original violinist. In other words, to you the listener it will sound exactly as if the violinist were playing at this location far behind the speaker array, because the wave front created by the speakers is the same as the wave front created by the violinist much further away (and of course the room echoes could be reproduced in the same way, too).

And this is the magic of the wave field synthesis system. It can produce sound waves which by the time they reach your ears are shaped and sound exactly like those of a far away violinist. Or a person standing right next to you, whispering in your ear. Or two people having a chat in front of you while there's a lot of background chatter going on further away. It all sounds completely realistic, because the sound waves reaching your ears are shaped in the same way as the ones that would reach your ears in the real scenario.

To this day I think this is one of the most impressive technologies I have ever encountered. You might think your 7.1 home cinema system sounds pretty sweet, and of course you would be right. But this takes it to the next level. It's the acoustic equivalent of watching a movie on a TV screen vs watching it on a gigantic IMAX screen which curves around you and takes up your entire field of vision. There is simply no comparison to a 'normal' surround sound system.

Practical applications

So what is this actually used for? As you will probably have guessed, this is not a product for the consumer market that you can easily set up in your living room. Even though the technology has existed for more than two decades at this point, it is slow to catch on, no doubt due to the high complexity. Sound needs to be specially mixed and processed to really take advantage of a WFS system, which requires a great deal of digital signal processing, and building a system like this in the first place is no small feat. Even a small system needs dozens of speakers, which require the same number of amplifiers, speaker cables, mounting hardware and so on. Not to mention the computer system needed to process the sound and drive all of these speakers. When was the last time you saw a 48 channel sound card?

You can still find some systems in use however:

• The TU (Technical University) Berlin has equipped one big auditiorium with a WFS system and has created some bespoke compositions to be played on this system. This article gives a good insight into the complexity of equipping an entire room with a WFS system.
• Here's a great use case: The Sphere in Las Vegas is equipped with a wave field synthesis system, built by a German company.
• I've actually come across one example of a practical implementation of a WFS system in my professional life. I worked as a developer for a hearing aid manufacturer for a few years and in the sound lab we had a WFS system to be able to simulate all kinds of acoustic environments in order to test the hearing aids under as realistic conditions as possible without actually going outside. And a WFS system is ideal for this. So there are definitely practical use cases, but it's still a specialised product at this point in time, and who knows if it will ever be more than that.

Sources

I wonder what became of the original speaker array at my University. The people who worked on it have long since retired or moved on to other projects. Are they still working with it? Did they disassemble it? Do they have it in storage somewhere? Would they sell it to me if I asked nicely? Let's maybe not think about this too much.

Footnotes