3D Audio with Stereo Headphones

Today I want to write about an audio experiment I did for a game that I’m developing using my Unity Accessibility Plugin. It’s an accessible cooking game in which you prepare food for customers.

Problem 

The game pans the voices of customers to the left or right, depending on where they are standing. But when playing the game blind, I find it hard to quickly tell where a customer is. It always takes me a second and it requires a little concentration. This might not be a show-stopper, but it keeps bugging me.

Spatial Audio could be the solution.

Woman with headphones on has her eyes closed and focuses on the whether the sounds are left, right or center.

One burger to the left, two hot dogs to the right…

3D Vision and 3D Hearing

Never heard of spatial audio? Don’t worry, I won’t delve into the details. In a nutshell, it is a clever way of playing back audio so that the listener has a 3D audio effect using only stereo headphones. It’s not even that complicated. Humans only have two ears, so stereo is technically all we need.

Just like your brain uses the differences in the images from your two eyes and gives you 3D vision, it uses the differences in sound between your two ears and gives you 3D hearing.

The picture shows sound sources that are placed at different locations around a head.

Even with only two ears, you can always tell where a sound is coming from. Image Source

Here Is The General Idea

Because sound waves travel through the air (at the speed of sound), a sound coming from your left would reach your left ear first, and then your right ear. Furthermore, your head is in the way of the sound and will block part of it. This is called the head shadow, which dampens some frequencies from the sound before it reaches your other ear. While the differences are very small, your brain picks up on them and uses this very subtle information to place the sound in the world around you. There’s more playing into it, like sound reflected off walls close to you etc, but I said I wouldn’t delve into the science too far.

If you haven’t yet, listen to the famous barber shop example (skip to 2:54 to get the idea):

 

Famous Games using Spatial Audio

A game can reproduce this 3D effect artificially, by delaying the playback on the far ear and filtering frequencies. The only requirement for it to work is that the player wears headphones.

Games for visually impaired or blind players sometimes use spatial audio to allow players to find their way in the dark. Famous examples are the Papa Sangre games and The Nightjar. If you want to check them out, get The Nightjar if you like Benedict Cumberbatch, or Papa Sangre II if you are a fan of Sean Bean (no need to play Papa Sange I first).

Actors Benedict Cumberbatch and Sean Bean inside a voice recording studio. They look cute.

The Nightjar and Papa Sangre II have pretty darn good voice acting in my totally biased opinion.

Unity and Spatial Audio

Somethin’ Else, the developers of Papa Sange and Nightjar, have written their own audio engine and their results are impressive. I wanted to give spatial audio a try in my Sandbox dev environment, before bringing it into my game. But I am fan of not reinventing the wheel. I didn’t want to implement my own spatial audio system when there are some out there already.

There are plugins for spatial audio available for Unity. Here are the three that I found in the Asset Store: Spatial Audio, AstoundSound and dearVR. The last one looked the best, but the first one was the cheapest. For just a quick test that would do. But dearVR is now on my WishList and I will be hoping for a sale in the future.

Direct Comparison

Which one works better?
The majority of people should be able to tell the direction of the voice much faster when listening to the spatial audio example.
But have a listen for yourself.

Put on your headphones and try to determine whether the voice is coming from the left, the right or from the front. How fast are you able to tell the direction? Do you need to focus on it?

Tip: After you’ve listened to the spatial audio example, listen to the stereo one for a second time. It seems a lot worse now, doesn’t it?

I know the spatial audio sounds like it is panned more to the side than the stereo one, but that is not the case – it’s just the effect that the brain creates from the sounds.

EDIT: I just learned that the SoundCloud player above doesn’t work with all screen readers. Here are alternative links to the audio files:
Stereo Results
Spatial Results

Conclusion

While the stereo panning works “good enough” for the simplistic setting of my cooking game, it seems to me that the spatial audio requires less effort to determine where the sound is coming from. For that reason, I will put in the work and switch over to the spatial audio library. The difference might not be huge, but I am hoping it will make the gameplay feel more intuitive, because blind players need to pay less attention to locating the customer. After all, the gameplay is supposed to be about cooking, not about telling direction.

Notes for those interested in the topic:

  • If you want to play around with spatial audio in Unity but are unable to spend money on plugins, good news: Unity has just released a demo Spatialization SDK. You can find it here. I gave it a quick test and it seems to be working fine. Hint: If you check out the demos, you need to select the HRTF demo.
  • If you want to read more about the topic of spatial audio, but are bummed out by all the “binaural beats for better sleep” nonsense search results, try searching for ‘HRTF’ instead.
  • The developer Somethin’ Else used to license their audio engine (Papa Engine) to other developers. While this is no longer the case, their website hints that there might be a Unity compatible version in the future (see here). There is also a forum thread that claims the engine might become open source entirely. You can read about it here.

 

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