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Virtual Reality Gaming… With Subwoofers! A Look At Optimizing Low-End Performance

Options for routing the game sound to the subs, and then deciding where to place the subs in the room.
Half-Life VR Gaming

Half-Life “3” is finally here, and at first I was skeptical about it being released as a virtual reality game — until I experienced it.

For those unfamiliar, the Half-Life series debuted in 1998. Half-Life 2 came out in 2002, and while the new game Half-Life Alyx may not be the official Half-Life 3, it’s the third game in the series. Alyx is the first AAA title to be released for SteamVR and is an action/survival/horror game built on the Source 2 engine.

Players assume the role of a 19-year-old woman named Alyx and battle aliens from another dimension as well as soldiers from an authoritarian ruling class. It looks and sounds fantastic.

One thing I noticed while watching the previews, however, is that the sound design would be very different when experienced on a headset versus a proper sound system. The HRTF (Head Related Transfer Function) algorithms that provide players with cues as to where the sound is coming from work similarly to our biological ears. The spacial awareness will be great on a headset or with headphones. Above 700 Hz, our ears look for differences in level to figure out where a sound is coming from.

Headphones can handle 700 Hz and up, so no problem. But the issue is, no matter how much “bass” a set of headphones claims to have, it would be completely incomparable to having a 12- or 15-inch subwoofer (or a few of them) in a VR room.

I realized this when a large, three-legged robot called a “Strider” came walking toward me in an online preview. The sub caused the room to shake with every step. It was such an awesome experience — extended low end is a must for great VR in my opinion.

The first issue to overcome is how to get the game sound out to subs, and the second issue is deciding where to put the subs in the room. Splitting the audio is a bit difficult because of how most VR headsets work. The Valve Index, for example uses USB 3, to connect to the host computer and the computer considers the headset a USB interface. In Windows, you can only use one interface for playback and recording at a time since one needs to be set as “default.”

One way to split the audio is to use a splitter cable from the headphone jack on the headset. One side goes the headphones and the other connects to a long cable that goes to the sub(s).

Another solution allows digitally routing the in-game audio to the headset and anywhere else at the same time. It’s called Dante Via (by Audinate) and is a software mixer that puts any input, output, USB interface, Dante device or desktop application on a virtual Dante network so audio can be routed anywhere. The audio from SteamVR can be sent to your existing audio interface and on to the sub(s) as well as the headset. Adding an inexpensive Dante line level output device could be a solution to get signal to the subs as well.

Once the signal is routed, it’s time to place the sub(s) for the best experience. Valve recommends having a 6.5- x 5-foot (2- x 1.5-meter) open space to use for gaming. For most folks this will be the center of a room.

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Subs are particularly prone to alteration of frequency response due to room modes. A room mode is a build up or cancellation of sound energy due to “standing [sound]waves.” Each frequency of a created sound wave has a physical distance that it needs to travel in order to cycle once. Depending on how the length of the sound wave correlates to the length of the physical room there will be an increase or decrease in volume or level at specific frequencies and in different spots in the room. Where the subs are placed and where the listener stands determines the response that’s perceived.

If a room dimension is the same length as a frequency’s wavelength, there will be an increase in level in the middle of the room. For subs we’re looking at around 40 Hz to 120 Hz which is a range of wavelengths of 28 ft (8.5 m) to 9.5 ft (2.9 m).

Now, room modal buildup can be either good or bad depending on the situation. If it’s a recording and mixing room, you don’t want large increases at modal frequencies. If it’s a hotel ballroom, you don’t want the modes to be excited because it may degrade speech intelligibility or make the low end muddy for a band.

For this application, a modal buildup at a certain bass frequency is like extra sub power for free. What we want to avoid is a big decrease in level in the bass range of 40 Hz to 120 Hz, as this condition will take away from the experience. Let’s look at a few examples.

The first room has the dimensions of 16 ft (4.9 m) long by 10 ft (3 m) wide and 8 ft (2.4 m) tall. There’s a single sub in the corner. For someone standing 5 ft 11 inches (1.8 m) tall and located in the middle of the room, the room’s modal tendencies would look like this:

70.4 Hz has the same wavelength as the length of the room, so the little head is where we see a big increase in level at 70.4 Hz. This program (free and linked below) takes three dimensions into account when it does its calculations. The user can move the head around the room and see the changes in response.

A program called MAPP XT (from Meyer Sound) lets us see it another way, showing a colorized plot of sound pressure level (SPL), which could roughly be considered “loudness” at any point in the room for most bass frequencies. This program works a bit differently because it works in two dimensions only but between both of them, we can get a pretty good idea of what’s happening in the room. The color plot follows this key — red is the loudest level and it gets lower from there:

The rectangle below shows the”play zone” recommended by Valve and the little “Y” represents a microphone in this app.

As we get farther from the sub, the level drops off. If we were to move the sub a quarter (1/4) of the way down the length of the room, the room mode at 70.4 Hz would cancel instead of increase, meaning it would lose impact in the center. To play it safe, try to keep any sub along a wall. Lets look at two subs.

With two subs, the big mode at 70.4 Hz stays but we’re still not getting the maximum level in the center of the room. Between these two subs it’s nice and loud, but remember, they can’t be moved toward the rear wall because around 70 Hz will start to cancel out. How do opposite corner subs work

Looking good! The big increase in level at 70 Hz is still there in the middle and we’re getting the full power of the subs in the play area. As we go away from the middle it actually gets quieter! Now let’s try middle of the wall for sub placement.

There’s a wider sweet spot in the middle and the nice boost from the room is still there! The polarity of the subs is very important in a setup like this. If one of the subs was reversed polarity, there actually a cancellation in the middle.

The ultimate configuration may be four subs in four corners!

Again minding the polarity of the subs, if the top left and bottom right subs were reversed, we would get:

With VR, we’re not usually standing at full height all the time, of course. If you crouched down to the ground to look under something, what would the response look like with your head 1 ft (.3 m) off the floor?

It should hold up pretty well down there.

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As you can see, there are many different possibilities for sub placement that can provide a great experience. The mode discovery tool is called Room Sim, and it’s part of a free app called Room EQ Wizard (Mac or Windows). You can download it and get the dimensions of your own room to see what’s happening. Here’s my living room.

I would need to move the subs around to clear that cancellation at 90 Hz; however, it may not go away completely due to room dimensions.

I’m looking forward to what VR will bring in the future, and I recommend Half-Life Alyx for anyone interested in Sci-Fi. It even has a scissor lift! No truss or loudspeakers though…

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