Virtual reality involves the maximum immersion of the user in the virtual world, which should be perceived as real as possible. If the VR system is designed skillfully, you can travel through time and space without getting up from the sofa, travel to the distant past of the Earth or explore Mars and the Moon, still without leaving the cozy living room.
The gaming industry has made significant progress in creating VR, but one of the main problems that developers have yet to solve is the so-called suppression of distrust of the virtual world: so that the user subconsciously perceives it almost as real (of course, he will know in any case that the world he is in is virtual, but “to know” and “perceive” are different things).
Whether you use VR to study an asteroid approaching the Earth, whether you play a game in which you shoot it with rockets, the more real your feelings are, the better. Of course, literature and cinema face the same problems, but VR has a characteristic feature — the need for a complete authentic immersion in the sound environment.
Audio immersion with Valve Index headphones
One of the recognized leaders of the gaming industry, Valve Corporation (creates games, gaming platforms and gaming equipment), set out to develop a Valve Index ® VR headset that would successfully suppress distrust of virtual reality. To do this, Valve engineer Emily Ridgeway and her team needed to figure out how to create an immersive audio environment (the buzzword immersive means “immersing in action”).
Usually players use stereo headphones to determine which side of the game the sound is coming from. If the source is to the left of the character, the player hears the sound through the left speaker, if on the right — through the right. Valve has decided to abandon traditional headphones, whose task is to isolate the ear from external sounds, suppress external noises and transmit sound completely cleared of them to the ear, but not to create a feeling of immersion. Ridgeway believed that a traditional headset could even prevent this. Ordinary headphones direct sound waves directly into the auditory canal, and the sound feels unnatural, imaginary, coming as if from the head of the person himself (the so-called internalized source). In addition, the headset itself can be uncomfortable, and this discomfort can “throw” the user out of the gaming experience.
Some players use speakers instead of headphones. This solves some problems, but also generates new ones. Firstly, the sound quality will depend on the geometry and acoustics of the room. Secondly, the best sound is created by speakers only in a certain zone, and when interacting with VR, a person usually moves.
What solution did Ridgeway propose? A pair of extra-aural (supra-aural) full-range headphones of the ultra-low field.
Ridgeway and her team studied several types of speakers. None of them fully met their goals until they came across speakers with Tectonic Audio Lab’s BMR technology. Ridgeway, as she wrote in her blog, “immediately noticed a number of advantages.” “They reduced the level of distortion in the event of a change in the orientation of the speakers, almost fit into our weight requirements, had excellent performance at high and medium frequencies (which is important for binaural simulation) and were much thinner than traditional speakers.” Valve has teamed up with Tectonic Audio Labs to take advantage of these advantages and develop custom speakers for its VR headset.
And what is BMR technology?
In traditional speakers, sound is generated by a cone-shaped diaphragm that performs piston movements. The diaphragm transmits energy along the axis of this movement, generating sound. BMR speakers work differently: they use both piston and bending vibrations of the diaphragm. This allows you to create a uniform acoustic field outside the speaker axis.
That is, high frequencies, for example, do not fade when the listener deviates from the acoustic axis. Such speakers do not require an ideal listening point (sweet spot) and, due to the ultralight membrane, provide the highest response speed, which means that the sound is natural. Traditional speakers have problems transmitting high frequencies, which can cause pulsation or bending of the diaphragm, known as “diffuser scrapping”. The resulting peaks and troughs reduce the sound quality and require more careful orientation of the speakers. Bending waves are avoided in most speakers, and BMR makes full use of them.
“We use bending vibrations, and we need them to occur. We can control the points of their occurrence, it is these bending vibrations that provide an off-axis signal output. We use resonant scrapping to our advantage,” said Tim Whitwell, vice president of engineering at Tectonic Audio. “BMR goes against traditional acoustic engineering concepts in many ways.”
BMR technology helps to use high-frequency vibration when optimizing certain characteristics, such as the material of the speakers or the mass of the air load.
Creating a Gold standard
So, the Tectonic Audio Labs team has started developing speakers for the Valve Index ® VR headset. “The starting point for us was the analysis of the diaphragm’s own modes,” says Whitwell. “For BMR technology, it is especially important that the bending mode is activated at the moment when the area of propagation of piston waves begins to narrow in the directional pattern.” When the narrowing is about to occur, the bending waves should begin to “fill” the zones on the sides of the central axis, into which the narrowed beam will not be able to get.
In order to optimize this process, the Tectonic team first needed to find out in which areas of the membrane the bending vibration mode occurs and how many such modes are available in the entire bandwidth. COMSOL Multiphysics ® software was used for numerical analysis of natural frequencies and operating modes of the speaker design. Thus, the team was able to control bending modes by optimizing the membrane material and its thickness. If the working modes are excited where necessary and when necessary, Tectonic Audio is able to provide a wide directivity of the output signal over the entire range.
Tectonic also performed a numerical analysis of the magnetic system in order to optimize it. “You can increase the number of turns of the conductor on the coil to enhance the conversion of electromagnetic energy into mechanical energy, but then the weight will increase, so there are limitations,” Whitwell explains. “We carry out all optimization using COMSOL ® .”
The mechanical and electromagnetic models were investigated and optimized separately. Then Tectonic Audio Lab combined them for joint analysis. Since almost everything in the combined model is axisymmetric, the engineers presented it in an axisymmetric 2D space, saving computational resources. The only exception was the diaphragm material.
“The diaphragm material is orthotropic. It has different stiffness in different directions,” explains Whitwell. “The Solid Mechanics interface in COMSOL Multiphysics ® allows you to simulate the orthotropic nature of the material in an axisymmetric 2D space, it’s just fantastic.”
After developing the combined model, the team added other elements to it, such as a spider mount that centers the coil and controls its movement. At the same time, optimization of the combined model continued to ensure a balanced aperture mode — this is a key moment in BMR technology, allowing it to work correctly in the Valve Index ® VR headset and guarantee an exciting experience for a wide variety of users.
