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determine their sound radiation into the environment, one by one. With this approach, we can create a new situation by extracting instruments, adding new ones, and changing the scene elements such as buildings, road surfaces, and source trajectories. This must, however, include all the dynamic and vivid atmosphere of Mardi Gras, creating a virtual Mardi Gras presented through an immersive VR device (Figure 1, right).
Another task that is quite different from the virtual Mardi Gras is to plan the renovation of an office space. People who will be working in the new space will want to know what it will be like. They are likely to expect an improved workplace that is more comfortable and perhaps quieter than they had before the renovation.
One way to help them envision the new workspace is to use VR to create a virtual office. This would not only include the renovated physical environment but also the sound sources
(e.g., HVAC, printers, copy machines, phone calls, conversa- tions). This VR office would thus include the room design from the architectural software as well as from the acoustic simulation. As a result, the virtual office could be seen and heard through a VR device. This virtual office can then be used for presentations by the architect and serve as the basis for discussions with the client as well as for decisions about the final design and materials. Of course, one of the most valuable features of the VR is that the user can move in the virtual office, thereby enhancing the realism of the experience.
The same technology can be used for planning the announce- ment system in a subway station, such as in New York City. Understanding speech announcements is not an easy task in this type of noisy space, particularly for hard-of-hearing people. A simulation in VR could not only be used for plan- ning purposes but also for listening experiments designed to provide a better understanding of how the auditory system performs in environments with a very complex setting of moving sound sources, reverberation, and interfering speech while concentrating on the announcement from what is gen- erally a rather poor public address system.
If you are curious to hear and see examples of applications of acoustic virtual reality (AVR), there are links to demonstra- tions at the end of the article.
Acoustic Virtual Reality
VR is a very powerful computer tool that is connected to 3-D devices for auditory-visual impressions. Cave automated
virtual environments (CAVEs) were invented by Cruiz-Neira et al. (1992). VR started with large and expensive hardware in setups of CAVE-like environments that can best be com- pared to the Holodeck that was featured in the TV show Star Trek. The Holodeck is a room where the user can experience a virtual world, including interactions with virtual objects and/or virtual lifeforms (see bit.ly/2OcHfZe).
Although the Holodeck is likely some years in the future, we can get a somewhat similar experience with head-mounted displays (HMDs) and headphones. Using such devices, audio- visual presentation became affordable for everybody, and thus entertainment, gaming, and social networks are rapidly entering the concept of virtual environments.
However, the acoustic component of the virtual environ- ment is generally rendered just as a sound “effect” and does not follow the strict principles of sound source calibration and sound propagation. The proper choice of sound in VR depends on the application, so it must be decided if the vir- tual sound should be as close to reality as possible or if a more or less plausible sound event is sufficient, such as in gaming.
In this context, a “virtual” sound is, in fact, a real audible sound that was created by simulation of a virtual scenario. This has many similarities to acoustic illusions, such as cre-
ating a specific auditory sensation of a musical experience.
In music production and audio engineering, stereo mixing or higher order spatial audio systems are designed to bal- ance the sounds into a spatial sound event that contains both localization and distance cues as well as the relative levels and spectral filtering to achieve a “good” sound. Although current audio systems share some similarities with AVR, the spatial audio technology used by the music industry has a different goal, which is to produce an artistically designed reproduction of an enjoyable music event. In doing this, the sound engineer can use many degrees of freedom and add, suppress, and enhance sound components until, in the end, it is heard as it is in “What Does It Sound Like, Baby?” (Ray Charles; see bit.ly/2KaW0e0).
Challenges of Acoustic Virtual Reality
In contrast to what sound engineers do, the VR sound in the virtual environment is created from computer data, compu- tational simulations, and audio signal-processing technology, with the goal of producing an auditory illusion that is, at its best, an exact copy of the real-world counterpart, if this exists.
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