Ever since they first put sound in movies, filmmakers (and later TV broadcasters) have looked for ways to use audio to make films and TV come alive and engage the audience. From stereo to 5.1 surround to 7.1 surround, audio engineers have continued to find ways to increase not just the speaker count but also the actual channel count of the audio track. By having more control over where the sound comes from in the room, engineers can create more realistic and, well, immersive experiences. This has given rise to the label “immersive audio,” a concept that goes beyond a 7.1 speaker layout to a three-dimensional audio experience with audio channels throughout the space.
While such a system has great benefits to audiences, it comes with a number of challenges to sound engineers, especially regarding the space where they create the mix. It is always a challenge for any recording engineer to create an ideal mix that will sound “right” in the variety of spaces where the final audio will be played. This is particularly true for TV and film engineers and most especially in immersive situations where the engineer is attempting to manipulate the perceptions of the physical space and re-create a new environment using sound. While films with immersive audio are often played in large theaters with hundreds of listeners sitting in different locations throughout the theater, post-production re-recording mixers often work in smaller studios that may not reflect the scale of a large format theater.
The goal of the engineer, then, is to work in a space that provides an ideal reference immersive experience, so the audio will have an increased likelihood of being effective for across a variety of different playback scenarios. This reference immersive audio solution should embody the concept of “immersion,” which speaks to diving into a pool of water. You are immersed, and the water is all around you. This is how immersive audio works. Sound envelops you from all around. You are not focused on the speakers, but on the experience.
Done wrong, speaker systems can feel like a sprinkler system, with sound coming at you from distinct directions. However, an immersive sound engineer should ultimately focus on the manipulation of sound in the 3D space, not on sound coming out of a particular speaker channel. When the coverage of the overall system is not consistent in volume and neutral (or “flat”) in its response, engineers will attempt to volume shape or equalize against perceived inconsistencies or flaws that may not exist in the audio itself. This ultimately would make the installation ineffective as a “reference” audio system.
There are a number of speaker technologies that help provide consistent coverage. One of the most important of these is the waveguide, which controls what you hear by affecting what sound levels and frequencies output in which directions. On the outer edges (or “off-axis”) portions of a speaker’s polar pattern, it is not uncommon for speakers with lower-quality waveguides to have very inconsistent frequency responses. In immersive situations, this results in noticeable tonal changes as sounds fade between speaker channels. If the sound moves from a left channel to a location between the left and center channels, the tone shifts and it doesn’t sound consistent, drawing attention and taking the listener out of the experience.
In order for coverage to be more consistent, speakers like the JBL M2 and 7 Series have specially-designed Image Control Waveguides that match the output of the high-frequency driver (or “tweeter”) to the low-frequency driver (or “woofer”). By ensuring high frequencies from the tweeters have the same coverage as low tones from the woofers, the speakers will have a more neutral frequency response at the off axis. In an immersive environment, this means you can add more speaker channels next to each other and still maintain a smooth coverage across all frequencies even as you fade between speakers.
A related element that effects listening consistency is the crossover, the point at which sound shifts from the woofer to the high-frequency driver within the cabinet. For all speakers, but especially immersive environments, speakers need a good crossover that offers a smooth experience. For the sound to be truly immersive, it is vital that you not hear the transition from the low to high drivers, a fact that is more complicated in film and television applications that are driven by dialog rather than music. The majority of dialog sound occurs in midrange frequencies, but the distinction and character of the individual voices often crosses over to low and high frequencies. If a loudspeaker does not have a smooth crossover, it will be very noticeable during dialog, due to where the crossover point will typically fall.
This is also why the JBL M2 and 7 Series are two-way speakers, rather than three-way speakers. While two-way speakers only have a low and a high-frequency driver, three-way speakers add a mid-range driver. Three-way speakers are great for many applications (including cinema), but for reference applications where perfect reproduction is the goal, especially immersive environments, two-way speakers are preferable as there is only one crossover point (low-to-high) rather than two (low-to-mid and mid-to-high). If the high-frequency driver has a wide enough frequency range to support a suitably low crossover point, a two-way speaker can provide a smooth response across all frequencies more readily than a three-way speaker.
Of course, there is a variety of technology both in the speaker and the system as a whole that helps ensure an effective reference immersive system. Having a higher compression driver output out of smaller speaker sizes, for example, allows speakers to be mounted ten feet away or more and still have sufficient volume without being too large. This ensures the system is loud enough but with space for enough speaker channels for an immersive installation. Power handling of the speaker as well as connectivity and signal processing capabilities of the amplifiers are also important, among other factors.
The true goal of an immersive audio solution is to make the audio system disappear. By deploying a solution that offers a consistent experience as you add additional speaker channels, audio engineers can focus their efforts on mixing the perfect experience rather than attempt to mix against audio flaws created by the installation itself.
Do you have experience with immersive audio post-production or broadcast? Tell us about it in the comments.