Spatially Enhanced Audio for Themed Entertainment
By: Michael King, Senior Design Engineer, and Joseph White, President and Principal Designer
What is Binaural Audio?
Many of us over the years have had some experience with recorded and reproduced binaural audio, mostly with headphones and select recordings that immerse you in a fully 3D sound field. Binaural audio is just how we as humans experience the world around us every day. Technically, the word “binaural” breaks down into ‘bi’ meaning two, and ‘aural’ relating to the ear, or sense of hearing. When we think of two-channel audio, one imagines the panning of instruments, voices, or sound effects from the left ear to the right ear. But binaural sound includes the ability to move the sound in all directions and at a distance - up, down, left, right, far away, or even as close as the intimate whisper on the back of your neck that makes your skin crawl.
Binaural audio has several components inherent to the process of recording and reproduction that create the fully 3D stereo effect: timing, loudness, and timbre. When sound arrives at one ear, it takes a few microseconds to reach the other ear. Your head and body also muffle and reflect the sound in a predictable way, changing the perception of frequencies, timing, and loudness arriving at each ear. Then, just like viewing a 3D movie from images fed separately into each eye, your brain “assembles” the audio from both ears in such a way that you perceive a realistic, immersive sound experience.
How Its Created
The year was 1881 when French inventor Clément Ader demonstrated a device known as the Théâtrophone, a distribution system for audio that would bring opera and theater performances over telephone lines to a remote listener, where a set of phone receiver pieces could be held to the ears to experience the performance at home or other remote locations. At the 1933 World’s Fair in Chicago, Bell Laboratories brought a dummy head with embedded microphones in its cheeks named Oscar, positioned in a glass room, while listeners could put on headphones and listen and watch what Oscar was experiencing remotely. It wasn’t until the 1970’s when the recording and microphone technology were able to create the first commercial binaural recordings. Technology has continued to improve in binaural audio recording, with many companies now producing high-quality binaural recording microphone setups.
In addition to recording binaural audio directly, the increasing power of computer digital audio workstations (DAW) and digital signal processing (DSP) software plugins have now been developed to process and place any recorded source in place a binaural sound environment. These plugins use customizable algorithms to reproduce an environment around a virtual listener. Two main parameters are critical to these plugins: the Head-Related Transfer Function (HRTF) and the Binaural Room Impulse Response (BRIR). The first of which, the HRTF, is essentially the “data” on how sound behaves around the head and torso of a human body, applying different effects in frequency, timing, and loudness on the sound file. In addition to getting these parameters correct, the sounds must also be placed in an environment, and that’s where the BRIR comes in – reproducing a real or imagined space’s reverb and delays from different objects around the space. These plugins give ultimate flexibility in placing the listener and the sound sources in any imaginable environment.
How Is It Reproduced?
One of the main criteria for sound binaural reproduction is the auditory separation between the left and right ears, with each ear only hearing its own signal. With traditional sound from stereo nearfield speakers, most of the sound heard from each ear corresponds to its nearest speaker, with significant overlap from the adjacent speaker. This is why headphones are often touted as the best way to reproduce binaural sound, as each ear has its own isolated sound field - but there is another way.
Placing high-quality, small speakers close to the listener’s ears, makes it possible to achieve a near-perfect binaural reproduction, but only if the speakers adhere to a few constraints. Because small speakers placed close to the listener do not have to be driven loud (thanks to the inverse square law), sufficient isolation can be maintained. To achieve full-range sound with a dynamic soundtrack, tuned nearfield subwoofers should accompany the full-range playback drivers. Most of our sense of directionality comes from the mid and high-frequency range, as lower frequencies tend to pass through and around objects in the real world without change to the original sound, making their use for localization negligible.
The use of multiple-source drivers (such as traditional two-way car audio speakers used in many attractions) can very quickly drift in both phase and frequency response as the listener moves off-axis from the “sweet spot”; this is not only bad for binaural reproduction, but even standard multi-channel audio playback. Many of these two, and three-way drivers also use poor quality, inexpensive polarity inversing crossover circuitry, further hindering the ability for proper, consistent frequency response, and intelligible sound reproduction.
In a non-headphone binaural reproduction setup, the listener must also not move too significantly during the experience, as too much offset from the center point of sound would pull them out of the binaural effect, even with the best of speakers. Thankfully, in the themed entertainment industry, it is easy to predict where a rider or theatergoer is going to be seated throughout an experience because of seat restraints or fixed seating positions for a show, which makes binaural audio reproduction without headphones a possibility.
How Can It Be Used?
At the heart of themed entertainment is immersion; Walt Disney once said, “I don’t want the public to see the world they live in while they’re in the Park. I want to feel they’re in another world.” In the ever-changing and constantly improving world of themed entertainment, operators are always attempting to bring new and unique experiences into their parks, to improve on the captivating story told throughout their attractions and parks.
On a dark ride, binaural audio can be used to enhance each scene and even guide the viewer to look in a particular direction to catch a glimpse of the action. In a theater stage production, binaural audio may be used to enhance environmental effects or even move performers virtually around the audience, creating an additional dimension to the show. Binaural audio also can have applications throughout park common areas; think of a fixed, binocular-like lookout point with integrated binaural audio and augmented reality overlays in the viewfinder, transporting a guest into the mythical world of the unseen.
We at Crafted believe that audio, especially binaural audio, is a novel tool to give guests something they haven’t experienced before in a theme park and could never get at home. It is even more unique than virtual reality, offering not just a single field of view into a world but a complete 360-degree auditory experience like none other. Combining the use of binaural audio in an attraction with practical, digital, or other virtual effects can anchor a guest to the scene like no other technology combination. Quoting Walt Disney again, “it’s kind of fun to do the impossible.” We want to bring your stories to life.
To learn more about how you can use binaural audio to enhance your guest experience, visit our website: www.craftedav.com