Written by: Lauren Lawson
About the Author: Lauren is a junior at USC, studying biomedical engineering. In her free time she volunteers for the Make-A-Wish USC chapter, participates in the executive boards of multiple on-campus organizations, and is in Alpha Gamma Delta.
Introduction
Imagine the ultimate headphone experience: just like listening to your favorite song at the front row of a concert. Drums slowly build from the back of the stage; the guitarist strikes a chord to your left; the lead singer’s voice is front and center. It’s almost as if you’re there. You see, communicate, touch, and interact with a 3-dimensional world every day, so shouldn’t your headphones also give you a 3D experience? Although the technology is possible, the reality today is that most widely-produced headphones on the market do not have 3D audio. The inability to hear sounds in different locations hinders the listening experience that music, movie, and virtual reality game producers intend the consumer to have. However, with the help of acoustic engineers, the ultimate headphone listening experience is closer than you think.
3D audio headphones are currently on the market as tech companies scramble to stay on top of the latest technology; but just how do these pieces of equipment work? Why are they just now being released on the tail end of the 3D era? Also, why are they so expensive? In order to fully understand the complicated inner workings of 3D audio headphones, one must first understand how sound is processed in the human ear.
Processing Sound Through the Ear
When a sound wave travels into someone’s ears it is altered by interaction with the listener’s head, torso, outer-ear, and external surroundings (such as walls). Although both processed by the brain, the waves that enter the left and right ear are slightly different. They are referred to as binaural sound [1]. Every binaural sound is a unique wave that you can locate; even as you move the brain is intuitive enough to remember the location of a binaural sound.
Timing is a factor that plays into the individuality of binaural noise. Sound waves hit your ears at different times, depending on the position of your head relative to the sound source. Timing can help your brain locate the position of an object (Fig. 1).
Figure 1 shows the transmission of sound waves from the laptop to the girl’s ears. The arrow on the left shows the path that sound waves will travel to enter her right ear. This path is much shorter than that of the left ear. The brain is able to register that sound waves are hitting her ears at different times, and can deduce that the laptop noise is closest to the girl’s right ear. The brain is also able to distinguish the left ear sound wave alteration since it hits the wall.
Each binaural sound that enters your ears can be quantified by a head-related transfer function (HRTF): a mathematical equation that identifies the unique relationship between the original soundwave and the manipulated wave that enters your ear. Figure 2 shows the frequency differences between the same sound being played in different positions relative to the ear. The differences shown on the graphs can be quantified by HRTFs. There is an HRTF for each individual ear [2]. The ability to relate an original sound wave to a binaural sound allows technology to mimic the manipulation of sound waves by an outside environment.
How Current Headphones Work
The problem with current headphones is not the quality of the sound, it’s how the sound is delivered to the listener. Figure 3 shows how sound waves travel from the headphone to the ear.
The manipulation of sound waves tells you where a noise is coming from. If a sound wave were to travel directly into your ear with no distortion your brain would not be able to process the location of the noise source. Sound waves travel directly from a speaker within the headphones straight into your ear (Fig. 3), leaving the brain with no spatial information to process.
Consequences can arise from prolonged non-binaural sound exposure, the most serious of all being listening fatigue. The brain receives spatial signals in daily life, and traditional headphones do not provide this spatial component of sound. As you hear audio from a headphone, the brain is over-stimulating the ears trying to make sense of the sound waves’ locations [3]. Strain has been known to cause fatigue, ear pain, and loss of hearing sensitivity. Prolonged periods of listening fatigue can even induce permanent hearing loss.
Listener dissatisfaction is another downside to audio from traditional headphones. With the rise of immersive technology through virtual and augmented reality, there is a new standard of entertainment experience that most headphones just aren’t providing. Name-brand over-the-ear headphones range from $200-$300, a price that is hard to justify considering the lack of technological advancement in comparison to the visual experience of games, movies, and television. Overall, there is a disconnect between the audio and video aspects of new technology that leaves consumers feeling underwhelmed with their purchase.
The 3D Difference
Headphones that have 3D audio effects are made tailored to each customer. Because binaural noise is processed based on the shape of your body, head, and outer-ear, the sound coming from these headphones must mimic spatial cues that your brain is used to processing. This is where audio engineering comes into play- more specifically, acoustic engineering. Acoustic engineers study the construction and manipulation of sound waves [4].
Within the engineering of 3D audio headphones there are two paths engineers can take: they can either reconstruct the interior shape of a headphone so that sound waves enter the ear as they would from different locations, or a personalized algorithm can be created that manipulates sound waves as they exit the headphone. HRTFs are able to quantify the relationship between regular and manipulated sound waves, which tells acoustic engineers how sound waves must be changed within the headphone. Thus, an algorithm can be specially rafted for individuals and packaged in a 3D audio headphone set. While this customized technology is expensive, it is much less time-consuming than completely rearranging the inner geometry of a pair of headphones for each customer.
The use of programs, such as MorrowSoundTrue3D, to create 3-dimensional audio in games and speakers is already taking place. This technology is included in microchips that convert non-binaural waves to binaural sound produced in natural environments [5]. The manipulation of sound waves can be achieved through the use of pre-programmed algorithmic functions that change the frequencies of audio as it is streamed from a device. The change of frequencies, pitches, and volumes of each sound wave is controlled mathematically.
3D audio must be mobile, meaning that the noise locations are able to adapt to head movement. An accelerometer, or device that measures one’s acceleration, is a small chip that can also be placed within a pair of headphones. This device will create an electric signal when linear movement takes place and will adjust the audio in the headphones accordingly.
Accelerometers use micro-electrical mechanical systems, or MEMS, to determine the linear movement of an object. Stationary silicon plates are set up on a small chip, and between those plates are mechanical springs that react to movement. A change in capacitance between the springs and plates creates an electric current that corresponds to the acceleration of the chip [6]. This current is sent to the microchip that distorts the audio through a series of wires within the headphones.
Vibrational gyroscopic sensors calculate rotational change within the headphones, and also offer an electric signal that can alter the audio. Rotational force applied to the sensor when the head turns changes the direction in which the gyroscopic sensor is vibrating. This difference in vibration is detected by the sensor, and is converted from kinetic energy to an electric signal [7]. This signal is also sent to the micro-chip to be alter the audio before it reaches the listener’s ear.
Products Out Now
Currently there are a few 3D audio headphones commercially available to consumers, for a fairly expensive price. Brands are pricing this technology anywhere between $275-350. These prices are near those of professional-quality traditional headphones. 3D Sound Labs has created a pair of headphones that encompass the standard of 3D audio on the market currently. As this technology becomes more common with music, movies, and virtual reality games, the price will decrease over time.
Looking Ahead
While 3D audio enhances the virtual reality experience in games, movies, and music, it can also be used to enhance the training of professionals in their career fields. For instance, medical schools are investing in simulations that allow residents to practice procedures. Architects can use VR to create buildings and structures. Future pilots can use flight simulators to learn the controls and technology behind flying aircraft. The implementation of this technology causes a need for 3D audio headphones to completely immerse the user in the virtual scenario. As the technology becomes less expensive the inclusion will continue to increase. The possibilities are endless, but for now just sit back and imagine a live performance from your favorite band- at the touch of your fingertips.
3D Sound Labs Website: http://www.3dsoundlabs.com/
References
[1] P. Geluso, A. Roginska. (2017, October). Immersive Sound [Online]. Available: http://proquest.safaribooksonline.com.libproxy2.usc.edu/book/audio/9781317480105
[2] B. Xie. (2013, July). Head-Related Transfer Function and Virtual Auditory Display [Online]. Avaiable: https://ebookcentral.proquest.com/lib/socal/reader.action?docID=3319556&query=
[3] (2016, June). 3D Audio on Headphones: How Does it Work? [Online]. Available: https://www.waves.com/3d-audio-on-headphones-how-does-it-work
[4] B.A. Auld. Acoustic Fields and Waves in Solids [Onine]. Avaiable: https://books.google.com/books?hl=en&lr=&id=_2MWAwAAQBAJ&oi=fnd&pg=PA33&dq=acoustic+engineering&ots=sCB_aenIe3&sig=B2glDTJuoYXUTVvkdTbAZbDFTOc#v=onepage&q=acoustic%20engineering&f=false
[5] Morrowsound Technical Info [Online]. Available: http://morrowsound.com/tech.html
[6] B. Baker. (2018, January). MEMS as Accelerators [Online]. Available: https://www.digikey.com/en/articles/techzone/2018/jan/apply-sensor-fusion-to-accelerometers-and-gyroscopes[7] Gyro Sensors- How They Work and What’s Ahead [Online]. Available: https://www5.epsondevice.com/en/information/technical_info/gyro/
