How Humans Sense Sound

The human auditory system can detect sounds within the 20Hz to 17kHz range for adults and up to 20kHz for infants, children, and adolescents. Sound is simply a vibration that propagates as a pressure wave moving through air. Sound doesn’t have any spatial information or dimensions, but the human ear is capable of processing sound based on the direction it comes from. 

Your ear can differentiate the relative horizontal, vertical, and distance information when you hear a sound. Your auditory system also processes the difference in sound levels (interaural level difference) and time of sound arrival (interaural time difference) between the two ears. For instance, if a vase falls behind you and to your right (120 degrees), your right ear will hear the crash louder and sooner than your left ear.

The Ear

The pinna is the external ear form with a helical shape, to funnel sound into the ear canal. Without your ear, sounds would enter the ear canal more directly, but with significant hearing loss. The auditory canal extends 2-3 centimeters into your ear and directs sounds to the tympanic membrane, which you know as the eardrum. Your ear canal curves right before the eardrum to protect this highly sensitive membrane and amplify sounds.

After the eardrum, there are three small bones and the Eustachian tube. Sound vibrates your eardrum, which then passes through these bones to the ‘oval window’. Sounds are again transmitted from the oval window to the cochlea and vestibular system. The cochlea has the important job of interpreting sound waves into electrical impulses that your brain can understand. 

What are HRTFs?

Head-Related Transfer Functions (HRTFs) describe the spectral changes and path of sound signals bouncing and scattering towards your head and into the ear canal. Notice the word Functions in Head-Related Transfer Functions, meaning they describe the shape of the ear in a mathematical way. HRTFs are unique to the ear, so no two HRTFs are the same. Headphones can apply HRTFs to offer directional sound experiences. HRTFs are applied to a mono sound source as a filter to dictate where you want the sound to come from. 

Human HRTFs have been estimated from the general human head and ear shape. These global templates are used pretty universally and offer a ‘coarse-grained experience’. Current research is focused on developing new methods to estimate HRTFs for individual high-resolution experiences. 

When it comes to HRTFs, we are most concerned about your outer ear, aka the pinna. Your pinna morphology is highly individualized. Research is trying to determine which morphological features could act as parameters for developing individualized HRTFs. A three-dimensional model is being used to systematically investigate pinna shape and the relative impact of different morphological features.

Binaural Experiences with HRTF

An HRTF can be multiplied by audio frequency to create an incredible 3D audio experience, with directionality. To experience HRTFs you must be wearing headphones. Not all headphones meet the mark though. Unfortunately, the science of headphones has not come as far as speakers. Even within the same manufacturer consistent tonal precision needed to get the HRFT tonal alteration right is uncommon. But the best way to start enjoying three-dimensional audio experiences with some degree of precision, is to pick up a pair of neutral headphones. Neutral headphones deliver every frequency at the same level. Bass, treble, and mid frequencies are experienced equally. Neutral headphones are the best way to experience an artist’s original sound without the addition of a headphone brand’s sound signature. 

Video Games and VR

Head-Related Transfer Functions have the capacity to extend into all kinds of audio experiences. Positional audio has been added to video games like Valorant. Valorant is a great game to include HRTF features because it’s a fast-paced shooter, where situational awareness can make the difference between winning or losing. Within Valorant you can hear an opponent approach you from behind and tell which direction the gunfire is coming from like you are actually there. This creates a more immersive experience and elevates the player's gameplay. 

Even generalized HRTFs can provide enhanced auditory spatial awareness within a virtual reality environment. However, a truly immersive VR experience won’t be realized without individualized HRTFs. 

Experience it Yourself

Want to experience a three-dimensional audio experience to learn what all the fuss is about? Remember to use headphones, close your eyes, turn the volume up, and sit back and enjoy. Check out the audio experiences below: 

1. Virtual Barber Shop

   
   

   
   

2. Virtual Dentist Binaural Recording

   
   

Apple has not yet developed an individualized HRFT experience, but it’s only a matter of time. Apple launched spatial audio in June 2020, which is a combination of 5.1, 7.1, and Dolby Atmos, plus a generalized HRFT to create a theatric listening experience for movies. You can enjoy the spatial audio experience with AirPods 3, AirPods Pro, and AIrPod Max. The earphones will gather information on your head movement and positioning relative to your viewing screen, to keep the audio centered around the screen even when you move about or turn your head. Apple’s spatial studio is a hit and great for watching movies at home, while still experiencing the sounds all around you. 

Personalized HRTFs

Conventional methods for creating individualized HRTFs are time-consuming and mostly impractical. Small changes in an individual’s pinna morphology can create large impacts on HRTFs, limiting the immersive 3D experience. Typical HRTF personalization requires sitting inside an anechoic chamber. With microphones in both your ears, speakers will rotate around you and play sounds at all angles, while the microphones capture the sound. 

Dolby has added HRTF support to their PHRTF Creator app, available on iOS devices. Dolby wants to provide an enhanced binaural experience that is only possible with personalized HRTFs. Non-personalized HRTFs depend on average physical characteristics, that may not apply to your unique ear and head shape. The PHRTF Creator app takes measurements of your ears and will create a Head-Related Transfer Function that is unique to you. Your shoulders, ears, and head are scanned and the app collects roughly 50,000 points to create a high-resolution acoustic map. 

HyperX, a gaming peripherals company, has included HRTF personalization in their top-of-the-line Cloud Orbit gaming headset. The peripheral software includes an HRTF Personalization section that requests your head circumference and interaural arc (that you must measure yourself using a measuring tape). This is an extremely low-resolution personalization of Head-Related Transfer Functions, but it’s great to see companies trying to provide an individualized audio experience. 

Recording Binaural Audio 

Experimenting with binaural audio started in the 2010s and has been rapidly increasing since. As the interest in this area grows, binaural audio projects, like BiLi Project backed by 9 French institutions including Radio France, become more common. According to Markus Noisternig with the Institute for Research and Coordination in Acoustics/Music or Ircam, the BiLi project’s goal is ‘improving the entire production chain–recording, streaming, decoding –for providing binaural 3D audio content to broadcasters.’

With the Neumann KU 100,  a $9,000 dummy head microphone, you too could get started recording binaural audio. It’s capable of producing highly-realistic three-dimensional binaural audio because of the microphones within each dummy ear. Using the Neumann KU 1000 for recording results in a sound image that matches the original recording location sound cues. This microphone is beyond the budget of most hobbyists, but its common applications are radio dramas, classical recordings, and experimental ambient recordings. 

Wrapping UP HRTF

HRTF adoption is expected to continue to climb as digital experiences such as movies, tv shows, virtual reality, and gaming become more and more mainstream. Companies investing in these digital experiences have a stake in making sure listeners have a great immersive experience and want to continue enjoying their content. But generalized HRTFs stand in the way. Based on the general human ear shape, HRTF templates work well for some users, but horribly for others. Your shoulders, head, and ears all contribute to how you perceive sound in a three-dimensional space. To create an accurate binaural experience, HRTFs should be tailored to your own head and ear shape. Research is focused on identifying methods of accurately and efficiently creating individualized HRTFs for the best experience possible.