Exploring Interaural Time Differences and Interaural Level Differences
Table of Contents
Introduction: Have you ever wondered how we can locate a sound source? The answer lies in our remarkable ability to perceive interaural time differences (ITDs) and interaural level differences (ILDs). In this blog post, I will give an insight into psychological acoustics, which is concerned with human sound localisation in the three-dimensional (3D) sound field. We will delve into the fascinating world of sound localisation, exploring the concepts of ITDs and ILDs and their crucial role in determining the spatial origin of sounds.
But before we take a closer look at 3D audio, let’s understand the basic elements of human sound perception. In this way, it will be possible to gain a better understanding of how 3D audio actually works.
Information on the anatomy of the human auditory system and how it functions can be found here: https://www.immersive-music-production.com/introduction-to-human-perception-of-sound/
3D audio (also: immersive audio or spatial audio) means sound coming from ALL directions around the listener. This means front and back, left and right, and most importantly, up and down. This is our normal state of natural hearing in an air environment.
Immersion in audio and acoustics refers to the psychological feeling of being surrounded and completely enveloped by certain sound sources and ambient sounds. And you can achieve this state of mind by listening to music in Dolby Atmos or by using headphones in the binaural format.
Binaural literally means ‘two ears’. With binaural hearing, a person can accurately determine the direction and the origin of sounds.
A binaural signal helps us understand the difference between the signals received by each ear. The three most important binaural parameters are the level difference and the time difference between the different frequencies of two signals. These are the interaural level difference (ILD) and the interaural time difference (ITD). The third factor for directional analysis is the difference in the timbre of the sounds.
Binaural signals are important for determining the direction of sound in the horizontal plane (“azimuth”). For example, if a sound source is on the right side of the head, the right ear will receive the sound immediately, while the left ear, i.e. the opposite ear, will receive it after a certain time. This difference is due to the distance between the two ears. In addition, the left ear receives the sound with more head shadow because the signal is deflected and reflected by the head, torso, auricle, etc. Essentially, the human ear recognises the direction of sound on a horizontal plane by the time difference (ITD) and the level difference (ILD).
Binaural cues do not give a complete picture of sound location because the human ear is parallel to the horizontal plane. In addition to binaural cues, humans also use monaural cues to determine the location and origin of a sound in space. Specifically, a monaural signal is used to determine the sound level because the frequency characteristics of an input signal vary with the angle of elevation.
Spatial hearing and sound localisation
Auditory perception is a complex phenomenon that cannot be discussed in its entirety here. As already mentioned, this perception is determined by the physiology of the auditory organ. But cognitive phenomena, i.e. the ability to perceive signals from the environment, to process them in the brain and to learn their variable sound properties, also have an influence.
To recap briefly: The position of a sound source is described by its direction and distance. In spatial hearing and directional perception, a distinction is made between the horizontal plane (“azimuth”), the vertical plane (“elevation”) and the frontal plane (“distance”). Differences in intensity, duration and tonality are crucial for accurate localisation. The position of the sound sources affects both binaural hearing – in the horizontal plane – and monaural hearing – in the median plane.
Interaural Time Differences (ITDs):
ITDs are caused by the fact that sound has to travel an extra distance. This means that the sound reaches the ear facing the sound source earlier than the other ear. This time delay provides vital information to our brains, allowing us to localize the sound source in the horizontal plane. Measured from a 90° angle, this is approximately 65 ms. The smallest perceptible difference is therefore 0.03 ms. This corresponds to a change in direction of 3°. The time difference as a localisation criterion is only significant in the frequency range between 100 Hz and 1600 Hz.
The brain analyses the phase differences between the sound waves received by each ear to calculate ITDs. The auditory system is extremely sensitive to these microsecond differences, which are analysed in the brainstem and auditory cortex. This allows our brain to determine the azimuth, or left-right location, of the sound source by comparing the arrival times of the sound at each ear.
Interaural Level Differences (ILDs):
ILDs refer to the differences in sound intensity or level between the two ears. If a sound source is off-centre, it will be louder in the ear closest to the source and softer in the other ear. ILDs provide valuable cues for sound localisation, particularly in the vertical plane.
In concrete terms, this means that differences in sound intensity (ILDs) in the horizontal plane are caused by the sound shadow of the head. At a lateral sound incidence of 90°, the sound on the opposite side of the ear is perceived as 7 dB quieter. Music is perceived as 7 to 10 dB quieter, depending on the frequency.
The shape of our head and the position of our ears create a unique acoustic filtering effect. As sound waves approach the head, they are diffracted and partially blocked, resulting in variations in sound intensity reaching each ear. The brain analyzes these ILDs, primarily for high-frequency sounds, to determine the elevation, or up-down location, of the sound source.
Combining ITDs and ILDs:
The brain combines the information obtained from ITDs and ILDs to create a comprehensive representation of sound localization. By integrating the precise timing differences and level disparities between the ears, our auditory system can accurately determine the three-dimensional position of a sound source in space.
Between 300 Hz and 1.6 kHz, time and intensity differences are evaluated, so that above 2 kHz, only intensity differences are important for locating a sound source.
Pitch difference or the timbre of a sound describes the phenomenon of sound being muffled on the side away from the source. This is because the head is a natural barrier to sound at higher frequencies. These frequencies are reflected off the head. The low frequencies are diffracted by the shape of the head, but still arrive on the other side.
The vertical plane
The most important indicator the human ear uses to determine the level of a sound source is the monaural spectrum, i.e. the change in frequency content of the sound in question. It is determined by its interaction with the shape of the pinna. The absence of an auricle and its cues, on the other hand, affects the accuracy of position perception.
The frontal plane
Distance estimation in the frontal plane is based on three main factors. Firstly, the quieter the signal, the further away it is. Secondly, the direct sound is amplified by reverberation and reflections as the distance from the sound source increases. Thirdly, differences in sound are perceived the further away the sound source is. The greater the distance, the fewer high frequencies reach the ear.
Challenges and Limitations:
While ITDs and ILDs play a significant role in sound localization, there are certain limitations to their effectiveness. For instance, ITDs are more useful for low-frequency sounds since higher frequencies tend to be absorbed and scattered, making timing disparities less discernible. ILDs, on the other hand, are more effective for high-frequency sounds, as they are affected by the head’s acoustic shadowing.
Moreover, sound reflections and complex auditory environments can distort ITDs and ILDs, making localization challenging. Our brains employ various mechanisms, such as spectral cues and head movement, to overcome these challenges and extract accurate spatial information. Try standing in an unfamiliar room with your eyes closed. Determine the size and shape of the room just by your voice and its reflections. It is amazing.
Applications and Significance:
The understanding of ITDs and ILDs has significant implications across various fields. In fields like audio engineering and virtual reality, precise sound localization enhances the immersive experience by creating a realistic audio environment. In addition, knowledge of ITDs and ILDs helps in designing hearing aids and cochlear implants, enabling individuals with hearing impairments to better perceive the spatial aspects of sound.
Interaural time differences and interaural level differences are integral components of our sound localization abilities. By perceiving the subtle time delays and intensity variations between our ears, our brains construct a spatial map of the world of sound. Through ongoing research and technological advancements, we continue to deepen our understanding of these mechanisms, unraveling the secrets of how we perceive and navigate the auditory landscape.