Understanding the Binaural Hearing Process

Have you ever wondered why we have two ears instead of just one? It’s not simply for symmetry. At Alta View Audiology in Sandy, UT, we understand that binaural hearing is fundamental to your overall hearing health and quality of life.

Binaural hearing refers to the brain’s ability to integrate sound information from both ears simultaneously. When sound reaches your ears, each ear sends slightly different signals to your brain. These differences are crucial for several key functions.

Use of both the ears to perceive the world of sound around us is defined as Binaural hearing. Just as we use two eyes to see in three dimensions, we use two ears for “dimensional hearing”. Binaural hearing is literally opposite of monaural hearing. It allows us to (a) ‘map’ the sound in space, (b) pick out soft sounds, (c) pick out distant sound or speech and (d) separate a single voice from surrounding background noise.

Among the mammals, human is considered to be the one gifted with most developed communication skill. One of the key factors that empowers him with his communication skill is spatial hearing. Spatial hearing provides cues as to the relative number and location of sound sources and objects in the environment, helps in determining the dimension and characteristics of rooms and enclosed spaces, and contributes to the "cocktail party effect", whereby the listeners are able to hear out against other interfering voices in crowded listening environments.

This article aims to help in understanding how the auditory spatial cues arising from individual outer and inner ears are computed and processed at specialized subcortical centres and lead to binaural hearing.

Key Benefits of Binaural Hearing

1. Sound Localization: One of the most important functions of binaural hearing is helping you determine where sounds are coming from. When a sound occurs, it typically reaches one ear slightly before the other. Imagine trying to cross a busy street with only one functioning ear.
2. Improved Speech Clarity in Noise: We’ve all experienced the “cocktail party effect” - the ability to focus on a single conversation in a noisy room. With two ears working together, your brain can focus on sounds coming from a specific direction while filtering out distractions from other directions.
3. Richer, More Natural Sound: Binaural hearing provides a richer, more natural listening experience. When you hear with both ears, sounds have more depth, clarity, and fullness.
4. Reduced Listening Effort: Using both ears requires less mental energy than straining to hear with just one. People with unilateral hearing loss (hearing loss in one ear) often report feeling exhausted after social interactions or work meetings.

The Science of Hearing: How Your Ears and Brain Work Together

How Binaural Hearing Works

When you use both ears for hearing, the term is binaural hearing, you give your brain additional auditory information to process. Better speech recognition occurs in noisy situations, and you can detect sound sources (directionality) through this mechanism. Your right ear transmits sounds to your left brain region before your left ear sends signals to your right brain region. Your brain processes sounds from both sides before the entire brain creates a unified interpretation of the heard information.

Your ability to recognize words and understand speech, along with your capacity to focus on specific sounds or voices, improves through this process. The brain's cooperative function enables selective listening. Your brain uses this method to block out unnecessary noises when you need to listen to a distant voice during meals, at restaurants, or when speaking with family members. The majority of individuals with hearing loss face difficulties hearing in noisy environments.

Binaural hearing enables individuals to hear more clearly by helping to pull what you want to hear out of background noise more successfully. When hearing aids are correctly fitted to both ears, your brain achieves better sound processing accuracy. Using two hearing aids requires adjusting the volume down for each device.

A sound stimulus is distinguished from another by its characteristics of frequency, intensity and time. Thus, two sounds with equal frequency and intensity, originating directly in front of an individual with normal hearing, which arrive at the ears at the same time may be literally indistinguishable from one another. This change could be (a) Inter-aural Time Difference (ITD), (b) Inter-aural Loudness Difference (ILD), (c) Inter-aural Frequency Difference (IFD) and (d) Head Related Transfer Function (HRTF).

The maximum time lag for sound generated at one side of the head is around 0.6 millisec. A trained ear can detect a time difference as slight as 30 microsec. ITD is a major factor in localizing lower frequency sounds of <1.5kHz. That means, for many terrestrial mammals (particularly human) localization of sound source in the horizontal plane is achieved by an exquisite sensitivity to difference in fine time structure of low frequency (<1.5kHz) components between the two ears.

Though the neurones in both the major nuclei of Superior olivary complex, namely Medial Superior Olive (MSO) and lateral superior olive (LSO) are capable of extracting ITD information from binaural inputs, MSO is considered the major site for ITD analysis in mammals. Normal ears use intensity of sound as the most common clue for locating sound sources. The moment a loud sound is heard on one side, a judgment is made that the source of the sound is on that particular side. Judgement is also made about an estimated distance of the source.

British physicist Lord Rayleigh [5], (1907). separated the auditory spatial processing into (a) low tone processing and (b) high tone processing. Distinguishing between the two, he proposed his “Duplex Theory” and suggested that ITD is the primary cue used to localize positions of low-frequency tonal sources (< 2KHz), while ILD is used for high-frequency tonal sources (>5KHz).

Anatomical and physiological studies have revealed two parallel brainstem pathways that appear to encode ITDs and ILDs separately. ITDs are extracted by MSO neurons in which ITD sensitivity results from the coincident arrival of excitatory inputs from the two ears ILDs are extracted by LSO neurons via a subtraction-like process resulting from inhibitory inputs from the contralateral and excitatory inputs from the ipsilateral ear.

Loudness of a sound depends on the number of action potentials triggered by it. It is integrated in brainstem. Each ear contributes substantially to the action potentials that reach the brainstem. The number doubles when the two ears are used instead of one ear for a sound coming from the front of the listener. This is called binaural loudness summation. It is, thus, another measurable dimension associated with binaural hearing.

It is common experience that if one is made to block one ear with an ear plug, the loudness of sound coming from a television in front immediately goes down which is restored as the ear is unplugged. Sound travels in waveforms. A sound wave consists of segments of compression and rarefaction. Number of cycles of compression and rarefaction occurring in a pure tone is defined as frequency.

The sound waves reach the ears either ‘in phase’ or ‘out of phase’. If the compressions of the waves of two pure tone signals arrive at the ears at the same time, they are said to have been ‘in phase’. Whereas if a compression wave arrives at one ear at the same time when a rarefaction wave arrives at the other ear, the two pure tone signals are said to have been presented ‘out of phase’.

A pure tone is received by the two ears either in phase or out of phase depending on the frequency and a complex sound tone is received by the two ears in both phases. Laboratory studies by Joris PX et al have already demonstrated the excellent, and often enhanced (relative to their auditory nerve inputs), phase-locking abilities of globular and spherical bushy cells of the cochlear nucleus that provide the inputs to the Medial Nucleus of Trapezoid Body (MNTB) and the ipsilateral inputs to the LSO.

Tollin et al also examined whether low-characteristic-frequency LSO and MNTB neurons exhibit phase-locked responses to pure-tone stimuli. Between the two ears, head acts as an effective acoustic block, reflecting and diffracting the sounds whose wavelengths are smaller compared to the dimensions of head. Depending on frequency, the sound pressure presented to the ears on either side of the head differs. The difference is related to the location of the sound in the free field.

Whether the sound source is located in the front or at the back, is not uniquely determined by time difference. It is rather determined by the pinna which reflects the sound differently for different positions of sound source in a listening atmosphere. A “Cone of confusion” also exists at each side of the head creating localization ambiguities for points located on the circumference of the cone. This cone is centred on the interaural axis with apex of the cone being the centre of the head. A sound source positioned at any point on the surface of the cone of confusion will have the same ITD values making sound localization difficult.

The head movement aided by reflection of sounds from pinna results in localization of sources and is described as ‘Head Related Transfer Function (HRTF)’. To determine the source location in vertical plane, front or back, the binaural cues of ITD, ILD and IFD fall short of perfection. The auditory system, in addition to binaural cues, hence also exploits HRTF.

Binaural Squelch and Redundancy

1. Binaural squelch: If two sound sources, one giving target signal and the other noise, are sited at the same place and their intensity well adjusted, the target signal will be effectively masked by the noise. When the noise source is moved to a different place, the target signal may become audible again, which indicates a release from masking in relation to spatial separation of the two sources. This effect is called binaural squelch, and is also known as binaural unmasking or Hirsh effect. This phenomenon helps the listener when he is receiving diotic sounds (dichotic sounds are the sounds heard independently by each ear) in any noisy environment e.g. coffee house, party, auditorium etc. In such environment, CNS is able to suppress the interfering noises while focusing on the speech of just one person by binaural selectivity.
2. Binaural redundancy: When the listener is receiving dichotic sounds, both ears send auditory information to the brain independently. Brain, acting as the central processor upon these dissimilar information arriving from each ear, perceives it much better than each ear separately. The listener gets the benefit of fusion of these information arriving from two ears separately and is able to filter out unnecessary parts of information by making them redundant. He not only perceives the sound signals louder, but is also able to understand the speech better. Binaural redundancy actually results in binaural enhancement. The effect of binaural redundancy refers to the improvement in speech reception when the same signal and noise are audible in both ears rather than in one ear alone.

Addressing Hearing Loss and Restoring Binaural Hearing

When hearing loss occurs in one or both ears, the benefits of binaural hearing can be compromised. At Alta View Audiology, we often recommend binaural hearing aids-hearing aids for both ears-even when hearing loss affects one ear more than the other.

Unilateral hearing loss or asymmetric bilateral hearing loss is literally equivalent to absence of good binaural hearing. This disadvantage puts a person under lot of strain. In any simple listening situation where the speech and noise are coming from different sources and are in obvious conflict with each other, the person loses the ability to filter out speech from noise.

Benefits of Binaural Hearing Aids

1. Two hearing aids work together to improve speech clarity, especially in challenging listening environments.
2. Wearing two hearing aids provides a more natural, balanced sound experience.
3. When hearing loss goes untreated, the auditory pathways in your brain can weaken from lack of stimulation-a condition called auditory deprivation.

While it might seem logical to treat only the ear with more significant hearing loss, this approach doesn’t restore the benefits of binaural hearing. Modern hearing aids are programmed to work together, providing appropriate amplification without overwhelming your auditory system.

The improved hearing outcomes and quality of life provided by binaural hearing aids make them a worthwhile investment for most people with hearing loss in both ears.

For most individuals with bilateral hearing impairment, the body of evidence collected across decades of research has also found that the provision of two compared with one hearing aid yields significant benefit for the user.

The level of disability caused by hearing loss is mainly determined by the hearing in the better hearing ear as assessed by pure-tone audiometry. Rehabilitation is required in these cases where a disability loss in the better ear is above 40dB hearing level (HL). In ideal situation with no cost issues, the patients should be habilitated by binaural fitting of hearing aids expecting them to extract the benefit of accurate localization, adequate segregating of competing sounds and better speech intelligibility.

Although, in these patients and in the recipients of cochlear implants, significant asymmetry may continue to appear across the frequency range over a long period of time. Fitting of binaural hearing aids should be such that aim to optimise the delivery of acoustic information and also to preserve the spatial cues.

Similarly, bilateral cochlear implants fitted to deserving candidates should also aim to fulfil the above two objectives as well as provide improved use of signal to noise ratio at the two ears.

Technological Advances in Binaural Hearing Aids

To achieve binaural hearing aid fittings, some data exchange is required between the left and right hearing aids. The rate and amount of data exchange impact the power consumption of the hearing aids and possibly the hearing aid size due to the need for a bigger battery and/or wireless antenna. Therefore, binaural processing might not be available in all form factors (e.g., completely-in-the-canal hearing aids).

Current technology for wireless binaural data exchange uses either near-field magnetic induction (NFMI) or 2.4 GHz wireless technology. Both approaches are robust and reliable. The NFMI approach can be optimized for low power consumption, but it is restricted in transmission bandwidth and increases the design complexity and size of the hearing aids.

In contrast, 2.4 GHz technology offers more bandwidth and reduces design complexity because it can integrate with standard Bluetooth wireless transmission protocols using a single antenna. Therefore, the activation of binaural signal processing should be strategically adjusted to the targeted perceptual benefits for the individual hearing aid user, which will depend on their auditory needs and residual hearing capabilities.

The chosen acoustic coupling and the listening environment also play a significant role in the achievable real-world benefit.

Each hearing aid processes the audio signals received from its own microphones (solid lines), then exchanges information (i.e., parameter data) with the other hearing aid (dashed lines) to synchronize filter parameters or program settings, for example. Depending on the rate and amount of data exchange between the hearing aids, binaural synchronization can offer substantial advantages for the user.

With higher investments in the data exchange rate, data volume, and power consumption, a higher degree of synchronized binaural system behavior can be achieved. Whether the high-rate synchronization of a specific signal-processing algorithm (e.g., noise canceller, beam-former, gain model, and limiting system) is beneficial for a user will depend on the details of its implementation and parametrization.

Additional Benefits of Binaural Hearing

• With binaural hearing, there is summation of intensity of sound, hence hearing requires less loudness.
• A voice that is barely heard at 10 ft with one ear can be heard upto 40 ft with two ears.
• Binaural hearing is less tiring and listening is more pleasant.

At Alta View Audiology in Sandy, UT, our experienced audiologists understand the importance of binaural hearing. If you’re experiencing hearing difficulties in one or both ears, don’t wait to seek help.