Binaural hearing is the ability to process sound in 3D space. Binaural hearing consists of localization: the ability to find the location and origin of a sound in an environment. Additionally, binaural hearing includes the ability to isolate speech in noise. Known as the “cocktail party effect”, it is the ability to selectively focus on one person’s speech in a noisy environment. Both abilities are crucial for social connection and communication, learning, and functional living. Therefore, it is imperative that researchers’ study how to improve spatial hearing in those with hearing deficits.
Below are examples of binaural hearing in everyday life:
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Localization
Crossing the street
You are walking across the street, and you hear someone shout “Look out!” from your left ear. Quickly, you look to your left and stop as a biker passes by. Binaural hearing allows us to avoid potential hazards by allowing us to attend to sounds from a specific direction and informing us if a sound is getting closer or farther away.
Finding a phone by ringtone.
You misplaced your phone, and while you were sure you placed it on the bedside table, you can’t seem to find it anywhere. Naturally, you ask your friend to call it for you. Using localization, your ears are able to locate a muffled ringing sound coming from the living room. As you slowly walk around the room, your ears are able to adjust to the sound and pinpoint that it is coming from the couch and that it is under a cushion.
Speech in noise
Listening to a friend in a restaurant
You and your friends are at a crowded restaurant. People are laughing, orders are being taken, and the live band is warming up on the stage to your left. By all accounts, it is loud. But somehow, even with all the noise, you are able to lean a little closer and tune out the background noise and focus your attention on your friend recounting every detail of her family vacation.
Airport Navigation
You are sitting at your gate in an airport. The environment is noisy — filled with people talking and random announcements. Suddenly, you hear over the intercom that your flight’s gate has been moved. All passengers are required to move to a different gate in order to board. Binaural hearing allows us to not only hear the intercom announcement, but also isolate the sound from the background noise. Without it, you may not be able to differentiate the announcement from the other noises in the airport, possibly missing your flight.
How does Binaural Hearing Work?
Binaural hearing occurs when our two ears work together to process complex sound environments, such as a busy restaurant or orchestra. When sound waves are produced, they reach each ear at different times and intensities (loudness). Our two ears can detect the tiniest differences in sound arrival and intensity, sending signals to the brain letting us know the location of the sound and which sounds to target. During binaural hearing, the auditory system looks for two main cues: Interaural Time Difference (ITD) and Interaural Level Difference (ILD).
Interaural Time Difference (ITD) is the difference in sound intensity (loudness) between the ears. Similarly, sound intensity depends on where the sound is produced. Intensity will be higher in the ear closer to the sound, while the other ear will experience less sound intensity.
Interaural Level Difference (ILD) is the difference in sound intensity (loudness) between the ears. Similarly, sound intensity depends on where the sound is produced. Intensity will be higher in the ear closer to the sound, while the other ear will experience less sound intensity.
Example of ITD and ILD
In Figure 1 (left), sound is coming from the right side of the head. The sound is reaching the right ear first because the path is shorter. It then moves on, wrapping around the head and into the left ear. The brain processes the signal in the right ear first, and then the left
In Figure 1 (right), the sound is coming from the right side. Sound waves reaching the right ear are perceived as louder because they are closer to the source and are not obstructed by the head. Conversely, our left ear experiences a quieter sound because the waves are blocked by the head: a phenomenon known as the Head Shadow Effect.
By processing these ITD and ILD cues, our brain is able to determine that the sound is coming from the right side in this example.
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Figure 1: The difference between ITD (left picture) and ILD (right picture) cues for sounds produced on the right side of the head.
Spatial Release from masing (SRM):
There is an auditory phenomenon called spatial release from masking (SRM). SRM occurs when competing sounds coming from two different locations (spatially separated) make it easier to separate the target sound from the background noise. The further apart the sounds are from each other, the easier it is to hear the individual sounds. Conversely, if the sounds are presented closer together (spatially), it is harder to tell the sounds apart. SRM is particularly important for speech recognition, especially in environments with multiple talkers. SRM is important to learning environments like classrooms and crowded social environments like restaurants. The Binaural Hearing and Speech Lab researches how individuals with Cochlear Implants benefit from SRM compared to typical hearing individuals.