How CROS Hearing Aids Work: The Science and Technology Explained
| Evolution of CROS Hearing Aid Technology | ||||
|---|---|---|---|---|
| Generation | Time Period | Transmission Method | Signal Processing | Key Features |
| First Generation | 1960s-1980s | Wired connection behind the neck | Analogue amplification with basic tone control |
• Basic sound rerouting • Visible wiring • Limited adjustability • Large battery consumption |
| Second Generation | 1980s-1990s | Radio frequency (RF) transmission or magnetic induction | Early digital processing with programmable settings |
• Wireless transmission • Smaller form factors • Limited noise management • Programmable via external devices |
| Third Generation | 2000s-2010 | Near-Field Magnetic Induction (NFMI) | Multi-channel digital processing with adaptive features |
• Improved wireless reliability • Directional microphones • Active noise reduction • Multiple listening programmes |
| Fourth Generation | 2010-2017 | Enhanced NFMI and early 2.4 GHz technology | Advanced digital signal processing with environmental adaptation |
• Automatic programme switching • Wind noise management • Feedback cancellation • Improved battery efficiency |
| Current Generation | 2017-Present | 2.4 GHz Bluetooth and proprietary protocols | AI-powered processing with machine learning adaptation |
• Smartphone connectivity • Rechargeable batteries • Real-time environment classification • Remote fine-tuning capabilities • Audio streaming from devices |
How CROS Hearing Aids Work: The Science and Technology Explained
Introduction
Modern hearing technology has revolutionised solutions for people with asymmetric hearing loss. Among these innovations, CROS (Contralateral Routing of Signals) hearing aid systems stand out for their unique approach to addressing single-sided deafness. Rather than attempting to restore hearing in a non-functioning ear, CROS technology employs a fundamentally different principle: rerouting sound from the impaired side to the functioning ear.
This comprehensive guide explores the intricate workings of CROS hearing aid systems, examining the technological mechanisms, signal processing methods, and the scientific principles that enable these devices to overcome the challenges of single-sided hearing.
The Fundamental Principle: Sound Rerouting
At its core, CROS technology is built on a simple yet ingenious concept: capturing sound from one side of the head and delivering it to the opposite ear. This approach addresses the primary challenge faced by those with single-sided deafness—the inability to perceive sounds originating from their impaired side.
Overcoming the Head Shadow Effect
The human head creates an acoustic barrier between our ears, known scientifically as the "head shadow effect." This natural phenomenon:
Attenuates high-frequency sounds by up to 15 dB when they must travel around the head
Creates interaural time differences (ITDs) as sounds reach one ear before the other
Produces interaural level differences (ILDs) with sounds being louder at the ear nearer to the source
For individuals with normal hearing in both ears, these differences provide crucial spatial cues that help with sound localisation. However, for those with single-sided deafness, the head shadow effect presents a significant barrier to hearing sounds from their impaired side.
CROS technology effectively eliminates this barrier by bypassing the physical obstacle of the head, creating an electronic pathway for sound to reach the functioning ear regardless of which direction it originates from.
The Technical Components of CROS Systems
Modern CROS hearing aids comprise several sophisticated components working in harmony to capture, process, and deliver sound:
1. Microphone Technology
The journey of sound through a CROS system begins with advanced microphones positioned on the device worn on the impaired ear:
Directional Microphones: Many systems utilise multiple microphones that work together to capture sound with directional sensitivity, preserving important spatial information.
Adaptive Directionality: Advanced systems can automatically adjust microphone directionality based on the acoustic environment, focusing on speech in noisy situations or providing wider awareness in quieter settings.
Wind Noise Reduction: Specialised microphone designs and protective covers minimise the impact of wind, which can be particularly problematic for hearing aid microphones.
These microphones are calibrated to mimic the natural sound-gathering properties of the human ear, capturing sound with fidelity to its original characteristics.
2. Digital Signal Processing
Once sound is captured by the microphones, it undergoes sophisticated digital processing:
Analogue to Digital Conversion: The acoustic signals are converted into digital information, allowing for complex manipulation and enhancement.
Noise Reduction Algorithms: Computational processes identify and reduce unwanted background noise while preserving speech and important sounds.
Sound Classification Systems: AI-powered technology identifies different listening environments (conversation, music, outdoors, etc.) and optimises processing for each scenario.
Frequency Shaping: Digital filters modify the frequency response to enhance speech intelligibility and listening comfort.
This digital processing occurs within miniaturised computer chips capable of performing millions of calculations per second, all powered by tiny batteries.
3. Wireless Transmission Technology
The processed sound information must then be transmitted from the impaired side to the device on the functioning ear. This critical step has evolved dramatically over time:
Early Systems: First-generation CROS aids used physical wires running behind the neck, later replaced by inductive neck loops.
Near-Field Magnetic Induction (NFMI): Many modern systems use magnetic field technology for reliable, low-power transmission between the ears.
2.4 GHz Technology: Advanced systems utilise the same radio frequency band as Bluetooth, offering extended range and higher bandwidth for better sound quality.
Proprietary Protocols: Manufacturers have developed specialised transmission methods optimised for low energy consumption, minimal delay, and resistance to interference.
This wireless transmission happens in near-real-time, with delays typically under 5 milliseconds—imperceptible to the user.
4. Receiver Integration
The final component in the signal path is the receiver on the functioning ear:
Sound Integration: The wireless receiver combines the transmitted signals from the impaired side with sounds directly captured at the functioning ear.
Binaural Coherence Processing: Some advanced systems coordinate the signals to maintain a more natural relationship between sounds from different directions.
Amplification Control: In BiCROS configurations, this component also manages the amplification of sounds for the better ear with hearing loss.
The Evolution of CROS Technology
The working principles of CROS hearing aids have undergone remarkable evolution since their introduction in the 1960s:
First Generation: Wired CROS
Physical connection between ear pieces via wire behind the neck
Analogue signal processing with limited customisation
Bulky designs with obvious visibility
Limited directional capabilities
Second Generation: Early Wireless
Elimination of the connecting wire for improved aesthetics and comfort
Introduction of digital signal processing
Improved sound quality and noise reduction
Still significant size constraints and battery limitations
Third Generation: Digital Wireless
Sophisticated digital signal processing with multiple channels
Improved wireless protocols with greater reliability
Smaller, more discreet designs
Introduction of environmental classification systems
Current Generation: Smart CROS Systems
Seamless connectivity with smartphones and other devices
AI-driven signal processing for optimised performance
Ultra-low-energy wireless protocols for extended battery life
Automatic adaptation to different acoustic environments
Rechargeable battery options eliminating the need for battery changes
This evolution reflects the continuous advancement in wireless technology, digital signal processing, battery technology, and miniaturisation that has transformed CROS systems from basic sound routers to sophisticated hearing instruments.
The Signal Journey: How Sound Travels Through a CROS System
To understand how CROS hearing aids work, it's valuable to trace the journey of sound from its origin to perception:
1. Sound Wave Capture
When sound waves originate on the side of the impaired ear:
They reach the microphones in the CROS transmitter
The acoustic energy is converted into electrical signals
These signals preserve the intensity, frequency, and timing characteristics of the original sound
2. Signal Processing Phase
The electrical representation of sound then undergoes:
Filtering to reduce unwanted noise and enhance speech
Compression to manage loud sounds while making soft sounds audible
Feature extraction to identify important acoustic patterns
Environmental classification to optimise processing for the situation
3. Wireless Encoding and Transmission
The processed signal is:
Encoded into a digital format suitable for wireless transmission
Modulated onto a carrier frequency (typically in the 2.4 GHz band or using NFMI)
Transmitted across the head to the receiver unit
This transmission occurs with minimal delay and energy consumption
4. Reception and Integration
At the receiving end on the functioning ear:
The wireless signal is demodulated and decoded
The recovered audio information is synchronised with locally captured sound
The combined signal is delivered to the ear canal of the functioning ear
The brain perceives sounds from both sides despite physically hearing through only one ear
This entire process—from sound wave to perception—occurs in milliseconds, creating the impression of natural, instantaneous hearing.
Behind the Technology: The User Experience
The technical workings of CROS systems directly translate to practical benefits in daily life:
Seamless Sound Awareness
The wireless nature of signal routing creates a continuous awareness of the sound environment. Users report being able to:
Hear someone approaching from their deaf side without turning
Detect traffic or alerting sounds from all directions
Participate in conversations without constantly repositioning
Adaptive Performance
Modern CROS systems automatically adapt to different environments:
In quiet settings, they may maintain omnidirectional awareness
In noisy restaurants, they might emphasise speech from specific directions
When outdoors, they can reduce wind noise interference
During music listening, they can preserve fidelity and tonal quality
Connectivity Integration
Contemporary CROS technology often includes:
Direct streaming from mobile phones for calls
Wireless connection to television and other audio sources
App-based control for personalised adjustments
Remote fine-tuning capabilities with hearing care professionals
Technical Limitations and Challenges
Despite their sophisticated design, CROS systems face inherent technical limitations:
Spatial Hearing Constraints
While CROS technology improves awareness of sounds from all directions, it cannot fully restore binaural hearing because:
All sounds are ultimately processed through one cochlea
The brain receives limited spatial cues compared to true binaural hearing
This affects precise sound localisation abilities
Power Management Considerations
Wireless transmission requires significant power:
Battery life is typically shorter than in conventional hearing aids
Rechargeable options address this challenge but require daily charging
Power consumption must be balanced with transmission quality
Signal Processing Trade-offs
Digital processing introduces certain compromises:
More aggressive noise reduction can affect sound naturalness
Wireless transmission bandwidth limitations may affect sound quality
Processing delays, while minimal, can never be completely eliminated
Technological Variations Among Manufacturers
While the fundamental principles remain consistent, different hearing aid manufacturers have developed unique approaches to CROS technology:
Proprietary Wireless Protocols
Major manufacturers have developed their own wireless transmission systems:
Phonak: Belongs to Sonova group and uses proprietary Binaural VoiceStream Technology
Signia: Utilises e2e wireless 3.0 for ear-to-ear communication
ReSound: Employs 2.4 GHz wireless technology with digital frequency modulation
Widex: Features WidexLink wireless technology for audio transmission
Signal Processing Differences
Manufacturers also differentiate their offerings through signal processing approaches:
Some prioritise speech enhancement in challenging environments
Others focus on sound naturalness and environmental awareness
Certain systems emphasise seamless switching between acoustic scenes
Various approaches to balancing transmitted sound with direct sound
Form Factor Variations
The physical implementation of CROS technology varies:
Behind-the-Ear (BTE) designs offering powerful capabilities
Receiver-In-Canal (RIC) options balancing visibility and performance
In-The-Ear (ITE) models prioritising discretion
Custom-moulded versions for optimal comfort and aesthetics
Conclusion: The Technical Elegance of CROS Solutions
CROS hearing aids represent a remarkable technical solution to the challenge of single-sided deafness. Rather than attempting to restore function to a non-working ear, they elegantly bypass the problem through sophisticated signal routing.
At its essence, the working principle of CROS technology exemplifies how innovative thinking can transform a seemingly insurmountable hearing challenge. By capturing sound, processing it digitally, and wirelessly transmitting it to where it can be perceived, CROS systems create a functional approximation of binaural hearing using just one functioning ear.
As wireless technology, digital signal processing, and AI continue to advance, we can expect CROS systems to become ever more sophisticated—offering increasingly natural, effortless hearing for those with single-sided deafness.
Understanding how these remarkable devices work not only illuminates their current capabilities but also provides appreciation for the intricate technology that works seamlessly to reconnect people with their complete auditory environment.
Frequently Asked Questions About How CROS Hearing Aids Work
How is sound quality affected when transmitted wirelessly in CROS systems?
Digital Sampling and Compression: Contemporary systems sample sound at 16-32 kHz with 16-20 bit resolution, capturing the full range of frequencies important for speech and environmental sounds. Advanced audio compression algorithms then optimise this data for wireless transmission while preserving critical acoustic information.
Bandwidth Management: Current 2.4 GHz wireless protocols used in premium CROS systems offer bandwidth of up to 3 Mbps, allowing for high-fidelity transmission of complex audio signals with minimal compression artifacts.
Latency Control: Modern CROS systems maintain transmission delays under 5 milliseconds—well below the threshold where the brain would perceive an echo or delay between direct and transmitted sound (approximately 20ms).
Signal Preservation: Advanced error correction protocols ensure sound integrity even in challenging environments with potential wireless interference.
While earlier generations of CROS technology had notable limitations in sound quality (particularly frequency response and dynamic range), current systems can transmit audio with fidelity nearly indistinguishable from the original acoustic signal. This enables natural-sounding awareness of the environment, though the inherent limitations of hearing through only one ear remain.
How do CROS hearing aids manage battery consumption with constant wireless transmission?
Low-Energy Wireless Protocols: Modern systems use custom wireless protocols specifically engineered for hearing aids, requiring significantly less power than standard Bluetooth. Near-Field Magnetic Induction (NFMI) technology, used in many systems, is particularly efficient for short-distance transmission across the head.
Smart Power Management: CROS systems incorporate dynamic power adjustment algorithms that modify transmission intensity based on need—using less power in quiet environments and allocating more energy in challenging listening situations.
Sleep Modes: Advanced CROS devices automatically enter low-power states when no significant sound is detected, and rapidly reactivate when sounds appear.
Rechargeable Solutions: Many current CROS systems use lithium-ion rechargeable batteries that eliminate the need for disposable batteries. These typically provide 18-24 hours of use from a single charge, with charging cases that can provide additional power when away from mains electricity.
Chip Technology: Modern CROS systems use application-specific integrated circuits (ASICs) manufactured with nanometer-scale processes (typically 16-28nm), dramatically reducing power consumption compared to earlier technology.
Despite these advancements, battery consumption in CROS devices typically remains higher than in standard hearing aids. Users can generally expect 3-7 days of use from disposable batteries or daily charging for rechargeable models, compared to 7-14 days for standard hearing aids using similar battery sizes.
What technological advances have most improved CROS hearing aid performance?
Wireless Technology Revolution: The transition from early magnetic induction to modern 2.4 GHz and proprietary wireless protocols has significantly improved transmission reliability, range, and sound quality. This advancement eliminated the need for physical connections and reduced interference issues.
Directional Microphone Arrays: Advanced multi-microphone systems can now focus on speech from specific directions while suppressing noise from others. These adaptive directional systems automatically adjust based on the acoustic environment, significantly improving speech understanding in noise.
Digital Signal Processing (DSP): The exponential increase in computational power within hearing aids has enabled sophisticated signal processing that can separate speech from noise, enhance important sounds, and reduce listening effort. Modern CROS devices contain processors capable of millions of calculations per second.
Artificial Intelligence: Machine learning algorithms now allow CROS systems to classify and respond to different acoustic environments automatically. These systems learn user preferences over time and optimise settings accordingly.
Miniaturisation: Advances in component miniaturisation have allowed CROS devices to become dramatically smaller and more discreet while incorporating more sophisticated technology. Modern systems can be nearly invisible when worn.
Smartphone Integration: Connectivity with smartphones has transformed how users interact with their CROS systems, enabling fine-tuning on the go, remote adjustments by audiologists, and direct audio streaming.
Together, these technological advances have transformed CROS from simple sound routing devices to sophisticated hearing systems that provide a more natural, effortless listening experience with greater awareness of the complete sound environment.
How does the brain adapt to hearing sounds from the deaf side through the functioning ear?
Initial Disorientation: When first using a CROS system, many users experience a sense of confusion as sounds that visually appear to originate from their deaf side are heard through their functioning ear. This creates a temporary sensory mismatch between auditory and visual information.
Cortical Remapping: Over time, the auditory cortex begins to reinterpret these signals. Neuroimaging studies show that the brain gradually develops new neural pathways that associate sounds captured by the CROS transmitter with their true spatial origin, despite being heard through the functioning ear.
Multimodal Integration: The brain begins to integrate visual cues (seeing a person speaking) with the transferred audio signals, helping to resolve the initial spatial confusion. This cross-modal sensory integration is a key part of adaptation.
Compensatory Head Movement: Many CROS users unconsciously develop subtle head movements to help localise sounds, which the brain incorporates into its spatial mapping strategy.
Timeline for Adaptation: Research indicates that basic adaptation occurs within 1-4 weeks for most users, with continued refinement over 2-3 months. Younger individuals typically adapt more quickly due to greater neuroplasticity.
Limitations of Adaptation: While the brain can learn to effectively use CROS-provided information, certain aspects of binaural hearing cannot be fully replicated. True sound localisation remains challenging because the brain cannot access the interaural time and level differences that require two functioning ears.
Consistent usage is critical for successful adaptation, as the brain needs regular exposure to this new pattern of sound input to establish and reinforce the necessary neural pathways.
What happens when environmental noise comes from multiple directions?
Signal Prioritisation: Modern CROS devices employ sophisticated algorithms to identify and prioritise speech signals over background noise. When sounds come from multiple directions, these systems analyse acoustic patterns to determine which sounds are likely to be speech versus environmental noise.
Directional Processing: Advanced CROS systems use adaptive directional microphone technology that can create "beams" of sensitivity in specific directions. When speech is detected from a particular direction, the system can enhance sounds from that direction while suppressing others.
Binaural Environment Classification: The CROS transmitter and receiver work together to analyse the complete sound environment, sharing information about sound characteristics from both sides of the head. This "binaural" processing helps the system make better decisions about which sounds to enhance.
Spatial Noise Reduction: Some premium CROS systems can identify the spatial origin of noise sources and selectively reduce them without affecting speech signals from the same general direction—a process called spatial noise reduction.
User Control: Many CROS systems allow users to manually adjust the balance between transmitter and receiver microphones, enabling them to focus more on sounds from their deaf side or their hearing side as the situation demands.
Limitations: Despite these advanced features, CROS users may still experience more difficulty than normal-hearing individuals in complex acoustic environments with multiple competing sound sources. This is an inherent limitation of having only one functioning ear to process all incoming sound information.
For particularly challenging environments, many CROS users benefit from supplementary assistive technology such as remote microphones that can be placed near desired sound sources.
References
- Appleton, J., & König, G. (2014). Improvement in speech intelligibility and subjective benefit with binaural beamformer technology. Hearing Review, 21(11), 40-42.
- Baguley, D. M., Bird, J., Humphriss, R. L., & Prevost, A. T. (2006). The evidence base for the application of contralateral bone anchored hearing aids in acquired unilateral sensorineural hearing loss in adults. Clinical Otolaryngology, 31(1), 6-14.
- Byrne, D., & Noble, W. (1998). Optimizing sound localization with hearing aids. Trends in Amplification, 3(2), 51-73.
- Carlsson, P. U., & Hakansson, B. E. (1997). The bone-anchored hearing aid: reference quantities and functional gain. Ear and Hearing, 18(1), 34-41.
- Dillon, H. (2012). Hearing Aids (2nd ed.). Sydney: Boomerang Press.
- Erdman, S. A. (2009). Counseling adults with hearing impairment. Adult Audiologic Rehabilitation, 595-635.
- Galster, J. A., & Galster, E. A. (2011). How to compare binaural benefits for those who wear one hearing aid. Hearing Journal, 64(3), 36-38.
- Harford, E., & Barry, J. (1965). A rehabilitative approach to the problem of unilateral hearing impairment: The contralateral routing of signals (CROS). Journal of Speech and Hearing Disorders, 30, 121-138.
- Hayes, D. E., & Chen, J. M. (2005). Bone-conduction amplification with completely-in-the-canal hearing aids. Journal of the American Academy of Audiology, 16(8), 592-599.
- Kuk, F., Keenan, D., Korhonen, P., & Lau, C. (2014). Efficacy of linear frequency transposition on consonant identification in quiet and in noise. Journal of the American Academy of Audiology, 25(5), 445-465.
- Lin, L. M., Bowditch, S., Anderson, M. J., May, B., Cox, K. M., & Niparko, J. K. (2006). Amplification in the rehabilitation of unilateral deafness: speech in noise and directional hearing effects with bone-anchored hearing and contralateral routing of signal amplification. Otology & Neurotology, 27(2), 172-182.
- Pumford, J. (2005). Benefits of CROS amplification for individuals with unilateral severe-to-profound hearing loss. International Journal of Audiology, 44(8), 414-420.
- Ryu, N. G., Moon, I. J., Byun, H., Jin, S. H., Park, H., Jang, K. S., & Cho, Y. S. (2015). Clinical effectiveness of wireless CROS (contralateral routing of offside signals) hearing aids. European Archives of Oto-Rhino-Laryngology, 272(9), 2213-2219.
- Schafer, E. C., Romine, D., Musgrave, E., Momin, S., & Huynh, C. (2013). Electromagnetic versus electrical coupling of personal frequency modulation (FM) receivers to cochlear implant sound processors. Journal of the American Academy of Audiology, 24(10), 927-940.
- Snapp, H. A., & Fabry, D. A. (2016). Objective and subjective comparison of CROS and BiCROS amplification options for unilateral hearing impairment. International Journal of Audiology, 55(5), 285-292.