A Contralateral Routing of Signal (CROS) hearing aid is a specialized electronic system designed to help individuals who have one ear with non-functional hearing and one ear with normal hearing. The acronym CROS describes its function: it takes sound from the non-hearing side and routes it to the side that can hear. This technology does not attempt to restore hearing to the deaf ear, but instead provides the brain with sound input from both sides of the head. The purpose of this system is to improve the user’s awareness of their surroundings and their ability to hear conversations, particularly in difficult listening environments.
The Purpose of CROS Technology
CROS aids are specifically developed to address the challenges created by single-sided deafness (SSD), which is characterized by a complete or profound hearing loss in one ear while the other ear maintains near-normal hearing. The most significant problem for people with this condition is the “head shadow effect.” When sound originates from the deaf side, the head acts as a physical barrier, blocking or attenuating the sound waves before they can reach the functional ear. This acoustic shadow causes high-frequency sounds, which have shorter wavelengths, to be substantially reduced by the time they cross the head. This makes speech coming from the deaf side sound muffled or completely inaudible to the good ear.
The inability to hear sounds originating on the deaf side forces the person to constantly turn their head to reposition their good ear toward the sound source. The CROS system overcomes this functional deficit by creating an electronic pathway that bypasses the natural obstacle of the head. It effectively eliminates the head shadow effect, allowing the user to hear sounds that would otherwise be entirely lost to them. This greatly reduces the listening fatigue that results from the constant need to focus and reorient oneself in social situations.
Components and Mechanism of Sound Transfer
A CROS system is comprised of two distinct devices that work together wirelessly. The first component is a dedicated microphone and transmitter placed on the ear with no functional hearing. This device captures sound from the deaf side.
The captured acoustic energy is converted into an electrical signal, which is then processed to preserve the original sound’s characteristics. This signal is then transmitted wirelessly across the head to the second component, which is worn on the functional ear. Modern systems primarily use advanced wireless technology to ensure a reliable and seamless transfer of the sound signal.
The second device, worn on the hearing ear, acts as a receiver and output unit. It receives the wireless signal from the deaf side and delivers that sound directly into the functional ear. Since the hearing in the receiving ear is normal, this device does not typically amplify the transferred sound; it simply presents it at a comfortable listening level. This continuous routing of sound allows the user to perceive sounds originating from either side of their body without having to manually adjust their position.
CROS vs. BiCROS: Addressing Dual Needs
The determination of whether a person needs a CROS or a BiCROS system depends entirely on the hearing status of the better ear. The CROS system is specifically indicated when the non-hearing ear is profoundly deaf and the other ear has normal or near-normal hearing. In this scenario, the better ear simply needs the signal routed to it without any additional amplification.
The BiCROS system, which stands for Bilateral Contralateral Routing of Signal, is necessary when the non-hearing ear is deaf, but the “better” ear also has a measurable degree of hearing loss. It is designed to address this dual need for both signal transfer and amplification.
A BiCROS system utilizes the same microphone and transmitter on the non-hearing side to capture and route the sound. However, the device worn on the better ear functions as a traditional hearing aid. This means it is programmed to amplify the sound it receives from the deaf side, as well as the sounds it captures directly.