The ability to hear among background noise occurs in the brain. The brain receives information from both ears, and "filters" out irrelevant information. If you are only wearing one hearing aid, the brain is unable to "filter" out noise effectively. If your primary complaint with your hearing is difficulty hearing in noise, wearing two hearing aids is a necessity as one hearing aid will likely increase your amount of difficulty.
Localization is the ability to figure out where sounds are coming from. To do this, the hearing in both ears must be the same. If you are only wearing one hearing aid, all sounds will appear as though they are coming from the aided ear. For example, if you are wearing a hearing aid in your right ear only, all sounds will appear to be coming from the right side, even though someone may be talking to you from the left side. This is particularly important in situations such as driving a car and being able to determine where the sirens from emergency vehicles are coming from.
Clarity Most people with hearing loss have a high-frequency hearing loss. The high frequencies are responsible for clarity of speech. People with high-frequency hearing loss typically find that they can hear people talking, however, they can't make out the words or find that speech is very muffled. Wearing one hearing aid will help to make speech clearer only if the speaker is near and on the same side as the aided ear. Two hearing aids allow you to hear much more clearly all around you, and from greater distances.
Most of us know someone who is constantly adjusting the volume on their hearing aids. These people are probably wearing only one hearing aid. To compensate for the lack of clarity typical of only wearing one hearing aid, people will turn up the volume. However, this also increases the volume of surrounding background sounds, causing frustration. Increased volume can also cause feedback or whistling from the hearing aid. People who wear two hearing aids generally require less volume and fewer adjustments to the volume control. Many of the hearing aids now available have an automatic volume control and can adjust based on the environment that you are in. These hearing aids are most successful when fit in both ears.
As the saying goes "if you don't use it, you lose it." If you only wear one hearing aid, only that hearing nerve gets stimulated with sound. The auditory nerve of the opposite ear does not get stimulated and can deteriorate. This does not mean that your hearing will get worse in the unaided ear, however, the deteriorating nerve can cause distortion of speech.
People with hearing loss will often find themselves avoiding certain situations and withdrawing from social events. Hearing loss can cause stress and fatigue from the strain of trying to hear and understand speech. Hearing with both ears helps the brain to hear more clearly and requires less strain, therefore resulting in less fatigue. Hearing from both ears also gives you a sense of balance.
Many people with hearing loss report tinnitus, a ringing in the ears or head. The use of hearing aids often helps to mask the tinnitus using natural environmental sounds. If you are using only one hearing aid, the tinnitus will be masked only in the aided ear. The tinnitus will still be apparent in the ear without the hearing aid. Best practices for finding a solution for treating tinnitus is to follow the advice of your Audiologist or hearing care provider.