Even mild hearing loss can significantly impact a child's ability to develop normal speech and language abilities. For this reason, children's hearing should be tested at a young age. In many states, it is required that babies have their hearing tested before they leave the hospital following birth.

The Centers for Disease Control and Prevention (CDC) recommends that babies have their hearing screened before they leave the hospital following birth, or before 1 month of age. Babies who do not pass this test should receive a follow-up screening at 3 months of age. If a baby is found to have a hearing impairment, treatment and other services should begin by the time he or she is 6 months old.

The CDC also recommends that children have their hearing tested again before beginning school, as some forms of hearing loss develop over time.

There are several types of hearing tests. Newborns are likely to undergo one of three types of electrophysiologic tests, which measure a child's ability to hear based on electrical information generated from the auditory nervous system. These tests include:

Auditory brainstem response (ABR). This test measures the function of a child's brainstem in reaction to sound and can reveal information about hearing sensitivity and the brainstem's response to sound. While a child is asleep, small earphones are inserted into the ear canals. Electrodes to measure the brainstem response are placed on the baby's head. These devices do not harm the child. A series of clicking sounds are introduced through the earphones, and the electrodes record the hearing nerve's responses to these sounds. These measurements are shown on a computer as a waveform and a pass-fail verdict is automatically generated - interpretation of the data by an audiologist is not required. An abnormal result may indicate hearing loss or another medical problem. In some cases, difficulties with the measurement process can cause a false abnormal result.

Auditory steady state response (ASSR). Often performed with the ABR. The sound is transmitted into the ear and a computer detects brain waves in the hearing section of the brain. The computer then measures the child's hearing capacity.

Otoacoustic emissions (OAE). Performed on sleeping newborns in a hospital or older children who are able to sit quietly. A tiny, flexible sponge microphone/probe is placed in the ear canal and a series of pulse sounds are introduced. The echo generated from the inner ear in response to these sounds is recorded and averaged by a computer. If no echo is measured, a hearing loss may be indicated. In some cases, an ear infection may prevent an echo from being measured even if the child's hearing is normal. Children who fail an OAE test often will take an ABR to confirm hearing loss. While these tests help measure the structural completeness of the auditory pathway - and thus the child's ability to detect sound - they do not actually measure the child's hearing. Instead, behavioral tests are used for this purpose. In these tests, an audiologist uses a technique called audiometry, in which tones are introduced to a child in a soundproof room, usually via headphones. The audiologist observes the child's responses to these sounds. Audiologists are trained to notice a child's bodily reactions to sound, such as changes in body movement, opening or widening of the eyes, and changes in the child's sucking rate. Behavioral tests can measure hearing thresholds at specific frequencies and reveal the degree of hearing impairment, if any, a child is experiencing.

Examples of these behavioral tests include:

Visual reinforcement audiometry. The child sits on the parent's lap in the middle of a soundproof room with speakers. When a tone is introduced, the child is supposed to turn toward the sound. Estimates are then made of the child's sensitivity to tones and speech sounds. This test is typically used in children between the ages of 6 months and 30 months.

Play audiometry. Also known as conditioned play audiometry (CPA), it involves training children to use play to respond to sounds they hear by performing a certain action. For example, a child may be asked to put a piece into a puzzle, drop a block into a bucket or put a ring on a peg whenever a particular sound is heard. This test typically is performed between the ages of 2.5 and 4 years.

Standard audiometry. Also known as pure tone audiometry. When a tone is introduced, the child presses a button or raises a right or left hand depending on which ear hears the sound. This is the standard method of hearing testing used for children who are age 4 and older, as well as for adults. In addition to these tests, tympanometry sometimes is performed to measure the eardrum's response to soft sounds or air pressure in the ear canal. A tympanometry cannot in itself reveal whether a child can hear. But it can be used to identify physical problems such as collected fluid behind the eardrum. The information gathered in this test is represented on a graph known as a tympanogram. A flat line indicates that the eardrum is not responding properly.

In addition, a new auditory testing tool that measures whether a child's nervous system is able to accurately translate sounds into brain waves is now available for professional use. The BioMAP is a user-friendly device that uses noninvasive electrodes placed on a child's scalp and an earpiece that delivers carefully calibrated sounds in one ear. As the child watches a movie or video, the tester is able to measure the brain's response to the sounds. Testing with BioMAP is relatively easy, painless and takes about 20 to 30 minutes to complete. The BioMAP device can help identify when a child has sound encoding problems related to learning disorders. These differ from hearing problems and would not be distinguished by standard hearing tests. Specialists (e.g., speech therapist) can then provide the child with auditory training that can improve their ability to distinguish sounds.