How to measure sound
One way to envisage hearing is to think about loudness. A person with normal hearing will be able to hear very quiet sounds; prefer sounds at a medium level, and be able to tolerate very loud sounds.
Tone-wise, sounds can also be described as “high pitched” or “low pitched”. The sounds of a violin or a bird singing are examples of high-pitched sounds - whereas a bass or certain male voices are examples of low-pitched sounds.
Hearing is measured both in terms of loudness (intensity) and pitch (frequency). The unit of measurement for intensity is Decibel Sound Pressure Level (dBSPL). For pitch it is Hertz (Hz). A person with normal hearing can typically hear sounds up to 120 dBSPL. Above 120 dBSPL, sounds become very uncomfortable and can be damaging. The same person can typically hear sounds within the pitch range of 20 to 20,000 Hz. However, during a hearing test, only the range from 125 to 8000 Hz is measured. Speech sounds fall within this range.
An audiologist can tell you what sounds your child can hear by performing a hearing test. This shows the level at which your child can or cannot hear different tones.
There are several types of measurement methods
These include:
- Otoacoustic Emissions (OAE), for newborns and infants. OAE are the very weak sounds that the cochlea makes when audible sounds stimulate it. Children without hearing loss produce emissions, while those with a hearing loss greater than 25-30 dB don’t.
- Auditory Brainstem Response (ABR), for newborns and infants. Electrodes are placed on the child’s head, and brainwave activity in response to sound is recorded.
- Visual Reinforcement Audiometry (VRA), for children between six months and two years of age. The child is trained to look towards a sound source and is ‘rewarded’ through a visual reinforcement such as a moving toy.
- Conditioned Play Audiometry (CPA), for children between the age of two and three years. The child is trained to perform an activity each time a sound is heard.
- Conventional audiometry, where the child raises their hand when hearing a sound. This method can be used for older children.
Your child's age and ability to cooperate will determine which methods the audiologist chooses to use. Performing a hearing test, especially on smaller children, can take some time. Often, more tests are required in order to define the degree of hearing loss.
