Understanding the process of normal hearing
To fully understand the reasons for your hearing loss or hearing impairment, it is firstly important to understand the process of normal hearing.
- Normal hearing occurs when sound enters the external ear, travels through the ear canal and reaches the eardrum.
- The eardrum is the start of the middle ear. The eardrum vibrates in response to the sound that it picks up and this causes vibration in the chain of the three bones in the middle ear.
- The vibration is transferred to the inner ear or cochlea. The cochlea consists of nerve fibres (hair cells) that receive sound. Different hair cells are responsible for receiving different pitches of sound.
- The signal is transferred via the auditory nerve to a nucleus in the brainstem, which, if it receives a signal from both ears, can:
- know the direction of sound
- boost speech above background noise
- help with perceptions of balance
- Then, it is transferred to the auditory cortex in the thinking brain, where the signal’s message is understood.
What do normal hearing test results look like?
We measure hearing in decibels which is a measure of sound energy (the loudness or intensity of the sound). Each pitch (or frequency, measured in hertz) across the speech range is measured to find the minimum amount of sound that is required to respond. A graph (audiogram) for each ear can be plotted.
Normal hearing will be found at between 0-10 decibels at all pitches.
Hearing impairment exists when the levels are elevated above 10 decibels.
- Mild hearing loss: A person can only hear a sound when it reaches at least 25 to 40dBHL
- Moderate hearing loss: A person can only hear a sound when it reaches 40 to 60 decibels
- Severe hearing loss: A person can only hear a sound when it reaches 60 to 80dBHL
- Profound loss: patients can only hear a sound of 80 dB or more