Jacob probably has a problem with the three little bones in his ears that transmit sound vibrations from the eardrum to the inner ear. Another possibility is an issue with the eardrum itself.
"...Deafness - a range of causes
The ear is our organ of hearing. At around 20 years of age, our hearing starts a gradual decline. Higher frequencies are usually the first to go. This age-related hearing loss is normal and doesn’t lead to total loss of hearing. Deafness can range from mild to profound and is caused by many different events including injury, disease and genetic defects. There are various ways to categorise deafness. The two main types of deafness are conductive deafness and nerve deafness. Deafness at birth is known as congenital deafness, while deafness that occurs after birth is called adventitious deafness. The most common cause of adventitious deafness is noise, which accounts for over one quarter of people affected by hearing loss.
The ear is made up of three different parts, including:
* Outer ear - includes the part you can see. Its shape helps to collect sound waves. A tube leads inward to the eardrum.
* Middle ear - separated from the outer ear by the eardrum. The middle ear contains three tiny bones called the malleus (hammer bone), the incus (anvil bone) and the stapes (stirrup bone). These bones amplify the movement of the eardrum produced by sound waves. The Eustachian tube attaches the middle ear to the back of the throat and helps to equalise air pressure.
* Inner ear - sound waves are picked up by a little spiral-shaped organ called the cochlear. Hairs on the cochlear sense the vibration and pass the message - interpreted into electrical impulses - on to the brain via the cochlear nerve.
Conductive deafness is caused by the failure of the three tiny bones inside the middle ear to pass along sound waves to the inner ear. Another common cause of conductive deafness is the failure of the eardrum to vibrate in response to sound waves. A build-up of fluid in the ear canal, for example, could dampen the movement of the eardrum. In many cases, treatment is available for conductive deafness and normal hearing will return...."