The second of the three stages of our hearing apparatus is the middle ear.

The middle ear transfers the vibration of sound from the ear-drum to the cochlea where it can be converted into nerve signals. This transfer of vibration from one place to another might appear to be a very simple task. But there is a subtlety to the task, and a remarkable solution.

Any material capable of carrying sound has a property called its acoustic impedance, which is the ratio between the pressure and the particle speed when sounds pass through it. Whenever the sound encounters a change from one material to another of different acoustic impedance, the sound energy is reflected as well as transmitted. The greater the difference in acoustic impedances, the more sound energy is reflected, and the less sound energy is transmitted.

The cochlea is filled with a fluid called lymph (for reasons we’ll see later), whereas the sounds we usually hear are travelling through air.

The acoustic impedances of air and cochlea lymph are very different, so if the ear canal simply ended at the oval window (another structure we’ll meet later), most of the sound energy (more than 99% of it) would simply bounce off, and we wouldn’t be able to hear very well at all.

What we need is a device to match the acoustic impedance of air to the acoustic impedance of the fluid in the cochlea — a kind of anti-reflection device. Unfortunately the idea of an anti-reflection coating, as used for spectacles, can only work over a very narrow frequency range, whereas the ear has to admit frequencies over a very broad range.

The design in the middle ear is more like using an electrical transformer to connect an amplifier to a loudspeaker, or using a gearbox to connect the engine of a car (or a truck or a tractor) to its wheels. The middle ear achieves this impedance matching by making an indirect connection between the eardrum and the cochlea. The eardrum is connected to the cochlea via three ossicles (bones): the malleus (hammer), incus (anvil) and stapes (stirrup). The arrangement of the ossicles acts as a lever, so that the stapes moves at about half the speed of the ear drum. The eardrum also has a much (about 18 times) larger area than the footplate of the stapes. These two effects (the leverage and the area ratio) combine to provide the right impedance transformation for energy to effectively enter the cochlea from the air. Incidentally, you may notice in the diagram that there is a round window (a membrane) as well as the oval window — though the round window has no ossicle connected to it. The purpose of the round window also relates to energy transfer. The fluid in the cochlea is essentially incompressible, and the volume of the cochlea cannot change. This means that the oval window would not be able to vibrate unless there is somewhere for the displaced fluid to go. The round window provides this: the round window moves out as the oval window moves in, and vice versa.

Of course, though we talk about sound energy, we’re not really interested in energy entering our ears; what we’re really interested in is the information carried by that sound energy — and in particular in words that are spoken. Hearing loss caused by problems in the middle ear is called conductive hearing loss. Conductive loss is generally more treatable than problems in the inner ear, but can be just as much a barrier to information flow.

Perhaps the spiritual analogue of conductive hearing loss is simply refusing to hear. The Bible connects refusing to hear with a stiff neck. There are many examples of this connection, especially in Jeremiah, such as in Jeremiah 6 and 7. But we could say that refusing to hear is a bit like having stiff ossicles! God’s ways and thoughts are very different from ours (Isa 55:8-9), just like the acoustic impedance of air is very different from that of water. Unless there is some willingness on our part to translate the message of God that He is trying to convey to us into terms that are meaningful to our own life, that message will simply bounce off us. It will also be likely to just bounce off other people with whom we try to share it.

Of course God’s message needs to do much more inside us than simply be heard. There must a response. We’ll look more at that next time when we consider the cochlea.