Our created ear
“The hearing ear and the seeing eye, the Lord has made them both” (Proverbs 20:12)
Looking at the ear and how it works, it becomes immediately obvious that there is an awesome and very intricate process underlying our auditory sense. In fact, the human ear is one of the most intricate examples of miniature and sophisticated engineering on the planet.
My background is in mathematics applied to acoustic engineering, having performed research on the role of pressure waves with combustion. This includes very small variations in pressure, called acoustic waves, which travel through the air when we speak. Though small, they can even affect standing flames. That work involves the safety of jet engines which can under a certain phenomenon called ‘resonance’ (when one object vibrates in sympathy with another) amplify acoustic waves such that the vibrations grow and even destroy fan blades in the rotor. As we will see, resonance is an important property in hearing, and in particular in hearing the human voice.
How we hear
Sound is a pressure disturbance in the air. Small cyclical vibrations of pulsing pressure (called ‘acoustic waves’) travelling through the air enter the opening of the ear canal and reach the eardrum (‘tympanic membrane’—see fig. 1).
The resultant small vibrations of the eardrum are then transferred, through three tiny bones called ‘ossicles’ in the middle ear cavity, into the cochlea in the inner ear. Each stage of this system is staggering in its complexity. All mammals have a system with these basic features (see fig. 1). However, there are large differences between mammals, even in the ear canal.
As well as transmitting sound, the canal amplifies it, by resonance. The frequencies at which it resonates depend on its length, shape and volume. (Consider how the air column in a flute vibrates at different frequencies as we change its length by opening it at different points.)
In humans the canal is about 20 mm (0.8 ins) long, while those of cats and dogs are longer and bent at nearly a right angle to give them a horizontal and vertical component; they are designed for amplifying different frequencies to ours.
Different hearing range
Humans hear over a wide range (9 octaves) from about 20 cycles per second (Hertz = Hz) to nearly 20,000 Hz. Dogs hear from about 65–44,000 Hz (again over 9 octaves but shifted upwards compared to humans), while cats have one of the widest ranges of all—over 10 octaves, from 55 to 77,000 Hz.
Even in our twenties we begin to lose the capability of hearing very high frequencies (around 12,000 Hz and above). The human voice carries with it a raft of ‘harmonics’ (multiples of the fundamental frequency at which it is emitted—typically 125–400 Hz), which firstly extends the range at which it is heard, and ensures that each individual voice is unique. This includes the voice of Christ Himself, which all creation obeys. This was demonstrated when He commanded the wind and the waves to obey Him in Mark 4—the same person who spoke all into existence in Creation Week!
The harmonics of the human voice are particularly important between 2,000 Hz and 5,000 Hz since this is the region where different vowel sounds are distinguished. The even higher frequencies enrich the quality of the sounds, particularly in music. The ear canal is just the right length and shape to resonate with ‘speech’ frequencies (that is, the ear canal air vibrations are in sympathy with typical speech modes).
The sound of water
Remarkably, there is another fascinating fact that has emerged in recent research. It has been shown that all sounds from water are produced by the popping of tiny little bubbles of air trapped in the water,1 Each bubble vibrates at a frequency that depends on its size, so flowing water produces a range of audible frequencies. So, the sound of a babbling brook, a flowing waterfall, and the crashing waves on the ocean shore are made by billions of very small air bubbles that are vibrating against the mass of the water that encases them. These countless bubbles all have slightly differing frequencies, but all lie exactly in the range that the human ear amplifies by the acoustic resonance provided by the ear canal.
And these same frequencies are also in the region where human speech is distinctive. It may be significant that God says in His Word that “The voice of the Lord is over the waters; the God of glory thunders, the LORD, over many waters (Psalm 29:3).”
God communicates with clarity and precision through the Scriptures, but also in a more general way through His creation. Which of us has not been moved as we have listened to the beauty of a sparkling stream, the majesty of ocean waves, or the awesome power of a rainstorm?
Furthermore, the frequencies used by songbirds are again in the same range! The remarkable way our ear canal naturally resonates with and amplifies the frequencies of human speech and song, flowing water, and bird song are a further witness to the design inherent in our bodies.
The middle ear’s three ossicle bones
The acoustic signal causes the eardrum to vibrate. This pushes on the malleus (hammer) attached behind, which itself then pushes onto the incus (anvil) bone, which then moves the stapes (stirrup) horizontally (see fig. 1). The first two of these bones are some 5 mm (0.2 inch) long, with the stapes (the smallest bone in our body) smaller still. In fact, all 3 bones will fit with ease on a British 1p coin (c. 20 mm across; see fig. 2).
These are the only bones in the body that do not grow in size after birth. Believers in evolution try to argue that upper and lower parts of the jaw bones of a reptile moved to become the malleus and incus bones, but quietly ignore one of the biggest hurdles to such a story, namely that the jaws of reptiles never stop growing!
The ossicle bones need to amplify the signal, because it is now going to pass into a liquid medium in the inner ear (being incompressible, liquid is an impediment to sound). Each of the three are specially shaped to form a lever mechanism such that the stapes (attached to a membrane called the oval window in the cochlea – see fig. 3), moves approximately three times the distance travelled by the malleus. There is also a tenfold smaller area being vibrated in the oval window compared to the tympanic membrane of the ear drum,2 so that the energy transfer involved is almost 100% efficient.
The cochlea of the inner ear
The stapes acts like a pump on the oval window membrane and, cleverly, the membrane of the round window (see fig. 3) expands to compensate for the movement of the liquid inside the cochlea.
If we were to unwind the cochlea (see fig. 4), we would see an ingenious basilar membrane which tapers for higher frequencies inside the cochlea, rather like a xylophone, so the combined frequencies that come in from the oval window vibration are immediately split up into their component frequencies, each causing different parts of the basilar membrane to vibrate. This is in effect an instantaneous frequency analyzer, which would make any electrical engineer marvel. It is rather like having a miniature gremlin (with concert pianist skills!) playing a keyboard in your inner ear!
The final part of the hearing system involves the organ of Corti (see fig. 3), running along the top of the basilar membrane. This has tiny little hairs (stereocilia) on it (fig. 5) which send an electrical signal according to each frequency excited by the incoming signal. It is astonishing that each tiny ‘hair’, called a cilium (0.00025 mm thick—less than 1/70th of the thickness of the thinnest (flaxen) human hair!), when disturbed by the tectorial membrane (which touches the cilia above), causes the operation of literally a mechanical spring attached to the top of one hair. This spring is only a few nanometres thick and stretches to about 100 nanometres long; a nanometre is a millionth of a mm, which is getting towards the molecular scale. The other end pulls on a tiny trapdoor at the side of an adjacent cilium (fig.5)—one of the smallest examples of mechanical springs!
This open trapdoor then allows charged ions in the fluid-filled cochlea to excite ganglion nerves to send the signal to different parts of the cerebral cortex in the brain, depending on whether it is music or speech. For low frequencies there is about one nerve for each change in Hz. In the upper range it is about 2–3 Hz per nerve ending.
Sometimes the hearing mechanism is damaged by listening to repetitive sounds of one particular frequency, such as in certain industries if someone is not provided with ear protection. Listening to loud music can also do this, because the springs at the tip of the cilia for a particular set of frequencies can literally snap.
Some people have a genetic defect in their ears such that the cochlear system is not working. A brilliant Australian surgeon, Graeme Clark, developed the cochlear implant, which bypasses the cochlea system with a microphone attached to the spiral ganglion nerves. Initially this at least enabled basic speech to be heard, but later developments have led to implants with greater frequency resolution so that even music can be heard. Achieving this feat of exquisite engineering required clever minds. The implications are obvious—the original design was indeed superb!
Summary and conclusion
Such an exquisite system involving air vibrations, mechanical, chemical and electrical engineering is frankly astonishing, and confirms the intelligent design of the ear. Surely we can say with the Psalmist, “I praise you, for I am fearfully and wonderfully made. Wonderful are your works; my soul knows it very well” (Psalm 139:14).