The way we hear
All human senses are remarkable and illustrate that we are fearfully and wonderfully made (Ps 139:14). The human ear is one of the most intricate examples of miniature and sophisticated engineering. The Lord clearly states in scripture that the ear and the eye are both made by Him (Prov 20:12 “The hearing ear, and the seeing eye, the Lord has made them both”) and it is striking that with both He stresses the hearing ear and the seeing eye. The very wording of this proverb is not without precision. It clearly underlines that there is a marvelous system going on that enables these two senses to work. It is inviting the reader to investigate further and in this article we look at the ear and how it works and it becomes immediately obvious that there is an awesome and such intricate process underlying our auditory sense!
The background of the author is in mathematics applied to acoustic engineering, having performed research on the connection of pressure waves with combustion. This includes what are very small perturbations in pressure called acoustic waves which travel through the air when we speak and though small, can actually even affect standing flames. That work is connected with the safety of gas turbines (jet engines) which can under certain conditions which we call “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. Later in this article it will become evident that resonance is an important property which enables us to listen to the human voice.
We hear sound as small cyclical pressure changes in the air which reach the ear drum inside the ear. Sound is actually a pressure disturbance in the air such that small cyclical vibrations of pulsing pressure (called “acoustic waves”) travel into the ear canal and reach the eardrum at the end of the auditory canal. This is called the tympanic membrane as shown in figure 1.
The small vibrations of the tympanic membrane are then transferred through tiny ossicle bones into the inner ear. Each stage of this system is staggering in its complexity. All mammals have a system which transfers acoustic waves through and eardrum and ossicle bones into the inner ear (figure 2). However there are large differences within the mammals. Certain length of tubes will vibrate at different natural frequencies. Depending on where you press open the keys, a flute will change its natural frequency. The ear canal in humans is about 2 cms (approx. 0.8 ins) long while the ear canals in cats and dogs are of a different shape such that there is a bend in them. Thus they have a vertical and horizontal ear canal which is designed for a different range of frequencies. Humans hear over a wide range (9 octaves) from approx. 20 cycles per second (Hz) to nearly 20,000 Hz. Dogs hear from approx. 65Hz – 44,000Hz (again over 9 octaves but shifted compared to humans), while cats have one of the widest ranges of all of over 10 octaves from 55Hz to 77,000Hz. Our ears are designed for low frequency as well as high frequency sound and resonate in the range of 4000 Hz which is right in the centre of the human speech range!
Human ears are such that when we are even in our twenties we are beginning to lose the capability of hearing very high frequencies (in the region of 12,000 Hz and above). The fundamental frequency range is from 125 to 400 Hz but what is called the “harmonics” of the vibration (multiples of the fundamental) extend the frequency range of human speech. In particular the human voice carries with it a raft of harmonics which means that any voice is unique for each individual. This includes the voice of Christ Himself which all Creation obeys as is demonstrated even when He commanded the wind and the waves to obey Him in Mark 4 – the same voice that spoke all into existence at Creation!
The harmonics of the human voice are particularly important in the region of between 2000 Hz and 5000 Hz since in this region different vowel sounds are distinguished, with the even higher frequencies enriching the quality of the sounds, particularly in music. What is remarkable is that the ear canal is just the right length to resonate (that is vibrate in sympathy) with these frequencies.
Furthermore it has been shown that all sounds to do with water are produced by the popping of tiny little bubbles of air entrapped with the water. Each bubble vibrates at a frequency depending on its size, so a range of frequencies is heard as water flows, due to air bubbles vibrating in the water. So the sound of a babbling brook, a flowing waterfall, and the crashing waves on the ocean shore are made literally by billions of very small air bubbles which are vibrating against the mass of the water which encases them. These countless bubbles all have a slightly different frequency 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. No wonder the Lord says in His Word that “The voice of the Lord is upon the waters: the God of glory thundereth: the Lord is upon many waters.” (Psalm 29:3)! No word is there by accident in Scripture. God does communicate through His creation, though clarity and precision of God’s voice is heard in the Scripture. But which one of us has not been moved as we have listened to the beauty of a sparkling stream, the majesty of ocean waves and the awesome power of a rainstorm?
What is more, the frequencies used by songbirds are again in the same range! Such remarkable connections between our ears with an ear canal which naturally resonates and amplifies the frequencies of human speech and song, flowing water and bird song is a strong witness to design inherent in our bodies.
The middle ear – Ossicle Bones
The acoustic signal causing the eardrum to vibrate, now pushes on the malleus (hammer) attached behind, which itself then pushes onto the incus (anvil) bone, which then moves the stapes (stirrup) horizontally (see figure 2). These bones are of the order of 5mm long with the stapes smaller still, and all three can fit on a 1p piece with ease! These are the only bones in the body which do not grow. They are the same size as when each of us was born as a baby. Those believing 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 the biggest hurdle to such a story which is that jaws of reptiles never stop growing!
The ossicle bones amplify the signal. Each of the three are shaped specially so there is a lever mechanism such that the stapes (attached to a membrane called the oval window in the cochlea), moves approximately three times the distance travelled by the malleus. There is also a ten fold smaller area being vibrated in the oval window compared to the tympanic membrane of the ear drum, so that the energy transfer involved in such that the system is almost 100% efficient.
Why the need to amplify the signal? This is because the signal is now going to pass into a liquid medium in the inner ear.
The inner ear - Cochlea
Liquid is a barrier to sound, and the stapes acts like a pump on the oval window membrane and cleverly the membrane of the round window (see figure 3 showing the Cochlea) expands to compensate for the movement of the liquid inside the cochlea.
If we were to unwind the cochlea (figure 3), we would see an ingenious basilar membrane which tapers for higher frequencies inside the cochlea, rather like a xylophone, so that whatever combined frequencies come in from the oval window vibration, are immediately split up into their component frequencies! This is effectively an instantaneous frequency analyser, so that whatever signal is coming in composed of many frequencies, immediately causes different parts of the basilar membrane to vibrate! Such an ingenious device will make any electrical engineer marvel, since unlike digital frequency analysers used often today, this instantaneously splits the incoming sound up into its component frequencies, rather like having a miniature gremlin (but a concert pianist!) playing a keyboard in your inner ear!
The final part of the hearing system is achieved by the organ of Corti (see detail of figure 3) running along on top of the basilar membrane. This has tiny little hairs (stereocilia) on it (figure 4) which send an electrical signal according to each frequency excited by the incoming signal. It is astonishing that each cilia (0.00025 mm thick – this is less than 1/70th of the thickness of a human hair! ) when disturbed by the tectorial membrane (which touches the cilia above), has a little trapdoor at its side which opens with a spring attached (see figure 5) to an adjacent hair! This allows electrical ions in the electrically charged fluid, within that part of the cochlea, to then 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 ear is damaged by listening to repetitive sounds of one particular frequency such as in industry when some are not provided with ear defenders. 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. Others have a genetic defect in their ears such that the cochlea system is not working and a brilliant Australian Surgeon Professor Graeme Clark (University of Melbourne) developed the cochlear implant which bypasses the system and attached a microphone to the nerves. Initially the progress 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. Such exquisite engineering required clever minds to achieve this. The implications are obvious -that the original design was indeed superb!
Such a system involving air vibrations, mechanical, chemical and electrical engineering is frankly astonishing and confirms the intelligent design of the ear. Surely we echo the glorious statement in Proverbs (Prov 20:12) “The hearing ear, and the seeing eye, the Lord has made them both” and say with the Psalmist (Ps 139:14) “I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.”
In the light of this wonder of hearing I trust readers will consider the depth of the miracles of Christ such as when He healed the deaf and dumb man (Mk 7:34,35) “Then looking up to heaven , He sighed, and saith unto him, Ephphatha, that is, Be opened. And straightway his ears were opened…” and surely there is a profound wisdom to the statement in John 18:37 “Every one that is of the truth heareth my voice.” We now hear Him spiritually, but one day we will literally hear His voice! What a day that will be!
A version of this article has also appeared in one section (pp. 180-183) of the book “Wonders of Creation – Design in a fallen world” co-authored with Professor Stuart Burgess and Brian Edwards and published by Day One, 2017.
(Please find it here: https://www.dayone.co.uk/collections/reduced/products/wonders-of-creation)
Professor Andy McIntosh
Leeds, Oct 2019