– our highest-precision
sense organ

Without the sense of hearing we would lack an important method of orientation. Like a bird in a small cage, we would then be largely excluded from everyday events,. Sounds intensify the sensory impressions of life. We can listen to the murmur of rippling wavelets at our feet, while at the same time hearing the booming of the mighty breakers rolling in from the ocean. When taking a stroll, we enjoy the humming of bees flitting from blossom to blossom, as well as the exuberant song of a lark. We can hear a wide range of sounds, from the soft whining of a mosquito to the earshattering noise of a jet plane taking off. The racket of pneumatic drills and other noisy machines are also part of our everyday experience. Although these sound signals reveal their origin, they do not convey any personal messages to us.

In addition to receiving sounds, we can also transmit them. Speaking and hearing comprise our basic method of communication. In this case the kinds of noises and their significance are completely different. Musical notes, songs, and the spoken word bear meaningful messages. The act of identifying their inherent meaning involves much more than merely processing the received sound waves. A special evaluation system, located in the brain, is required for this purpose. Without the brain we would not have been able to hear. Our souls are stirred by what we hear, as expressed by the poignant French proverb: “The ear is the way to the heart.”

In our contact with the world our sense of hearing is just as important as our vision. Sounds are air vibrations which are detected by our ears, where they are first converted into hydro-dynamic vibrations, and subsequently into electrical nerve impulses. Finally the brain identifies these signals as information.

Did you know that the human ear is a detection device which utilises a level of technology that no science has as yet been able to attain, or even (in many aspects) to understand? For the purpose of describing this, we need a few technical terms, which will first be explained:

Sound level: Vibrating objects (like a tuning fork, the cone of a loudspeaker, or human vocal chords) induce vibrations in the surrounding air. Adjacent air molecules are accelerated, causing waves travelling at a speed of approximately 330 m/s. This phenomenon is called sound, and in a sound field there are zones where air molecules are more densely packed than in other zones. Air pressure is greater in the denser zones, and smaller in the less dense zones. Sound vibrations can be depicted as wave-shaped graphs. The distance between two adjacent locations where the air pressure is the same, is known as the wave length. The maximum deviation from the neutral value is called the amplitude. If the wave length increases (implying a decrease in the number of vibrations per time unit), the pitch of the sound becomes lower. And, vice versa, when the number of vibrations per time unit increases (shorter wave length), we hear a higher pitched sound. The pitch of a tone – its frequency – is measured in Hertz (1 Hz = one vibration per second). If the amplitude increases, the sound becomes louder, and a decrease in amplitude results in a softer sound. Everyday sounds comprise a mixture of different frequencies and amplitudes.

The amplitude of a sound is called sound pressure, which, in the same way as any other type of pressure, can be measured in N/m2 (Newtons per square metre). But in acoustics, another unit is preferred, called sound level or noise level, which is measured in dB (deciBels). To convert sound pressure px into the corresponding dB number, we use the quotient px/p0 where p0 = 2 x 10-5 N/m2 represents an arbitrarily selected reference value. It is actually the pressure of a sound which one can just detect at the threshold of audibility. The logarithm (base 10) of the ratio px/p0 is multiplied by 20 so that the formula for the noise level L in dB is

L = 20 x log(px/p0).

This seemingly random definition has a number of advantages:

– Instead of using cumbersome powers of ten for pressure, these values are expressed by one, two or three digit numbers.
– The following simple relations hold for the indicated physical units:
– A tenfold increase in sound pressure is expressed as a change of 20 dB.
– If the sound pressure is doubled, we have an increase of 20 x log(2) = 20 x 0.30103 = 6 dB.
– In the case of a three-fold increase in sound pressure the formula produces a dB change of 9.54, which can be rounded off to 10 dB.
– The energy level of a sound is proportional to the square of the sound pressure, meaning that when the energy is doubled, the decibel level is increased by 3 dB.

Structure of the human ear. Sound vibrations travel through the external acoustic meatus to the ear drum, and then via the malleus, the incus, and the stapes through the oval window into the liquid-filled cochlea. The round window allows the pressure between the cochlea and the air-filled middle ear to be equalised. The three arc-shaped structures (semi-circular ducts) are part of the organ of balance. Sounds are detected in the two spirals of the cochlea which contain the organ of Corti. This has some 15,000 sensory “hair” cells. A thick “cable” of nerve fibres (the cochlear branch of the 8th cranial nerve), leads from the cochlea to the brain.

Loudness: The noise level of a sound is a purely physical measure, expressed in either N/m2 or dB. But this gives no indication of the subjective experience of the loudness of a sound. Sound waves at different frequencies, exerting the same pressure, are not perceived subjectively as being of equal volume. A sound at a level of 20 dB, having a frequency of 63 Hz, would have to be made about 30 times stronger in order to sound as loud as a 1,000 Hz signal at the same dB level. From the formula already given, that means it would have to be increased by 20 x log(30) = 29.5 dB.

By connecting the points of equal loudness at different frequencies on a dB-Hz diagram, curves known as isophones result. By definition, the measured sound pressure in dB at a frequency of 1,000 Hz is the loudness, expressed using a unit known as the phon. So for example, if one wants to find the 50 phon isophone, a test subject listens to a 1,000 Hz signal having a sound pressure of 50 dB. At all other frequencies the person adjusts a control indicating dB values, until it sounds just as loud as the 1,000 Hz signal. In this way one can plot the dB values corresponding to each of the frequencies to obtain the 50 phon curve. Only at a frequency of 1,000 Hz is the phon scale numerically equal to the decibel scale.

The pressure at which a sound becomes audible is called the threshold of audibility. This corresponds to the 4 phon isophone. If a sound is so loud that it causes pain, the threshold of pain is reached. Its isophone is 130 phon. If our ears were purely mechanical sound detectors, then all isophones would have been horizontal lines.

We are able to distinguish very clearly between the loudness of two sounds. At low sound intensities at a given frequency, a difference of 1 dB is sufficient. At louder levels this difference is even less.

12 orders of magnitude without switching: The ear has the amazing ability of detecting a range of sound pressure extending over 120 dB. Keeping in mind that 6 dB represents a doubling of sound level, this means that the human ear can handle intensities ranging over 20 powers of 2 (120/6 = 20; 220 = 1,048,576 = approximately one million). In the case of sound energy, doubling occurs every 3 dB because of the physical relationships involved. The human ear thus has the unique ability of detecting differences in sound energy over a very wide range. The relevant factor is 40 powers of 2 (120/3) which is equal to 12 powers of ten (240 = 10244 = 1.099 x 1012). Expressed differently: The range between the pain threshold and a barely audible sound encompasses an energy ratio of one million million to one. This is an astonishing feat, since it is accomplished with just one range of measurement. No known technical measuring apparatus can do this without switching from one range to another. If, for example, we want to measure voltages in the range from 1 volt to 10,000 volts (4 powers of ten), it can only be done with a single instrument by switching the measuring range.

Range of normal human hearing The threshold of audibility is a curve, meaning that the ear is more sensitive to some frequencies than to others. The optimal range is between 1 kHz and 5 kHz, where sound pressures as low as 2 x 10-5 N/m2 can be detected. This is equivalent to an intensity I (sound energy) of 10-16 W/cm2. The intensity-frequency ranges for speech and music are shaded. The maximum range of hearing lies at about 2 kHz. At this frequency, the range of sound energy we are able to detect spans an almost unimaginable 13 orders of magnitude (powers of ten).

Furthermore, the human ear is an optimally constructed measuring system whose sensitivity reaches the limits of physical possibility. Sound waves are pressure waves having very small amplitudes. The pressure exerted by a barely audible 1,000 Hz tone is 2 x 10-5 N/m2. At the same frequency the pain threshold is about six million times greater. The performance of the ear encompasses several powers of ten (Diagram, page 23).

A sound at the threshold of audibility causes the ear drum to vibrate with an amplitude of only 10-10 cm. We need to use an extraordinary comparison in order to visualise such a minute displacement. If our body height were increased by a factor of 200 million, it would extend from the earth to the moon. Even at this enormously magnified scale, the ear drum would only vibrate over a distance of 2 mm.

The frequency range of the human ear is approximately 10 octaves. One octave comprises the notes from middle C to C’ (or A to A’, G to G’, etc). This does not imply absolute values, but indicates a doubling of the frequency. Two octaves (e. g. from C to C’’) thus range from a given frequency f1 to a frequency f2 = 4 x f1 – a fourfold increase. Similarly three octaves imply an eightfold increase: f3 = 23 x f1. Human hearing ranges over 10 octaves, from 20 Hz to 20 kHz, involving the factor 210 = 1024 = approximately one thousand.

The ability to distinguish between different tones is astonishingly good. Around a frequency of 1,000 Hz we can detect frequency differences as small as 3 Hz or 0.3 %.

Auricle and sound paths Auricle (left): Anatomical names of the various parts of the auricle (outer ear) are shown in the illustration above left. Possible sound paths (right): Two possible paths, 1 and 2, are indicated in the diagram above right: 1 shows the route from the antihelix to the acoustic meatus (auditory canal), while route 2 follows the S-shaped curve of the rim of the helix. Route 2 is about 66 mm longer than route 1, so that the time lag is 0.2 milliseconds (0.066 m / 330 m/s = 0.0002 seconds). The result is that the brain has four different sound inputs from the two ears (it’s as if there were four ears!): Two of the four are located somewhat higher, and further out, than the other two. The brain receives the same signal at four slightly different instants.

Loudness, duration, and frequency are the properties of a sound which inform us of its character and its origin. But the direction of the sound is also important. The problem of direction finding was solved by the Creator when He endowed us with two ears. To locate the position of a sound source, two factors are involved: the difference in intensity, and the time lag. The ear turned away from a sound source receives the signal a little softer and somewhat later than the other one. The brain can also measure the difference in relative loudness between both ears, and thus estimate the distance of the sound source. Although very small, these time and volume differences are evaluated by the brain in such a way that the direction can be ascertained with some precision. This measuring process is so accurate that a time lag of 0.00003 seconds between the two ears can be detected clearly. In accoustic orientation terms this means that a sound source located only about 3° from the centre line of the head is recognised as being off-centre.

Measured sound levels: The dB values of various sounds are given in the table below. Sounds louder than 90 dB can cause hearing damage, while a continuous sound of 155 dB can burn the skin. Some sound sources and their dB levels are:

 

Source Limit of audibility Rustling of leaves Whispering Spacious office Motor car cruising smoothly Thunder Noisy street traffic Typewriter Waterfall Freight train Sawmill Jet plane (at 600m height) Disco Prop-driven plane starting up Boilermaking Rock concert Pneumatic drill Artillery fire Testing airplane engines Jet engine starting up

dB 15 18 25 50 50 65 70 70 90 98 100 105 114 120 120 125 130 130 140 145

Different computational comparisons of sounds by the brain.

Six different values: As can be seen from the diagram, the four separate locations result in six different sound values to be computed and compared.

Sound reception areas (right): The positions of the sound reception areas can be clarified by means of this diagram. If a sound source is located in a direction 45° below one's head, then the sound waves reach the upper reception area along a route which is 9.1 mm longer. The height of the right-angled triangle would then be 13 mm. The entry to the tunnel-like rim of the helix is located below Darwin's bulge. The aperture for the shorter route 1 is located where the rim of the antihelix becomes a flume. The sound waves are also reflected into the helix rim by the thickened antihelix from whence it follows the longer route to the ear canal. It could also travel along the shorter route. The shortest link between the two apertures makes an angle of 45°, so that the difference between the two routes is about 18 mm (based on J. Maximilian, E. Irrgang, and B. Andresen).

Let us now consider the structure of the ear.

The auricle: The human auricle, with its attractive relief of ridges, hollows, bulges, curves and grooves, is unmistakeably the same in its basic features, yet is a little different, in each one of us. The fact that these complex and beautiful forms play an important part in the hearing process has only been discovered in recent years.

Sounds are conducted to the external hearing canal along two different paths, with the result that the sound travelling along the shorter route arrives one five thousandth of a second before the other signal. Given that sound travels at a speed of 330 m/s, this means that the difference in path length is about 6.5 cm. This is quite separate from the time difference between the two ears, which enables us to locate the source of various sounds. Such refined accoustic analysis is fairly essential for us. In effect we are able to analyse sounds in three dimensions, in such a way that we can recognise the direction of all incoming sounds, as well as the location and motion of their sources.

Vocal communication requires very accurate identification of the position and movement of someone who is speaking, as well as of all the complex sound sequences involved. Since there are two sound paths for each ear, we virtually possess four ears. This ingenious system is so subtly and cleverly designed, that it all happens without us ever being aware of any doubling or quadrupling of sounds.

Darwin’s book, The Descent of Man and Selection in Relation to Sex was published in 1871. In it he denigrated the external human ear, with all its convolutions, as being pointless and useless, a degenerate left-over from some alleged evolutionary history. The little protrusion on the upper outside edge of the auricle has been known since then as “Darwin’s tubercle”. Generations of researchers after Darwin blindly accepted his verdict on the ear. But in reality, the whole beautiful, convoluted labyrinth that is our auricle is a precise, genetically programmed device which delivers an identical additional signal to the brain after a time lag of one five thousandth of a second ( = 0.0002 s, see the diagram on page 24). In effect, one has four ears, two of which are located slightly higher than the other two. The result of this finely tuned system is that the brain is able to process six different values, two of them being the differences between the two upper ”ears”, two more are those between the two lower “ears”, and the third pair is that between the lower “ear” on one side, and the upper “ear” on the other side (see the diagram on page 25). The required computations are carried out at lightning speed in the brain to give us a very sophisticated “sound image” of our surroundings. This structure is also crucial in our astounding ability to voluntarily suppress some sounds to the enhancement of others.

Middle ear: After travelling along the external acoustic meatus, incoming sound waves strike the ear drum, which is set vibrating. The energy transferred in this way is passed on, as the three tiny, connected bones in the middle ear (malleus, incus, and stapes, or hammer, anvil and stirrup) transmit the sound vibrations through the oval window into the inner ear. Weighing only about 10 mg, a tiny percent of the mass of the smallest of coins, these minute bones are the smallest in the entire human body.

The hearing process involves the transfer of air vibrations into the liquid medium of the inner ear. In the normal course of events, the greater part of the sound energy would be reflected at an air/liquid boundary, and such losses would play havoc with the hearing process. To circumvent this, the Creator used a very ingenious interface structure which limits reflection losses to a negligible level.

This complex mechanism, comprising the ear drum and the three middle ear bones, exactly matches the sound wave impedance in air to that of the inner ear. The effective vibrational area of the ear drum is about 0.65 cm2, which is 20 times that of the oval window (only 0.032 cm2). This is equivalent to an amplification factor of 20. In addition, the leverage afforded by the malleusincus- stapes linkup provides a further amplification factor of 3.

Inner ear: It is clear that a massive amplification factor is involved in the conversion of air waves (at the ear drum) to the vibrations of the liquid in the cochlea. The inner ear, comprising both the balance organ and the cochlea (Latin cóchlea = snail), is housed in the solid bone of the skull. A second conversion takes place here; mechanical vibrations are changed into electric neural (nerve) impulses.

The cochlear duct is filled with a highly viscous liquid (Latin viscum = birdlime, sticky, or thick), called the endolymph. There are two more liquidfilled spaces on either side, the scala vestibuli (Latin scala = steps; vestibuli = forecourt), and the scala tympani (Latin tympanum = drum). Both of these cavities are filled with a somewhat less viscous liquid, called the perilymph, and they are connected at the apex of the cochlea (by the helicotrema). The scala vestibuli begins at the oval window, and the scala tympani ends at the membrane of the round window.

The cochlear duct and the scala vestibuli are separated by an elastic sheet, called the vestibular membrane. This membrane reproduces the waveshaped changes in volume caused by the incident sound. The endolymph then transmits the vibrations to the basilar membrane which lies between the cochlear duct and the scala tympani. Eventually the round window is reached via the perilymph. Because of this shortcut the vibrations do not have to travel all the way round through the helicotrema. The vestibular and basilar membranes thus vibrate in unison.

The spiral-shaped bulge of the organ of Corti is located above the basilar membrane. It consists of sensory cells, twelve thousand of which, arranged in rows of three to five, make up the exterior hair cells. The interior 3,500 hair cells as well as the structural support cells lie in one row. The 12,000 sensory cells are arranged on a 32 mm long lamina in four parallel rows having a total width of only 1/20 mm. Their geometrical sequence and distribution resemble that of piano keys in a linear scale. At one end the cells are tuned to a maximum frequency of between 10 and 20 kHz, which descends to about 30 Hz at the other end.

When sounds are received, the basilar membrane vibrates in sympathy, but the amplitude is inconceivably minute, only about 10-11 m. This is equal to 100 picometres or one thousand millionth of a cm (one million million pm = 1 metre) which approaches the size of a few atoms. The tips of the outer hair cells penetrate a covering membrane (the tectorial membrane) which projects into the cochlear duct. Volume changes in this passage cause the basilar membrane to move relative to the tectorial membrane so that the sensory hairs experience a slight shearing pressure. These stimuli are then transmitted as electrical signals along the cochlear (auditory) nerve to the brain. It should be noted that these signals not only travel to the brain, but also in the reverse direction. For this purpose there are two types of neural tissue at the base of the hair cells, namely the afferent (leading to the brain) and the efferent fibres leading to the hair cells. The reason for this feedback is not yet understood and this is only one of the many unsolved puzzles.

There are about 15,000 receptor cells (hair cells) in the cochlea and they are sensitive to different sound frequencies (Diagram, page 27). The hair cells are located in ordered rows on the basilar membrane, which is a thin wall extending through the entire cochlea, following all its convolutions. An incident sound image is separated into its single component frequencies, each of which stimulates only a small fraction of the 15,000 sensory cells located at a specific position on the basilar membrane. The functioning of the cochlea is highly complex, and its ingenious structure is not yet fully understood.

Special abilities of the ear: The ear is the most sensitive human sensory organ. Sounds with frequencies between about 20 Hz and 16 kHz are audible. Lower frequencies are felt rather than heard. All natural sounds are highly complex; pure tones consisting of a single sinusoidal frequency are not found in nature. But when they are produced artificially, they are of great experimental use. Sounds and noises can be considered as a mixture of sinusoidal tones having different frequencies and amplitudes. A tone can be regarded as being the elementary unit for natural sounds and noises. A 3 kHz note having an energy level of only 4 x 10-17 W/cm2 can be heard, and in general audible sound intensities lie in the range between 10-16 and 10-4 W/cm2 (Diagram, page 23).

Speech detection: Amongst all living things the gift of speech is unique. Only man has been endowed by the Creator with this exceptional means of communication. It essentially requires four independent organ systems:

– The larynx which produces the sounds (phonation). – The mouth and throat which modulate the sounds produced by the larynx to form recognisable vowels and consonants. This process is called articulation. – The brain’s motor speech centre controls both the above processes. – Hearing is essential for the continuous feedback control which orderly speech requires; hearing ourselves as we speak, the so-called aural-vocal cycle. This cycle depends on the intact physiological operation of all the auditory mechanisms and pathways leading to perception of speech in the brain’s sensory speech centre. It also involves the psyche and the intellect. It should be obvious that the ear is much more than a technologically sophisticated physical detection device. It is in fact an integral component of a system involving the transmission of meaningful information, of thoughts, ideas, and intelligence, and the beauty of music.

Origin of the ear: From whence does the ingenious construction of the ear (and the eye) derive? The Psalmist gives a concise and striking answer: “He that planted the ear, shall he not hear? he that formed the eye, shall he not see?” (Psalms 94:9). This is also affirmed in Proverbs 20:12: “The hearing ear, and the seeing eye, the LORD hath made even both of them.” The ear did not originate in some evolutionary process, but it was made by the almighty Creator. Jesus calls those people blessed who hear the Word of God (Matt 13:16) and He urges the bystanders to listen: “He that hath ears to hear, let him hear.” (e. g. Matt 11:15, Matt 13:9,43). All the messages which the resurrected and ascended Lord Jesus Christ sent to the seven churches end with the admonishment “He that hath an ear, let him hear …” (Rev 2:7,11,17,29; Rev 3:6,13,22). The Creator blessed us with ears which are indispensable organs for the reception and processing of accoustic information in this life, and He desires that His Word should be given its proper place.