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There is the serene chant of worship that uplifts the soul, and the magic of the mantra with its occult significance. There is the melody of music filling us with delight and the hearty laughter that reflects a happy feeling. There is the cooing of birds, the gurgling of brooks, the shriek of the frightened, the moan of the dejected, and the wail of the bereaved. There is the noise of machine and the roar of thunder. There is the hush-hush of gossip, the abrupt knock on the door, the chime of church bell, the bleating of a goat, the call of a playmate, and the beat of a drum. One can go on and on listing all the wondrous variety of sounds that fill our world of perceived reality.

Every sound enriches life in a different way, and connects us invisibly to our surroundings. Some evoke thoughts, others incite feelings. Some create emotions and others produce a jerky response. Some give us delight, others cause pity. Some convey information, others nothing at all.

We rarely pause to consider how we are affected by the sounds that reach us because we rarely reflect on the commonplace. Yet, sound is a major feature of perceived reality that adds depth and meaning to the world of experience in myriad ways. Its rhythmic forms in poetry and prayer put us in communion with realms of reality, real or imagined, and its magnificent expressions in music are divine as any God or angel.

What is this sound that we rejoice in and respond to? There are two sides to this question: First, what is there in the external world that strikes us as sound? Second, what is happening in the internal world underneath the skull that creates this sensation? Perceived reality always has two parts: reality and perception. And there is its impact on the core of our being.

Physics is concerned with the reality aspect, and neuroscience with the perception aspect. The deep impact is part of religious reflection.

Sound is immaterial: What we hear as sound is not substance by state of motion. Sound is a consequence of vibrations brought about in the surrounding air. The vibrations are longitudinal pressure waves, incredibly subtle so as not to cause undue turbulence. They could be set up by any vibrating body: a drum, a string, a bell, the vocal cord, or whatever.

Air is always disturbed to one degree or another by the motions occurring in it. By simply swinging a rope or a baton in air, we can set up vibrations. But we do not hear every swing because our sensory mechanisms respond only to frequencies within a certain range. No human hand can shake a cane at 20 or more cycles a second, for that is the minimum frequency for vibrations to become audible. On the other hand, a mosquito can flap its wings this fast and more, which is why we hear their annoying hum as they hover in our vicinity. Nor do we hear the much faster vibrations of the atoms in a solid, for there is also an upper limit to audible frequencies, about 20,000 hertz.

So what we perceive as sound consists of waves: in air, in water, in solids, or wherever, but waves in a material medium, for there is something that vibrates and causes the waves.

The magic occurs when the waves reach our ears. The surface area of the normal eardrum is barely a centimeter square, smaller than a thumbnail. When the slight sound waves strike the eardrum, it begins to vibrate. Then through the wiring made up of neurons, powered by potassium and sodium ions, and assisted by fluids and bone-structure, the stimuli reach the brain where the physical processes get transformed into the experiential mode: We hear.

The sweet whisper of a beloved one and the firm order of the army commander are both sounds. The soft rustling of leaves in breeze and roaring noise of the Niagara are sounds, too. But there are differences in loudness. Loudness is a feature of sound that strikes us most. Up to a point, loudness is necessary for sound to be audible. Beyond that, to an extent, it may be necessary for clarity. In excess, loudness becomes a downright nuisance though it seems to augment the enjoyment of some types of music.

Waves transport energy. The loudness of a sound is a reflection of the amount of energy the wave carries. Compared with the energy involved in other common contexts, the energy carried by sound waves is pitifully small. The marvel of our ear is that it detects stimuli as low as a tenth of a quadrillionth of a joule per second.

The clap of thunder reaches our ears only a few seconds after the flash of lightning blinds our eyes: We conclude that sound travels slower than light. Indeed, we conclude that sound takes time to travel even if light may not. When we talk to a person, even at the far end of a hall, unlike the thunder from the distant sky, our words seem to be heard right away. We do not see any lack of synchronization between the movements of the lips and the sound therefrom. This suggests that sound must be traveling quite fast: In fact, it does, at more than a thousand feet a second in air.

Think of what would happen if sound traveled at a much slower pace, say a few millimeters a second, When the professor is lecturing, students in the back rows will be hearing her a few minutes later than those in the front rows! The thud of thunder would be heard perhaps an hour after the flash of lightning.

The wavelengths of sound are not very small. When we hear a baby crying in her room, sound has turned around the walls and reached us. In the language of physics, sound waves diffract around obstacles. One may suspect from this that the lengths of the sound waves are of the same order of magnitude as the doors and the walls. Indeed they are. When we strike the middle C note on a piano, we generate a sound whose frequency is 262 Hz. This corresponds to a wavelength of about 4.3 feet. The same note five octaves higher has a frequency of 8,384 Hz, hence a wavelength of only 1.7 inches.

It is good that sound waves have wavelengths of a few inches and feet. Imagine for a moment that the wavelength of sound, like that of light, is of the order of a few millionth part of a millimeter. Then, we would not be able to hear a person calling us from right behind us, our dog barking outside, or a prowler’s stealthy steps, for the waves would not bend around corners to reach our eardrums.

This is one example of how certain quantitative features of physical phenomena make us perceive the world the way we do. It is not simply the physical laws that create the impressions we receive, but equally the numerical values of their measurable aspects.

This recognition has provoked in the minds of some that the world was created such as it is for our benefit. Some have contended that the great architect of our world designed sound waves commensurate with our dimensions so that we might be able to hear the baby’s cry from the next room. Aside from the fact that microbes survive without this benefit—they don’t even hear, let alone listen to music being played in the next apartment—this argument is as valid as the view that our noses were made with bridges so that we might be able to rest our spectacles on them.

We are grateful for our faculties of sight and hearing, of touch, smell, and taste. However, these make us aware of only small portions of the physical world. More often than not, our faculties are even deceptive in how they portray whatever there is. Were it not for the instruments we have devised, and our imagination, we wouldn’t be able to recognize anything beyond what we get from our direct perceptual modes.

There are a great many things not directly accessible to our sensory perceptions: for example, pressure waves of frequencies greater than 20,000 Hz and lower than 2,000 Hz. We call these ultrasonic and infrasonic waves. Yet, these have been used for practical purposes. We have all heard of sonar and ultrasound images of fetuses. By injecting ultrasound into water containing certain toxic chlorinated compounds, the pollutants can be broken down.

Unearthing the roots of perceived reality of sound has opened up possibilities never before dreamed of. If someone of an earlier century had been told that the sex of an unborn child can be accurately told by just exploring the expectant mother’s body externally, or that the precise location of an underwater ship could be determined without the use of light, it would have sounded like pure fantasy.

At the experiential level, the difference between music and noise is clear: We enjoy one and find the other to be annoying. Both are composites of waves. Musical sounds consist of discrete frequencies, whereas noise is made up of a continuous set of waves. It is somewhat like the difference between a can of pebbles and a can of oil. In the one case, we can separate out the components, literally count them as so many; in the other case, it is one continuous stream with indistinguishable parts. In a musical sound, discrete frequencies are present. In noise, practically all frequencies are present. It is remarkable that these differences can have such varying effects on our perceptions.

According to an ancient idea, associated with the celestial spheres is a cosmic music that permeates the universe. Just as strings plucked in proper proportions produce the various octaves on the scale, planetary motions in conformity with celestial arithmetic was believed to create a divine music.

The imagery is beautiful: The spheres are heavenly wheels on which stand sweet sirens who create musical notes. The combination of these notes merged to form the music of the spheres. These became the Muses after Plato’s school was formed, and Christian mythology transformed these into angels and other beings who formed the celestial orchestra. The Pythagoreans believed that their master was one of the few who could hear this music. Others, perhaps in the mother’s womb or in early infancy, recognized it, but age and corruption soon deafened ordinary mortals to this universal harmony.

Aristotle, who believed heavenly bodies to be perfect crystalline material, feared that the delicate perfection of cosmic crystals would be shattered by such loud notes in heaven.

Ancient Hindus imagined the universe to be pervaded by magical mantras: divine poetry with spiritual prosody and esoteric significance. These nuggets of eternal wisdom were revealed to the Himalayan sages as the Vedas, the scriptural treasures of the Hindu religion. When Moses heard the Commandments and Prophet Mohammed the Holy Quran, they too were privy to this cosmic music, it is believed. Chants and mantras, like the drone of the Hindu om, are human efforts to resonate with the cosmic vibrations.

Underneath the poetry of the ideas of the ancients, there is this insight: Music is interlinked with mathematics, and mathematics to knowledge. If mathematics pervades the world, so much music and knowledge. And the enlightened ones receive that knowledge from that cosmic music.

From the perspective of physics, it is difficult to imagine music in stellar space, if only because an elastic material medium is necessary for sound to propagate. No one can sing on the moon, for there is no air to vibrate. Even if one were to beat a drum with full force, no sound would emerge there since there is no atmosphere on the moon.

And yet, in a strange sort of way, the idea of a cosmic vibration has modern analogy. In 1966, astronomers discovered a microwave radiation that is cosmic in scope and has been there ever since the birth of the universe. This surely is not sound, nor music in the usual sense, but there is an all-pervading vibration in the heavens that began with Genesis, the birth-cry of the baby universe, as it were. This is a notion that would have seemed strange and unacceptable to the physicists of the 18th and 19th centuries.