Sound Waves Explained: According to Physics
The phenomenon our minds perceive as sound is created by physical—albeit invisible—waves originating at a source and traveling outwards (away from the source) in all directions through a medium like air or water. Waves are produced by a disturbance of some kind, and if the frequency of those waves is within the audible range, then those waves will be perceived as sound (provided there is a functional set of auditory organs around to receive those waves).
There are four main types of waves that carry information through a medium such as the earth or atmosphere: pulse, continuous, transverse, and longitudinal waves. These wave forms differ in how they travel, but on the microscopic level, all waves are similar in their particle interaction: the movement of waves means that an energy transfer is occurring between particles, and that transfer keeps the wave in motion. This energy transfer is more intense the closer to the point of origination of the wave, and as some energy is lost in each transfer, eventually the wave form will dissipate. Sound waves travel as longitudinal waves.
The way that sound waves create sound is via pressure waves: longitudinal waves cause the air (or water) to compress and expand, alternately, thereby creating areas of higher pressure (known as compression) and lower pressure (known as rarefaction). These pressure waves then cause a receptive mechanism such as a diaphragm in a microphone or the ear drums in our ears to vibrate. These vibrations then cause the particles in the surrounding medium to vibrate via a microscopic energy transfer, and the waves that are generated are then translated into audio data in the microphone and into nerve impulses by our brains.
Sound waves are described by physics as possessing seven main characteristics: amplitude, frequency, velocity (speed), wavelength, phase, harmonic content, and envelope. These qualities are related to each other and together they “define the sound”. The amplitude of a wave is the height of the wave between the alternating peaks and valleys. The wavelength is the distance between two consecutive peaks of a wave; and this distance constitutes one wavelength, or cycle. The frequency of a wave is measured by how many times the wave cycles per second, designated as Hertz. Velocity, or how fast the wave moves through a given medium is the product of the wavelength multiplied by the frequency. A wave will travel more slowly through a denser medium such as water than it will through air. The interaction of two similar waves with each other is known as phase. This interaction can be either constructive or destructive, meaning two similar waves can combine and increase their amplitude, or they can meet and cancel each other out, as in the case of noise-cancelling headphones. The other two characteristics of sound waves, harmonic content and envelope, attempt to account for the quality of the sound as perceived by the listener.
The pitch of a sound corresponds to the frequency of the wave: the higher the frequency, the higher the pitch. The average range of human hearing is 20 to 20,000Hz. Sounds that are higher than this range are called ultrasonic, and sounds that are lower than this are called infrasonic. The loudness of a sound is directly proportional to its intensity: the higher the intensity of a sound, the higher the loudness. Bels is the mathematical measure of intensity; Bels convert sound intensity to a logarithmic scale, whereby each new level of the scale is 10 times as higher than the previous level. For instance, 1 picowatt is equal to zero Bels, and 1 Bel is 10 times as intense as zero Bels. A decibel is one-tenth of a Bel. There is much more math involved in calculating sound waves and their intensity, but I would like to use the rest of this paper to revisit these basics of sound by way of practical application.
There is an old Zen koan, or riddle, that presents the following dilemma: if a tree falls in the forest and no one is around to listen, does it make a sound? According to the explanation given by modern physics of how the phenomenon known as sound actually occurs, the answer to the riddle is yes. Because sound is the product of physical waves traveling through space in every direction from the point of origin until the waves loose their energy and decay, or dissipate into the atmosphere, then the tree that falls in the forest would indeed produce a sound when it hits the ground. Shock waves would radiate through the air and even through the ground (albeit with less intensity because the earth is a denser medium than the air) and, according to the science of acoustics, that is what sound is. Sound is a disturbance in the atmosphere, and whether or not someone is present to experience it doesn’t factor into the equation. Where the relative nature of phenomenal experience does play into the question is when we look at who can hear what. For instance, the normal range of human hearing is 20 to 20,000 Hz. Elephants can hear much lower frequencies than this; meanwhile dogs, cats, dolphins, and bats have a higher range of hearing, extending well beyond 20k. So in conclusion, if a tree falls in the forest it will make a sound, and if some creature is within the appropriate range (distance) and possess the capacity to hear into the necessary range of frequencies, then the waves oscillating through the air will be changed into mechanical vibrations by the creature’s auditory organs and then translated into nerve impulses in the brain and the “sound” will be “heard”.
It is important to note, however, that modern science is itself a product of the modern era and this question has existed in the Zen tradition since when ancient ways were à la mode. Could it be possible that, perhaps, facts that were known to traditions long dead and buried have been lost and forgotten in the course of the earth’s tumultuous history of fighting over resources and religion often motivated by nothing more than arrogant prejudice? The fundamental concepts of light and sound and time and space that comprise both the boundaries and the essence of the matrix that supports all of Life are so all-encompassing and, at the same time, so elusive that I believe the mystery of their nature cannot be so easily explained. And music, when it is well made, is pure magic. The above explanation of sound leaves much to be desired in relation to the question: what is music? Perhaps physics is still in its infancy of understanding the fundamentals of sound, in which case, I look forward to the possibility that the mystery is still unfolding and there is much for us to learn.
