Unheard Sounds

Sound is defined, in simplest terms, as the transmitted vibrations of any frequency. These transmissions are a constant presence in the lives of all individuals, and play an active role in their perceptions of art, physical surroundings, and interpersonal communication. Hearing is the physiological process of perceiving sound. Most humans rely heavily on both the presence of sound and their ability to hear in order to function effectively on a daily basis. For example, persons crossing the street depend on the sounds of oncoming traffic or sirens for safety purposes, the sound of a telephone ring for that type of communication, or the sound of a song for appreciation of the meaning that the work presents. Each of these examples demonstrates the conscious auditory process that people employ to effectively apply the sounds they hear in the best manner with respect to their individual goals.

Sounds that cannot be heard by the human ear present a different realm of auditory processing that is not commonly addressed. The term sound is said to represent all transmitted vibrations. Sound vibrations, which fall outside of the range of 20Hz to 20,000Hz, do not present an audible noise to the human ear, however these vibrations still fall under the definition of sound, as they are transmitted vibrations (Berendt, 56). In addition to vibrations that do not produce an audible noise to the human ear, the intersection of two conflicting sound waves creates another type of noiseless situation. In these situations, sound exists by definition, however individuals are not consciously processing the sound and therefore do not recognize the ramifications therein. Analysis of how individuals both process these types of sounds as well as how they practically apply them in the world, will yield insight into the possibilities of heightened aural experiences for humans and a stronger comprehension of the human auditory system.

The concept of unheard sounds can best be explored by first looking at how the human ear absorbs sound under normal conditions. The explanation of the inner workings of the outer, middle, and inner ear in auditory processing provides a basis for understanding the fundamentals of hearing and listening. Next, this knowledge can be built upon in understanding how it is possible for sound to be present without being heard. The concepts of noise cancellation and ultrasound are the two most common forms of unheard sound to be explored. Taking this knowledge a step further, discussion of the application of these sounds and their value will lead to conclusions regarding the hearing capabilities of humans.

The Human Ear

The human ear is the apparatus by which individuals perceive sound. The manner in which the ear works to accomplish this goal is fundamental to understanding any type of sound absorption or auditory process. Listening depends on three things: filtering and absorption, perception, and environment (Moore, 109). The ear is responsible for the first two elements, and therefore should be further examined when studying the complex nature hearing. The ear can be divided into three key structures. The outer ear is primarily responsible for reception and filtering. The middle ear works to transmit vibrations to the inner ear, which is responsible for sending sound signals to the brain and aiding in perception. Specifically examining the functions of each part of the ear demonstrates how they work together to promote hearing and indirectly describe the circumstances for non-audible sound to take place.

The outer ear serves as the initial receptor and filter for all transmitted vibrations that exist in the environment (Moore, 112). This part is the most external, and is composed of three parts: the pinna, the auditory canal, and the eardrum. The pinna is the outer most segment of the outer ear. The pinna is generally referred to as the ear lobe. The purpose of the pinna is to receive sounds in the surrounding environment. The pinna is shaped like a horn, and amplifies the sounds that it collects. The pinna also filters the sound to absorb only the frequencies of human speech, and directs these sounds to the inner portions of the ear (Blauert, 327). Once through the pinna, the auditory canal serves as a path for sound to travel from the pinna to the eardrum. As sound travels through the auditory canal, a more thorough filtering process takes place in an effort to protect the ear from infection. The outer most part of the auditory canal is made up of nerve fibers that constantly refresh the ear with new skin. Farther in are groupings of hairs that also filter the sound. The last section of the auditory canal is composed of wax strands known as cerumen strands that help to catch the tiniest dust particles, and allow only sound to proceed through to the eardrum. The eardrum plays a key role in hearing. The eardrum is approximately 1 cm in diameter, very thin and translucent. When faced with sound waves transmitted by the auditory canal, the eardrum vibrates and transfers these vibrations to the middle ear (Moore, 117).

The middle ear consists of the three smallest bones in the body, and serves to effectively transfer sound vibrations from the eardrum to the inner ear. These bones are known as the ossicles. Individually, these bones are referred to as the malleus, incus, and stapes, and sometimes informally as the hammer, anvil, and stirrup due to their physical shape (Handel, 228). Regardless of what name is used for description, these bones will continually perform the same function. The malleus is attached to the eardrum and is the first bone of three to transfer vibrations from the outer ear. The malleus sends the vibrations to the second bone in the sequence, the incus, which passes the vibrations on to the stapes. The stapes is attached to the oval window, which is a part of the inner ear. Along with the ossicles, the middle ear is connected to the Eustachian tube that works to equalize pressure in the ear (Handel, 230). The Eustachian tube is connected to the cavity of the middle ear and leads to an opening in the back of the mouth. When excess fluids build up in the ear, these fluids are drained through the Eustachian tube. Also, when air pressure builds up in the ear, excess air is released through this tube. The Eustachian tube both drains excess fluid from the ear and equalizes pressure on both sides of the eardrum (Handel, 236).

The final portion of the ear is the inner ear. This part of the ear is very important to sound absorption and perception, as this is the part of the ear responsible for sending auditory information to the brain (Wever, 86). The inner ear is made up of the oval window, semicircular canal, cochlea, and the auditory nerve. The oval window is a small membrane in the wall of the cochlea, and is responsible for transferring vibrations from the stapes of the middle ear to the fluid of the cochlea in the inner ear. The cochlea is a coil shaped structure that is internally divided by the basilar membrane. The primary responsibility of the cochlea is to convert physical vibrations into electrical impulses. The vibrations in the fluid of the cochlea cause standing waves to form on the surface of the basilar membrane. The location of the peaks of these waves depends on the frequency of the sound vibrations in the fluid of the cochlea. Tiny hairs known as stereocilia detect the waves on the basilar membrane and the vibrations in the fluid, and convert the information to electrical impulses that are sent to the brain via the auditory nerve. Once the neural signals from the stereocilia have reached the brain, the signals from both ears are processed and hearing takes place (Handel, 210). The semicircular canals are a part of the inner ear, however they do not perform any function in auditory processing. The semicircular canals are present to maintain balance and alert the brain when an individual’s head has shifted its position.

The process of hearing in the human ear is accomplished by the interworkings of the outer, middle, and inner ear. The outer ear collects, directs, and filters sound vibrations to the middle ear. The middle ear passes these vibrations along, via the ossicilies bone structure, and equalizes pressure on both sides of the eardrum with the Eustachian tube. Finally, the inner ear receives the vibrations from the middle ear and converts them to electrical impulses to send to the brain through the auditory nerve. The brain then processes neural signals obtained from both ears to generate the sensation that people perceive as hearing. Human auditory processing allows individuals to hear audible sounds and enjoy the many positive benefits associated with that ability. The hearing process, as explained by the functions of the three parts of the ear, implies that all sound vibrations are uniformly absorbed and generally produce the similar types of hearing sensations. While in most cases this assumption can be applied, circumstances exist in which sound is not perceived in the standard form. The absorption of these types of sounds, which are not heard by humans, present benefits that are similar to those associated with audible sounds. However, these benefits can often be overlooked due to the fact that these sounds are not consciously processed. Before discussing the advantages and uses of unheard sounds, the manner in which noiseless sound can occur and be absorbed should first be examined.

Hearing More Than We Think

Vibrations that are absorbed by the human ear, but not “heard” provide productive uses, which tend to be unnoticed due to the manner in which the sounds were realized. This section of the paper seeks to investigate the circumstances under which sounds are processed but not heard. Noise cancellation, and ultrasonic vibrations are the two main types of unheard sound (Berendt 117). Noise cancellation represents destructive interference of sound waves resulting in sound that is not audible. Ultrasonic vibrations are those sounds which occur at frequencies above 20,000Hz, which is the maximum frequency that can be heard by the human hear. The dynamics of how these situations can occur should be further analyzed for the purposes of understanding how they can be applied to enhance human auditory capabilities.

Noise cancellation is based on the concept of destructive interference (Moore, 215). Destructive interference occurs under distinct criteria. Sound vibrations occur when molecules of a substance vibrate back and forth through that substance. Every sound produces a sound wave that consists of numerous alternating regions of increased and decreased pressure of these molecules. The period of increased pressure of molecules is known as condensation, while decreased pressure periods are known as rarefactions. Each cycle of a sound, or the individual wave, is composed of one period of condensation and one rarefaction. Sound frequency is measured in hertz (Hz). A hertz measures the number of cycles of condensation and rarefaction that pass over a given point per second (Blauert, 253). For example, a frequency measure of 500 Hz would imply that 500 condensations and rarefaction cycles are occurring each second.

In order to demonstrate the concept of sound interference, a hypothetical situation is created. Two sources each produce a sound wave, which are identical to one another. With the waves being the same, the amplitude, frequency, and pitch are also identical. As these sounds are being emitted, the condensations of each wave are colliding with the condensations of the other wave, and the rarefactions collide with the rarefactions of the other wave. As this occurs, the molecules of each wave double the amplitude of the wave. The listener then absorbs a sound wave with a greater amplitude and pitch and thus perceives the sound as louder. This type of interference is known as constructive interference. Destructive interference is the exact opposite. Instead of the condensations colliding with the condensations and the rarefactions colliding with the rarefactions, assume the same sound environment as stated above still exists only the sound waves being emitted are exact opposites of each other. In such a situation, the condensations of one wave are meeting with the rarefactions of the other wave. The sound waves cancel each other out. The listener absorbs the combined effect of these vibrations and subsequently hears nothing.

Unlike noise cancellation, ultrasonic vibrations are not the combination of two sound waves from different sources, but rather one wave that travels at a frequency above the range of human audibility. As mentioned before, the frequency of a sound wave measures the number of cycles of condensation and rarefactions of molecules pass over a point in a second, and the pitch of a sound is how the brain interprets this frequency. When the frequency of a sound is above 20,000 Hz, the cycles of the sound wave are occurring at a rate that is too fast to be interpreted by the human brain. Also, if it were possible for the human auditory system to effectively process sound waves at this high frequency, studies have shown that the perception would likely be very painful (Moore, 189). As a result, 20,000 Hz is also known as the human auditory threshold of pain. As it is, the frequency of 20,000 Hz is too rapid of a vibration to be processed in the ear. Humans, therefore, are unable to hear these vibrations even though they are present in the environment. On the other side of the human auditory range is sound that occurs below 20 Hz, or the lowest level of detectable sound that can be heard occurs at a frequency of 20 Hz. At a level of 20 Hz, the wavelength of a sound wave is extremely long and the pitch is extremely low resulting in a very low sound. Below the level of 20 Hz would not be painful to listen to as ultrasound, however sound at this level does travel at a vibration rate or frequency too slow to be processed by the human auditory system (Handel, 114). Often, individuals can absorb sounds at very low frequencies through their sense of touch or by feeling for the vibrations generated by the sound. The deaf community commonly relies on other sensations such as touch to absorb sounds.

The concept of sounds being present in the environment and not being heard is not only a strong possibility, but also a clearly supported actuality. Noise cancellation occurs when two sound waves of the same frequency, amplitude, and pitch intersect each other and cancel each other’s condensations and rarefaction periods out resulting in silence. Ultrasonic sounds are vibrations traveling at frequencies too high to be audibly absorbed by the human ear. Sounds traveling at frequencies below 20 Hz are also unheard as their vibrations are moving too slowly creating a frequency and pitch too low to be processed by the human auditory system. In each case, sound vibrations are present in the environment, but for one reason or another, these sounds are not heard. Further investigation into the uses of these types of sounds will demonstrate both the benefits they provide as well as the possibilities they present.

Using Unheard Sounds

Examining the manners in which individuals can and do employ noiseless sounds in everyday life, is insightful in that this knowledge promotes awareness of heightening the human auditory experience. Destructive interference and noise cancellation can be used in sound proofing techniques and to further audio enhancements. Ultrasonic waves are perhaps the most useful of all noiseless sound as they can be employed in forecasting weather, navigation, tracking objects, and for animal survival. Specifically analyzing both of these areas of noiseless sound, will promote greater comprehension of human auditory capabilities.

To begin with, destructive sound interference results in noise cancellation, which is perceived as silence to the human ear. This silence, while seemingly serving no auditory purpose, is actually a key element employed by individuals for both soundproofing as well as sound enhancement. Sounds that travel at extraordinarily high frequencies are extremely loud. When the ear is continually exposed to vibrations with such high frequencies, elements of the ear can be damaged resulting in decreased auditory ability in the future. In an effort to prevent such disabilities from happening to individuals who work in environments of high frequency sound, noise cancellation can be employed preventatively to protect these individuals from encountering hearing difficulties in the future (Berendt, 98). For example, employees in the airline industry are surrounded by the loud noises of jet engines. To compensate, these individuals wear specialized headphones that absorb outside sounds of the engine and create a sound wave that is the inverse of the one absorbed thus creating noise cancellation and providing a quieter environment for the individual. This not only protects the auditory system, but also facilitates more efficient performances from employees (Moore 206). In this same manner, noise cancellation is also used in automobiles to create a quieter environment for the passengers. In addition to soundproofing, many musicians and audio artists use the property of noise cancellation to tune their instruments. Audio artists also take advantage of noise cancellation in creating artwork. Destructive interference creates beats in music, which demonstrate that an instrument is out of tune. A person listens for the destructive interference or beats when tuning an instrument. Destructive sound interference or noise cancellation is one example of sound that is unheard, but also heard in the form of the functions that can be provided due to its existence. Noise cancellation generates the opportunity for people to safely function in damaging environments and enhances auditory experiences by providing a basis for differentiation among audible sounds (Blauert, 316).

Ultrasound is the most commonly used form of noiseless sound. Ultrasound is utilized to assist with navigation, forecasting the weather, and tracking various objects. An ultrasonic wave travels at such high frequencies that the wave is perfect for use in measurement as the frequency has few gaps leaving a very small margin for error. The term SONAR, stands for sound navigation ranging, and works by bouncing sound waves off of objects to determine their location or distance from a specified point (Berendt, 254). A sonar device consists of a transmitter and a receiver. The transmitter emits an ultrasonic sound wave that travels through the air until coming into contact with another object. When this occurs the sound wave bounces off of the object and returns to the receiver. The time elapsed between the initial transmission and the reception of the sound wave represents the distance from the transmitter to the object. Sonar is used to measure depths of water, traveling vehicles, and even moving air masses. While not audible, ultrasonic waves are absorbed and perceived to provide valuable information that is important for safety and communication.

Despite the fact that humans cannot audibly absorb certain sounds, these sounds are present in the environment and are processed and utilized in many of the same manners as audible sounds. The human auditory process allows vibrations to travel through the ear and be transformed into electrical impulses that are sent to the brain and perceived as the sense of hearing (Wever, 88). Individuals are conscious of hearing and therefore recognize the various functions and advantages that go along the use of that particular sense. Conversely, humans are not consciously aware of the hearing that occurs with sounds that do not produce an audible noise, and therefore do not acknowledge the value or possibilities that these sounds provide. Individuals in the form of beats, which aid in the enhancement of auditory experiences, and soundproofing, which protects the auditory system, perceive noise cancellation. Ultrasound is used in navigation, tracking systems, and weather forecasting to supply valuable information. Both noise cancellation and ultrasound make available some important benefits, but more importantly, acknowledging these benefits and taking advantage of the possibilities that go along with them, will lead to positive progression in auditory technology, which will enhance the experience for everyone.