Lindsay's Wheel of Acoustics, which shows fields within acoustics
Acoustics is a branch ofphysics that deals with the study ofmechanical waves in gases, liquids, and solids including topics such asvibration,sound,ultrasound andinfrasound. A scientist who works in the field of acoustics is anacoustician while someone working in the field of acoustics technology may be called anacoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio andnoise control industries.
Hearing is one of the most crucial means of survival in the animal world andspeech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society—music, medicine, architecture, industrial production, warfare and more. Likewise, animal species such as songbirds and frogs use sound and hearing as a key element of mating rituals or for marking territories. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge.Robert Bruce Lindsay's "Wheel of Acoustics" is a well-accepted overview of the various fields in acoustics.[1]
The word "acoustic" is derived from theGreek word scratching (acoustic), meaning "of or for hearing, ready to hear"[2] and that from Christmas (acoustic), "heard, audible",[3] which in turn derives from the verb ἀκούω(akouo), "I hear".[4]
The Latin synonym is "sonic", after which the termsonics used to be a synonym for acoustics[5] and later a branch of acoustics.[5]Frequencies above and below theaudible range are called "ultrasonic" and "infrasonic", respectively.
Thefundamental and the first 6overtones of a vibrating string. The earliest records of the study of this phenomenon are attributed to the philosopherPythagoras in the 6th century BC.
In the 6th century BC, the ancient Greek philosopherPythagoras wanted to know why somecombinations of musical sounds seemed more beautiful than others, and he found answers in terms of numerical ratios representing theharmonicovertone series on a string. He is reputed to have observed that when the lengths of vibrating strings are expressible as ratios of integers (e.g. 2 to 3, 3 to 4), the tones produced will be harmonious, and the smaller the integers the more harmonious the sounds. For example, a string of a certain length would sound particularly harmonious with a string of twice the length (other factors being equal). In modern parlance, if a string sounds the note C when plucked, a string twice as long will sound a C an octave lower. In one system ofmusical tuning, the tones in between are then given by 16:9 for D, 8:5 for E, 3:2 for F, 4:3 for G, 6:5 for A, and 16:15 for B, in ascending order.[6]
Aristotle (384–322 BC) understood that sound consisted of compressions and rarefactions of air which "falls upon and strikes the air which is next to it...",[7][8] a very good expression of the nature ofwave motion.On Things Heard, generally ascribed toStrato of Lampsacus, states that the pitch is related to the frequency of vibrations of the air and to the speed of sound.[9]
In about 20 BC, the Roman architect and engineerVitruvius wrote a treatise on the acoustic properties of theaters including a discussion of interference, echoes, and reverberation—the beginnings ofarchitectural acoustics.[10] In Book V of hisDe architectura (The Ten Books of Architecture) Vitruvius describes sound as a wave comparable to a water wave extended to three dimensions, which, when interrupted by obstructions, would flow back and break up following waves. He described the ascending seats in ancient theaters as designed to prevent this deterioration of sound and also recommended bronze vessels (echea) of appropriate sizes be placed in theaters to resonate with the fourth, fifth and so on, up to the double octave, in order to resonate with the more desirable, harmonious notes.[11][12][13]
Principles of acoustics have been applied since ancient times: aRoman theatre in the city ofAmman
The physical understanding of acoustical processes advanced rapidly during and after theScientific Revolution. MainlyGalileo Galilei (1564–1642) but alsoMarin Mersenne (1588–1648), independently, discovered the completelaws of vibrating strings (completing what Pythagoras and Pythagoreans had started 2000 years earlier). Galileo wrote "Waves are produced by thevibrations of a sonorous body, which spread through the air, bringing to the tympanum of theear a stimulus which the mind interprets as sound", a remarkable statement that points to the beginnings of physiological and psychological acoustics. Experimental measurements of thespeed of sound in air were carried out successfully between 1630 and 1680 by a number of investigators, prominently Mersenne. Inspired by Mersenne'sHarmonie universelle (Universal Harmony) or 1634, the Rome-based Jesuit scholarAthanasius Kircher undertook research in acoustics.[16] Kircher published two major books on acoustics: theMusurgia universalis (Universal Music-Making) in 1650[17] and thePhonurgia nova (New Sound-Making) in 1673.[18] Meanwhile,Newton (1642–1727) derived the relationship for wave velocity in solids, a cornerstone ofphysical acoustics (Principia, 1687).
Substantial progress in acoustics, resting on firmer mathematical and physical concepts, was made during the eighteenth century byEuler (1707–1783),Lagrange (1736–1813), andd'Alembert (1717–1783). During this era, continuum physics, or field theory, began to receive a definite mathematical structure. The wave equation emerged in a number of contexts, including the propagation of sound in air.[19]
In the nineteenth century the major figures of mathematical acoustics wereHelmholtz in Germany, who consolidated the field of physiological acoustics, andLord Rayleigh in England, who combined the previous knowledge with his own copious contributions to the field in his monumental workThe Theory of Sound (1877). Also in the 19th century, Wheatstone, Ohm, and Henry developed the analogy between electricity and acoustics.
The twentieth century saw a burgeoning of technological applications of the large body of scientific knowledge that was by then in place. The first such application wasSabine's groundbreaking work in architectural acoustics, and many others followed. Underwater acoustics was used for detecting submarines in the first World War.Sound recording and the telephone played important roles in a global transformation of society. Sound measurement and analysis reached new levels of accuracy and sophistication through the use of electronics and computing. The ultrasonic frequency range enabled wholly new kinds of application in medicine and industry. New kinds of transducers (generators and receivers of acoustic energy) were invented and put to use.
Acoustics is defined byANSI/ASA S1.1-2013 as "(a) Science ofsound, including its production, transmission, and effects, including biological and psychological effects. (b) Those qualities of a room that, together, determine its character with respect to auditory effects."
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.
The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of causes, both natural and volitional. There are many kinds of transduction processes that convert energy from some other form into sonic energy, producing a sound wave. There is one fundamental equation that describes sound wave propagation, theacoustic wave equation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about anearthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.
The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. Influids, sound propagates primarily as apressure wave. In solids, mechanical waves can take many forms includinglongitudinal waves,transverse waves andsurface waves.
Acoustics looks first at the pressure levels and frequencies in the sound wave and how the wave interacts with the environment. This interaction can be described as either adiffraction,interference or areflection or a mix of the three. If severalmedia are present, arefraction can also occur. Transduction processes are also of special importance to acoustics.
In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. While this disturbance is usually small, it is still noticeable to the human ear. The smallest sound that a person can hear, known as thethreshold of hearing, is nine orders of magnitude smaller than the ambient pressure. Theloudness of these disturbances is related to thesound pressure level (SPL) which is measured on a logarithmic scale in decibels.
Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencies, partly because this is how ourears interpret sound. What we experience as "higher pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand the acoustic phenomenon.
The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20Hz and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths which allow better resolution in imaging technologies. Medical applications such asultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomena such as earthquakes.
Analytic instruments such as thespectrum analyzer facilitate visualization and measurement of acoustic signals and their properties. Thespectrogram produced by such an instrument is a graphical display of the time varying pressure level and frequency profiles that give a specific acoustic signal its defining character.
An inexpensive low fidelity 3.5 inchdriver, typically found in small radios
Atransducer is a device for converting one form of energy into another. In an electroacoustic context, this means converting sound energy into electrical energy (or vice versa). Electroacoustic transducers includeloudspeakers,microphones,particle velocity sensors,hydrophones andsonar projectors. These devices convert a sound wave to or from an electric signal. The most widely used transduction principles areelectromagnetism,electrostatics andpiezoelectricity.
The transducers in most common loudspeakers (e.g.woofers andtweeters), are electromagnetic devices that generate waves using a suspended diaphragm driven by an electromagneticvoice coil, sending off pressure waves.Electret microphones andcondenser microphones employ electrostatics—as the sound wave strikes the microphone's diaphragm, it moves and induces a voltage change. The ultrasonic systems used in medical ultrasonography employ piezoelectric transducers. These are made from special ceramics in which mechanical vibrations and electrical fields are interlinked through a property of the material itself.
There are many types of acousticians, but they usually have aBachelor's degree or higher qualification. Some possess a degree in acoustics, while others enter the discipline via studies in fields such asphysics orengineering. Much work in acoustics requires a good grounding inMathematics andscience. Many acoustic scientists work in research and development. Some conduct basic research to advance our knowledge of the perception (e.g.hearing,psychoacoustics orneurophysiology) ofspeech,music andnoise. Other acoustic scientists advance understanding of how sound is affected as it moves through environments, e.g.underwater acoustics, architectural acoustics orstructural acoustics. Other areas of work are listed under subdisciplines below. Acoustic scientists work in government, university and private industry laboratories. Many go on to work inAcoustical Engineering. Some positions, such asFaculty (academic staff) require aDoctor of Philosophy.
Archaeoacoustics, also known as the archaeology of sound, is one of the only ways to experience the past with senses other than our eyes.[21] Archaeoacoustics is studied by testing the acoustic properties of prehistoric sites, including caves. Iegor Rezkinoff, a sound archaeologist, studies the acoustic properties of caves through natural sounds like humming and whistling.[22] Archaeological theories of acoustics are focused around ritualistic purposes as well as a way of echolocation in the caves. In archaeology, acoustic sounds and rituals directly correlate as specific sounds were meant to bring ritual participants closer to a spiritual awakening.[21] Parallels can also be drawn between cave wall paintings and the acoustic properties of the cave; they are both dynamic.[22] Because archaeoacoustics is a fairly new archaeological subject, acoustic sound is still being tested in these prehistoric sites today.
Aeroacoustics is the study of noise generated by air movement, for instance via turbulence, and the movement of sound through the fluid air. This knowledge was applied in the 1920s and '30s to detect aircraft beforeradar was invented and is applied inacoustical engineering to study how to quietenaircraft. Aeroacoustics is important for understanding how windmusical instruments work.[23]
Architectural acoustics (also known as building acoustics) involves the scientific understanding of how to achieve good sound within a building.[25] It typically involves the study of speech intelligibility, speech privacy, music quality, and vibration reduction in the built environment.[26] Commonly studied environments are hospitals, classrooms, dwellings, performance venues, recording and broadcasting studios. Focus considerations include room acoustics, airborne and impact transmission in building structures, airborne and structure-borne noise control, noise control of building systems and electroacoustic systems.[27]
Bioacoustics is the scientific study of the hearing and calls of animal calls, as well as how animals are affected by the acoustic and sounds of their habitat.[28]
This subdiscipline is concerned with the recording, manipulation and reproduction of audio using electronics.[29] This might include products such asmobile phones, large scalepublic address systems orvirtual reality systems in research laboratories.
Environmental acoustics is the study of noise and vibrations, and their impact on structures, objects, humans, and animals.
The main aim of these studies is to reduce levels of environmental noise and vibration. Typical work and research within environmental acoustics concerns the development of models used in simulations, measurement techniques, noise mitigation strategies, and the development of standards and regulations. Research work now also has a focus on the positive use of sound in urban environments:soundscapes andtranquility.[30]
Examples of noise and vibration sources include railways,[31] road traffic, aircraft, industrial equipment and recreational activities.[32]
Many studies have been conducted to identify the relationship between acoustics andcognition, or more commonly known aspsychoacoustics, in which what one hears is a combination of perception and biological aspects.[34] The information intercepted by the passage of sound waves through the ear is understood and interpreted through the brain, emphasizing the connection between the mind and acoustics. Psychological changes have been seen as brain waves slow down or speed up as a result of varying auditory stimulus which can in turn affect the way one thinks, feels, or even behaves.[35] This correlation can be viewed in normal, everyday situations in which listening to an upbeat or uptempo song can cause one's foot to start tapping or a slower song can leave one feeling calm and serene. In a deeper biological look at the phenomenon of psychoacoustics, it was discovered that the central nervous system is activated by basic acoustical characteristics of music.[36] By observing how the central nervous system, which includes the brain and spine, is influenced by acoustics, the pathway in which acoustic affects the mind, and essentially the body, is evident.[36]
Structural acoustics is the study of motions and interactions of mechanical systems with their environments and the methods of their measurement, analysis, and control. There are several sub-disciplines found within this regime:
Ultrasonics deals with sounds at frequencies too high to be heard by humans. Specialisms include medical ultrasonics (including medical ultrasonography),sonochemistry,ultrasonic testing, material characterisation and underwater acoustics (sonar).[39]
^AkouoArchived 2020-01-23 at theWayback Machine Henry George Liddell, Robert Scott,A Greek-English Lexicon, at Perseus
^abKenneth Neville Westerman (1947).Emergent Voice. C. F. Westerman.Archived from the original on 2023-03-01. Retrieved2016-02-28.
^C. Boyer andU. Merzbach.A History of Mathematics. Wiley 1991, p. 55.
^"How Sound Propagates"(PDF). Princeton University Press.Archived(PDF) from the original on 2022-10-09. Retrieved9 February 2016. (quoting from Aristotle'sTreatise on Sound and Hearing)
^Whewell, William, 1794–1866.History of the inductive sciences : from the earliest to the present times. Volume 2. Cambridge. p. 295.ISBN978-0-511-73434-2.OCLC889953932.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
^Greek musical writings. Barker, Andrew (1st pbk. ed.). Cambridge: Cambridge University Press. 2004. p. 98.ISBN0-521-38911-9.OCLC63122899.{{cite book}}: CS1 maint: others (link)
^ACOUSTICS, Bruce Lindsay, Dowden – Hutchingon Books Publishers, Chapter 3
^P. Findlen,Athanasius Kircher: The Last Man who Knew Everything, Routledge, 2004, p. 8 and p. 23.
^Athanasius Kircher,Musurgia universalis sive Ars magna consoni et dissoni, Romae, typis Ludovici Grignani, 1650
^Athanasius Kircher,Phonurgia nova, sive conjugium mechanico-physicum artis & natvrae paranympha phonosophia concinnatum, Campidonae: Rudolphum Dreherr, 1673.
^Pierce, Allan D. (1989).Acoustics : an introduction to its physical principles and applications (1989 ed.). Woodbury, N.Y.: Acoustical Society of America.ISBN0-88318-612-8.OCLC21197318.
^Technical Committee on Musical Acoustics (TCMU) of the Acoustical Society of America (ASA)."ASA TCMU Home Page". Archived fromthe original on 2001-06-13. Retrieved22 May 2013.
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