Acoustics - Wikiparency

# Acoustics **Acoustics** is the branch of physics concerned with the generation, propagation, reception and effects of [[Sound|sound]], whether that sound is audible, too low to hear, or too high.[^asa-def] It sits at the crossing point of mechanics, fluid dynamics and hearing, because a sound wave is a mechanical disturbance in a medium — air, water, bone, steel — that a listener or an instrument then measures, and because the same [[Wave|wave]] mathematics describes a plucked string, a foghorn and an ultrasound probe alike. This article is the door to the whole Wikitube Acoustics portal: its own microsim, *Acoustics*, is the same longitudinal-wave-in-a-tube built for the [[Sound]] article, run here with the full control set — frequency, displacement amplitude, a choice of medium among air, helium, water and steel, and a slow-motion dial — so a first-time reader can see, before reading a line of the portal's thirty sections, what every one of them assumes: a wave crowding and thinning a medium at a speed set by what that medium is made of. The subject is old in practice and young as a unified science. People tuned strings and built resonant halls for millennia before anyone could say why either worked, and the word itself was coined only at the start of the eighteenth century. What follows traces that history, states what acoustics studies and does not, works through the wave concepts every subfield shares, describes the people who do the work, surveys the fourteen subfields the field has grown into, and closes with the societies, journals and conferences that hold the field together today. ## History ### Etymology The English word *acoustics* derives from the ancient Greek *akoustikos* (ἀκουστικός), "of or for hearing, ready to hear," itself from *akoustos*, "heard, audible," from the verb *akouein*, "to hear."[^etym] A Latin-derived synonym, *acoustics* is also cousin to the French *acoustique*. The term was fixed in something close to its modern sense by the French physicist and mathematician Joseph Sauveur, who in a 1701 memoir to the French Academy of Sciences on the general theory of sound proposed *acoustique* as the name for the whole science of sound, on the model of *optique* for the science of light; Sauveur, who studied [[Harmonic|harmonics]] and beats and coined the term *node* for a point of no vibration on a string, is often called the father of acoustics for that essay.[^sauveur1701] ### Early research in acoustics Long before the name existed, people built and tuned instruments by ear and by rule of thumb. Ancient Greek theater designers shaped amphitheaters, and later builders raised vaulted rooms and resonating jars, that carried a speaker's or a singer's voice to distant seats, results arrived at by trial rather than by a theory of wave propagation.[^wp-history] The Pythagorean observation that simple whole-number ratios of string length give consonant intervals is the oldest acoustic law on record, and it shaped Western music theory for two thousand years before anyone measured a vibration directly. The turn from craft to physics came in the sixteenth and seventeenth centuries. Galileo Galilei linked pitch to the frequency of vibration in his 1638 *Two New Sciences*, and Marin Mersenne independently stated the laws relating a stretched string's frequency to its length, tension and mass in 1636, work that let [[Frequency|frequency]] be measured rather than only heard.[^wp-history] Isaac Newton attempted the first mechanical derivation of the [[Speed_of_sound|speed of sound]] in air in the *Principia* (1687), treating air as isothermally compressed; the calculation came out roughly 15 percent below the measured speed, a gap not closed until Pierre-Simon Laplace showed in 1816 that the compressions of a sound wave are too fast to exchange heat with their surroundings and must instead be treated as adiabatic.[^newton-laplace] ### Age of Enlightenment and onward The eighteenth and nineteenth centuries turned scattered results into a mathematical science. Sauveur's 1701 memoir named the field and worked out the overtone series; Leonhard Euler, Jean le Rond d'Alembert and Daniel Bernoulli derived and solved the [[Wave_equation|wave equation]] for a vibrating string at mid-century, giving acoustics its first differential-equation footing. In the nineteenth century Hermann von Helmholtz's *On the Sensations of Tone* (1863) joined the physics of vibration to the physiology of hearing, and John William Strutt, Lord Rayleigh, gathered and extended the whole mathematical theory of vibration, waves and sound in his two-volume *The Theory of Sound* (1877–1878), still cited as a foundational text.[^rayleigh1877] Rayleigh's own 1877 paper on progressive waves is one entry point into that theory. The last piece of classical acoustics fell into place in 1900, when the American physicist Wallace Clement Sabine, having measured how long sound lingers in a room by ear and a stopwatch across dozens of Harvard lecture halls, published the reverberation equation that founded [[Architectural_acoustics|architectural acoustics]] as a predictive science rather than a craft.[^sabine1900] The twentieth century added electronics — the microphone, the loudspeaker, the vacuum-tube amplifier — and then digital signal processing, which turned acoustic measurement from a matter of ears and mechanical recorders into one of transducers and computed spectra. ## Definition Acoustics is defined, in the standard the field itself keeps, as "the science of sound, including its production, transmission, and effects, including biological and psychological effects."[^asa-def] That definition deliberately covers two different things under one name. First, sound as a physical event: an oscillation in pressure, stress, particle displacement or particle velocity, propagated in a medium with internal forces such as elasticity or viscosity, or the superposition of such oscillations.[^asa-def] Second, sound as the auditory sensation such an oscillation evokes.[^asa-def] The first sense reaches far outside anything a person can hear, from the infrasonic rumble of an earthquake to the megahertz pulses of a diagnostic ultrasound probe; the second sense is bounded by the human ear, roughly 20 Hz to 20 kHz, and by the physiology and psychology of hearing. Acoustics as a discipline studies both, and treats the physical wave and the perceived sensation as two ends of the same causal chain rather than as separate subjects. A useful working distinction is between acoustics as pure physics — the equations that govern any mechanical wave — and acoustics as an applied and engineering discipline, where those equations are put to work designing a concert hall, quieting a factory floor, or imaging a fetus. Most of what a reader meets in the rest of this article, and in the thirty-section Acoustics portal it introduces, sits on the applied side, because that is where the field's practical identity — and its professional societies, journals and conferences — actually lives. ## Fundamental concepts Acoustics rests on a short list of concepts that recur, in different clothing, in every one of its subfields. A [[Sound|sound]] wave is a [[Mechanical_wave|mechanical wave]]: a disturbance that needs a material medium to exist and that carries energy without carrying the medium itself along with it. In a fluid — air or water — the disturbance is longitudinal, meaning the medium's particles move back and forth along the same line the wave travels; in a solid it can also be transverse, because a solid, unlike a fluid, resists shear.[^up17-1] For a plane wave in one direction, the displacement and the pressure of the medium each vary sinusoidally in space and time, one a quarter cycle out of step with the other, and both are tied together, and to the source, by the same handful of quantities: frequency, wavelength, speed and amplitude. ### Wave propagation: pressure levels The pressure fluctuation that a sound wave produces is minute next to the surrounding atmospheric pressure — a loud voice a meter away raises and lowers the local pressure by only a few pascals out of roughly 100,000 — and the ear's dynamic range spans more than a millionfold in pressure, from the threshold of hearing near 20 micropascals to the threshold of pain. Because that range is so wide, acoustics reports levels logarithmically as [[Decibel|decibels]] of sound pressure level, `L_p = 20 log10(p_rms / p_ref)` with the reference `p_ref = 20 µPa`.[^up17-3] On that scale, every tenfold increase in intensity adds 10 dB, and every doubling of intensity adds about 3 dB; ordinary conversation runs near 60 dB, and pressure at the source falls off as 1/r with distance for a small source radiating into open space, so [[Sound_pressure|sound pressure]] itself carries the geometry of a room or a battlefield as much as the strength of the source. ### Wave propagation: frequency The [[Frequency|frequency]] of a sound wave is fixed by its source and does not change as the wave crosses from one medium into another; what changes is the [[Speed_of_sound|speed]] and, with it, the wavelength, tied together by the one relation every subfield of acoustics reuses: `c = f λ`.[^up17-2] Speed itself depends on the stiffness and inertia of the medium — `c = √(K/ρ)` in a fluid, with *K* the bulk modulus and ρ the density, and `c = √(γRT/M)` in an ideal gas, with γ the adiabatic index, *R* the gas constant, *T* the absolute temperature and *M* the molar mass — which is why sound runs at about 343 m/s in air at 20 °C, nearly three times faster in helium, about 1,480 m/s in water and roughly 5,000–6,000 m/s in steel.[^up17-2] The microsim built for this article puts that relation in front of the reader directly: switching its medium control from air to helium to water to steel re-races the same frequency through four different speeds, and the wavelength and period readouts move accordingly while the frequency control does not. *Try: set frequency and displacement amplitude, then step the medium selector from air to helium to water to steel and watch the wavelength stretch as the speed-of-sound readout climbs from 343 m/s toward several kilometers per second; use slow motion to watch a single particle sway about its resting position while the compressions it belongs to march past at that speed.* ### Transduction in acoustics Every acoustic measurement or reproduction depends on [[Transducer|transduction]]: converting acoustic energy into some other form, usually electrical, and back again. A microphone converts a sound wave's pressure fluctuations into a varying voltage, most often by moving a diaphragm against a fixed plate (a condenser microphone) or past a coil and magnet (a dynamic microphone); a [[Loudspeaker|loudspeaker]] runs the same physics backward, driving a cone with a voice coil in a magnetic field so that an electrical signal becomes a pressure wave in air. How efficiently a transducer couples energy into or out of a medium is governed by [[Acoustic_impedance|acoustic impedance]], `Z = ρc`, the ratio of pressure to particle velocity that a medium presents to a wave; a large impedance mismatch, as between air and water or between air and the skin at the mouth of a hearing-aid receiver, reflects most of the incident energy, which is why transducer design and the choice of coupling material are as much a part of acoustics as the wave equation itself.[^us1-6] ## Acoustician An **acoustician** is a person who works professionally in acoustics, applying the physics of sound to a specific problem: designing a concert hall's reverberation time, quieting an aircraft cabin, characterizing a material's sound absorption, or developing a medical ultrasound protocol. Because acoustics reaches across mechanical and electrical engineering, physics, architecture, biology, and psychology, acousticians are trained through several different routes rather than one standard credential, and much of the field's professional identity is carried by its societies (see Research, below) rather than by a licensing regime as tightly defined as, say, structural engineering. ### Education Acousticians typically hold a degree in physics, mechanical or electrical engineering, or architectural engineering, followed by graduate study or on-the-job specialization in acoustics itself; a number of universities offer dedicated graduate programs or concentrations in acoustics, vibration or audio engineering. Professional certification exists in some jurisdictions and specialties — for example, board certification in specific areas of noise control or in medical ultrasound physics — but the more common route into the field's applied branches is a general engineering or physics degree paired with membership in a professional society such as the Acoustical Society of America, which itself functions as a locus of continuing education through its meetings, tutorials and standards work.[^asa-def] ## Subdisciplines Acoustics divides into more than a dozen recognized subdisciplines, each defined by the medium it works in, the kind of source or receiver it studies, or the application it serves. All of them share the wave concepts above; each earns its own microsim and its own Main article on the Wikitube Acoustics portal, and the sections below name that article for each subdiscipline in the order Wikipedia's own outline lists them. ### Archaeoacoustics Archaeoacoustics is the study of the acoustic properties of archaeological sites — caves, tombs, temples and other ancient structures — and of what those properties may have meant to the people who built or used them. Researchers measure a space's [[Acoustic_resonance|resonant]] modes and reverberation and ask whether a chamber's dimensions were chosen, deliberately or not, to reinforce a particular tone, chant or drum pattern; the field sits between acoustics, archaeology and anthropology, and its claims about intent are typically more tentative than its acoustic measurements, which can be made with the same instruments used in any other room-acoustics survey. ### Aeroacoustics Aeroacoustics studies sound generated by turbulent fluid motion or by aerodynamic forces interacting with a surface, rather than by a vibrating solid alone — the noise of a jet exhaust, a wind turbine blade, or wind moving past overhead wires. The wind's own contribution to the last of these, the [[Aeolian_sound|aeolian]] tone produced as air sheds vortices from a wire or a cable, is among the oldest aeroacoustic phenomena on record, audible long before the field had a name. Aeroacoustic theory, developed largely from the 1950s onward for jet engine noise, treats the turbulence itself as a distributed source term in the wave equation, which is what makes the subject distinct from the vibrating-structure sources that dominate the rest of acoustics. ### Acoustic signal processing Acoustic signal processing applies the tools of digital [[Fourier_analysis|Fourier analysis]] — filtering, spectral analysis, correlation — to recorded or synthesized sound: speech recognition, [[Noise|noise]] cancellation, audio compression, sonar signal detection and room-response measurement all depend on decomposing a waveform into frequency components and manipulating them. The subject is as much digital electrical engineering as acoustics proper, but every one of its algorithms answers to the same physical wave the rest of the field studies, which is why acoustics and signal processing schools of thought have grown up largely intertwined since the mid-twentieth century. ### Architectural acoustics Architectural acoustics, also called building or room acoustics, designs and analyzes the acoustic behavior of enclosed spaces: how long sound persists after a source stops (its [[Reverberation|reverberation]] time), how evenly it is distributed through a hall, and how well one room is isolated from noise in the next. The field's founding result is Wallace Sabine's 1900 reverberation equation, developed from measurements at Harvard's Fogg Lecture Hall and still the starting point for [[Room_acoustics|room-acoustic]] design a century later.[^sabine1900] Concert halls, courtrooms, classrooms and open-plan offices are all, in different ways, architectural-acoustics problems. ### Bioacoustics [[Bioacoustics|Bioacoustics]] studies how animals produce, receive and use sound: bird and whale song, insect stridulation, bat and dolphin echolocation, and the ways sound structures animal behavior and habitat use. Because many animals hear and vocalize well outside the human range — bats and toothed whales into the ultrasonic, elephants and some whales into the infrasonic — bioacoustics regularly extends the working frequency range of acoustics research beyond what a person can perceive directly, using the same wave physics with different instruments. ### Electroacoustics Electroacoustics is the branch concerned with converting acoustic energy into electrical energy and back — the physics and engineering of the [[Loudspeaker|loudspeaker]], the microphone, the headphone and the electronic amplifier. It underlies recorded and reproduced sound of every kind, from a telephone earpiece to a stadium public-address system, and its central design problem is impedance matching: getting as much acoustic energy as possible across the boundary between a mechanical diaphragm and the medium it drives or is driven by. ### Environmental noise and soundscapes This subdiscipline studies unwanted or intrusive sound in the outdoor and indoor environment — traffic, aircraft, industrial and neighborhood [[Noise|noise]] — and the broader concept of a soundscape, the acoustic environment as perceived and understood by people in context. [[Noise_control|Noise control]] is its applied, engineering-facing half: acoustic barriers, quieter machinery, land-use planning and regulatory noise limits, most of which trace back to the same barrier-diffraction and absorption physics used in architectural acoustics, applied outdoors instead of inside a room. ### Musical acoustics [[Musical_acoustics|Musical acoustics]] studies how musical instruments and the singing voice produce sound, how that sound is perceived, and how the two connect — why a violin sounds different from a viola playing the same note, and why certain intervals sound consonant and others dissonant. Musical acoustics leans on the same vibrating-string and vibrating-air-column physics as the fundamental-concepts section above, applied specifically to the harmonic series an instrument produces and to the physiology and psychology of musical hearing. ### Psychoacoustics Psychoacoustics is the scientific study of how a physical sound wave is perceived — how physical quantities such as intensity and frequency map onto perceived loudness and pitch, and how listeners resolve where a sound is coming from. [[Hearing|Hearing]] itself, the physiological and perceptual side of the chain from eardrum to brain, and [[Sound_localization|sound localization]], the specific problem of judging a sound's direction from the small differences that arrive at the two ears, are the two Main articles on the Acoustics portal that carry this subfield forward; both descend from nineteenth-century work, including Lord Rayleigh's own study of directional hearing. ### Speech The acoustics of speech studies how the vocal tract produces the sounds of spoken language and how a listener recovers linguistic information from the resulting waveform. Its central concept is the [[Formant|formant]], a resonant frequency of the vocal tract that shapes the spectrum of a vowel; the source–filter model that ties a vibrating glottis (the source) to the vocal tract's resonances (the filter) is speech acoustics' equivalent of the loudspeaker-and-room model used elsewhere in the field. ### Structural vibration and dynamics This subdiscipline treats sound and noise as a consequence of the mechanical [[Vibration|vibration]] of solid structures — machines, vehicles, buildings, bridges — rather than of a fluid medium directly. Because a vibrating structure radiates sound into the air or water around it, structural dynamics and acoustics are two views of one physical system, and controlling a structure's vibration (with damping, stiffening, or isolation) is usually the most direct way to control the noise it makes. ### Ultrasonics Ultrasonics is the study and use of sound above the range of human hearing, roughly above 20 kHz. Medical [[Ultrasound|ultrasound]] imaging typically operates in the low megahertz range, where short wavelengths in tissue (well under a millimeter) resolve fine anatomical detail, while industrial ultrasonic testing uses similar frequencies to find flaws in welds, castings and composite materials without damaging the part.[^us1-6] Ultrasonics shares its underlying physics with audible acoustics entirely; only the frequency, and the technology needed to generate and detect it, differ. ### Underwater acoustics [[Underwater_acoustics|Underwater acoustics]] studies the propagation, reflection and scattering of sound in the ocean and other bodies of water, where sound — not light or radio — is the medium that travels farthest, because water absorbs light and radio far more strongly than it absorbs low-frequency sound. Sonar, marine mammal communication and echolocation, and the sound channels formed by the ocean's temperature and pressure structure are all underwater-acoustics subjects, and the field depends heavily on the same speed-of-sound relation given above, evaluated for seawater instead of air. ## Research Acoustics research is organized, published and presented much like any other physical science, through a small number of societies, journals and recurring conferences that most subdisciplines share. ### Professional societies The Acoustical Society of America (ASA), founded in 1929, is the principal professional society for acoustics in the United States and publishes the field's standard terminology as well as its flagship journal.[^asa-def] Counterparts exist worldwide — the Institute of Acoustics in the United Kingdom, the European Acoustics Association, and national acoustical societies across Europe and Asia — and these bodies jointly coordinate through the International Commission for Acoustics, which sponsors the quadrennial International Congress on Acoustics. ### Academic journals The *Journal of the Acoustical Society of America* (JASA) is the field's principal peer-reviewed journal, covering the whole span of acoustics from physical theory to psychoacoustics, bioacoustics, underwater acoustics and noise control. Other significant venues include *Applied Acoustics*, the *Journal of Sound and Vibration*, and *Acta Acustica*, the journal of the European Acoustics Association, each somewhat more focused on the engineering or vibration side of the field than JASA's broader remit. ### Conferences Beyond the International Congress on Acoustics, acousticians meet through the twice-yearly meetings of the Acoustical Society of America, the Institute of Acoustics' annual meetings in the United Kingdom, and specialist meetings for individual subdisciplines — underwater acoustics, noise control engineering, and ultrasonics among them — usually organized under the umbrella of the relevant professional society. These meetings function much like the conferences of any other physical science: a venue for presenting unpublished results, for standards committees to meet in person, and for the field's applied and academic halves to talk to each other. ## See also - [[Sound]] - [[Wave]] - [[Vibration]] - [[Reverberation]] - [[Ultrasound]] - [[Bioacoustics]] - [[Noise_control]] - [[Acoustic_resonance]] - [[Acoustic_impedance]] - [[Loudspeaker]] - [[PORTAL_Acoustics]] ## References [^asa-def]: Acoustical Society of America (2013, reaffirmed 2020). *ANSI/ASA S1.1-2013 Acoustical Terminology*, entries "acoustics" and "sound." ASA Standards term pages: https://asastandards.org/terms/ [^etym]: Liddell, Henry George; Scott, Robert. *A Greek-English Lexicon*, entries ἀκουστικός and ἀκούω. Perseus Digital Library: https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057 [^sauveur1701]: Sauveur, Joseph (1701). "Système général des intervalles des sons, et son application à tous les systèmes et à tous les instrumens de musique." *Mémoires de l'Académie Royale des Sciences*. Paris. Sauveur's proposal of *acoustique* as the name of the general science of sound, and his coinage of *node*, are widely reported in histories of acoustics; the present author has not directly verified the page reference within the 1701 *Mémoires* and flags the exact citation as *Citation needed*. [^wp-history]: Beyer, Robert T. (1999). *Sounds of Our Times: Two Hundred Years of Acoustics*. New York: Springer/Acoustical Society of America. General history of the field from the seventeenth century onward; specific claims above (Galileo on pitch and frequency, Mersenne's string laws) follow the standard historical account given in this and comparable acoustics histories. [^newton-laplace]: Newton, Isaac (1687). *Philosophiæ Naturalis Principia Mathematica*, Book II, Section VIII. London: Royal Society. — Laplace, Pierre-Simon (1816). "Sur la vitesse du son dans l'air et dans l'eau." *Annales de chimie et de physique* 3: 238–241. English translation: https://personal.lse.ac.uk/robert49/ebooks/philsciadventures/img/Laplace_VitesseEN.pdf [^rayleigh1877]: Strutt, John William, Lord Rayleigh (1877–1878). *The Theory of Sound*, 2 vols. London: Macmillan. [^sabine1900]: Sabine, Wallace Clement (1900). "Reverberation," collected in *Collected Papers on Acoustics* (1922). Cambridge, MA: Harvard University Press. Originally published serially in *The American Architect and Building News*, 1900. [^up17-1]: OpenStax (2016). *University Physics Volume 1*. Ling, S. J.; Sanny, J.; Moebs, W. Rice University. §17.1 "Sound Waves," pp. 808–810. Portal Books 077. [^up17-2]: OpenStax (2016). *University Physics Volume 1*. §17.2 "Speed of Sound," Table 17.1, pp. 810–818. Portal Books 077. [^up17-3]: OpenStax (2016). *University Physics Volume 1*. §17.3 "Sound Intensity," pp. 818–826. Portal Books 077. [^us1-6]: *Ultrasound Physics and its Application in Medicine* (2024). Chapter 1, "Basic Principles of Ultrasound," §1.6 "Audible and Ultrasound Waves." Portal Books 091.  **Microsim — three.js (Wikitube framework):** *Acoustics* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/acoustics/Acoustics.html" data-title="Acoustics"></div> *Built from `MICROSIM_GUIDE/specs/acoustics/variants/Acoustics.json`; part of the [[PORTAL_Acoustics|Acoustics portal]] spine (section sims and See-also variants).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Acoustics) : [Wikitube](https://en.wikitube.io/wiki/Acoustics) - skeleton pinned to revision 1362070797 (2026-09-11).