The New Student's Reference Work/Acoustics. Acoustics (a-kōōs'tĭks). Those phenomena which one detects by the ear are generally studied together under the head of acoustics. But whenever any sound is heard we find that somewhere in the neighborhood there is what we call a sound- ing body, and this is always found to be a body in rapid vibration. Besides this we find that if the sounding body be supported on a bit of cotton wool, placed under the receiver of an air pump, and the air exhausted, the sound is almost entirely extinguished. We are thus led to believe that two things are always essential to the production of sound, viz., a rapidly vibrating body and an elastic medium, generally air, between that body and the ear. Accordingly the subject of acoustics is made to include a study of vibrating bodies, such as a piano wire, a violin string, an organ pipe, etc., and also of the bodies which transmit vibrations to the ear, such as air, wooden rods and other elastic media.
A VIBRATING STRING[edit] SOUND A WAVE MOTION[edit] 1. 2. Acoustics - Wikibooks. Acoustics (from Greek ακουστικός pronounced akoustikos meaning "of or for hearing, ready to hear") is the science that studies sound, in particular its production, transmission, and effects. The science of acoustics has many applications which are dependent upon the nature of the sound that is to be produced, transmitted or controlled. In the case of a desirable sound, such as music, the main application of acoustics is to make the music sound as good as possible. In the case of an undesirable sound, such as traffic noise, the main application of acoustics is in noise reduction. Another major area of acoustics is in the field of ultrasound which has applications in detection, such as sonar systems or non-destructive material testing. In order to add an article to this Wikibook, please read the How to contribute? Fundamentals Applications Applications in Transport Industry Applications in Room Acoustics Applications in Psychoacoustics Musical Acoustics Applications Miscellaneous Applications.
Wave equation. Spherical waves coming from a point source. The wave equation is an important second-order linear partial differential equation for the description of waves – as they occur in physics – such as sound waves, light waves and water waves. It arises in fields like acoustics, electromagnetics, and fluid dynamics.
Historically, the problem of a vibrating string such as that of a musical instrument was studied by Jean le Rond d'Alembert, Leonhard Euler, Daniel Bernoulli, and Joseph-Louis Lagrange.[1][2][3][4] In 1746, d’Alambert discovered the one-dimensional wave equation, and within ten years Euler discovered the three-dimensional wave equation.[5] Introduction[edit] where ∇2 is the (spatial) Laplacian and where c is a fixed constant. The equation alone does not specify a solution; a unique solution is usually obtained by setting a problem with further conditions, such as initial conditions, which prescribe the value and velocity of the wave.
Scalar wave equation in one space dimension[edit] . . Wave. From Wikipedia, the free encyclopedia Waves most often refers to: Plural form of wave, a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves may also refer to: Topics referred to by the same term. Surface acoustic wave. Experimental image of surface acoustic waves on a crystal of tellurium oxide.[1] Discovery[edit] SAWs were first explained in 1885 by Lord Rayleigh, who described the surface acoustic mode of propagation and predicted its properties in his classic paper.[2] Named after their discoverer, Rayleigh waves have a longitudinal and a vertical shear component that can couple with any media in contact with the surface. This coupling strongly affects the amplitude and velocity of the wave, allowing SAW sensors to directly sense mass and mechanical properties. SAW devices[edit] SAW devices use SAWs in electronic components to provide a number of different functions, including as delay lines, filters, correlators and DC to DC converters.
Application in electronic components[edit] This kind of wave is commonly used in devices called SAW devices in electronic circuits. Schematic picture of a typical SAW device design. SAW device applications in radio and television[edit] SAW in geophysics[edit] Soundproofing. Sound reflection board Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.
Distance[edit] The energy density of sound waves decreases as they spread out, so that increasing the distance between the receiver and source results in a progressively lesser intensity of sound at the receiver. Damping[edit] Absorption[edit] Absorbing sound spontaneously converts part of the sound energy to a very small amount of heat in the intervening object (the absorbing material), rather than sound being transmitted or reflected. Porous absorbers[edit] Porous absorbers, typically open cell rubber foams or melamine sponges, absorb noise by friction within the cell structure.[1] Sound pressure. Sound pressure diagram: 1. silence, 2. audible sound, 3. atmospheric pressure, 4. instantaneous sound pressure Sound pressure level (SPL) or sound level is a logarithmic measure of the effective sound pressure of a sound relative to a reference value.
It is measured in decibels (dB) above a standard reference level. The standard reference sound pressure in air or other gases is 20 µPa, which is usually considered the threshold of human hearing (at 1 kHz). Instantaneous sound pressure[edit] The instantaneous sound pressure is the deviation from the local ambient pressure caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square (RMS) of the instantaneous sound pressure over a given interval of time (or space).
Total pressure is given by: where: = local ambient atmospheric (air) pressure, = sound pressure deviation. Intensity[edit] In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. Where is. Pressure Wave. Plane P-wave Representation of the propagation of a P-wave on a 2D grid (empirical shape) Velocity[edit] where K is the bulk modulus (the modulus of incompressibility), is the shear modulus (modulus of rigidity, sometimes denoted as G and also called the second Lamé parameter), is the density of the material through which the wave propagates, and is the first Lamé parameter. Of these, density shows the least variation, so the velocity is mostly controlled by K and μ.
The elastic moduli P-wave modulus, , is defined so that and thereby Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s.[2] The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core.[3] Seismic waves in the Earth[edit] Velocity of seismic waves in the Earth versus depth.[4] The negligible S-wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S-wave velocity is non-zero. Noise pollution. Noise pollution is the disturbing or excessive noise that may harm the activity or balance of human or animal life. The source of most outdoor noise worldwide is mainly caused by machines and transportation systems, motor vehicles, aircraft, and trains.[3][4] Outdoor noise is summarized by the word environmental noise.
Poor urban planning may give rise to noise pollution, since side-by-side industrial and residential buildings can result in noise pollution in the residential areas. Indoor noise can be caused by machines, building activities, and music performances, especially in some workplaces. There is no great difference whether noise-induced hearing loss is brought about by outside (e.g. trains) or inside (e.g. music) noise. High noise levels can contribute to cardiovascular effects in humans, a rise in blood pressure, and an increase in stress and vasoconstriction, and an increased incidence of coronary artery disease. Health[edit] Human[edit] Wildlife[edit] Noise mitigation[edit] Noise mitigation. Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors.
The main areas of noise mitigation or abatement are: transportation noise control, architectural design, urban planning through zoning codes,[1] and occupational noise control. Roadway noise and aircraft noise are the most pervasive sources of environmental noise worldwide, and little change has been effected in source control in these areas since the start of the problem,[citation needed] a possible exception being the development of hybrid and electric vehicles.
Social activities may generate noise levels that consistently affect the health of populations residing in or occupying areas, both indoor and outdoor, near entertainment venues that feature amplified sounds and music that present significant challenges for effective noise mitigation strategies. Types[edit] Noise control techniques include: Roadways[edit] Aircraft[edit] Noise control. Doppler effect. Change of wavelength caused by motion of the source An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The pink circles are sound waves. When the car is moving to the left, each successive wave is emitted from a position further to the left than the previous wave. So for an observer in front (left) of the car, each wave takes slightly less time to reach him than the previous wave. Doppler effect of water flow around a swan For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted.
Development[edit] General[edit] In classical physics, where the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency and emitted frequency is given by:[5] where is the velocity of waves in the medium; and given by. Diffraction. Diffraction pattern of red laser beam made on a plate after passing a small circular hole in another plate Diffraction refers to various phenomena which occur when a wave encounters an obstacle or a slit.
In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. Similar effects occur when a light wave travels through a medium with a varying refractive index, or when a sound wave travels through a medium with varying acoustic impedance. Since physical objects have wave-like properties (at the atomic level), diffraction also occurs with matter and can be studied according to the principles of quantum mechanics.
Richard Feynman[3] wrote: Examples[edit] The effects of diffraction are often seen in everyday life. History[edit] Mechanism[edit] where and. Audiology. Audiology (from Latin audīre, "to hear"; and from Greek -λογία, -logia) is the branch of science that studies hearing, balance, and related disorders. Its practitioners, who treat those with hearing loss and proactively prevent related damage are audiologists. Employing various testing strategies (e.g. hearing tests, otoacoustic emission measurements, videonystagmography, and electrophysiologic tests), audiology aims to determine whether someone can hear within the normal range, and if not, which portions of hearing (high, middle, or low frequencies) are affected and to what degree.
If an audiologist determines that a hearing loss or vestibular abnormality is present he or she will provide recommendations to a patient as to what options (e.g. hearing aid, cochlear implants, surgery, appropriate medical referrals) may be of assistance. Audiologist[edit] History[edit] The use of the terms "Audiology" and "Audiologist" in publications has been traced back only as far as 1946. Australia[edit] Acoustic wave equation. In physics, the acoustic wave equation governs the propagation of acoustic waves through a material medium. The form of the equation is a second order partial differential equation. The equation describes the evolution of acoustic pressure or particle velocity u as a function of position r and time . For lossy media, more intricate models need to be applied in order to take into account frequency-dependent attenuation and phase speed.
In one dimension[edit] Equation[edit] Feynman[2] derives the wave equation that describes the behaviour of sound in matter in one dimension (position ) as: where is the acoustic pressure (the local deviation from the ambient pressure), and where is the speed of sound. Solution[edit] Provided that the speed is a constant, not dependent on frequency (the dispersionless case), then the most general solution is and are any two twice-differentiable functions. ) travelling up the x-axis and the other ( ) down the x-axis at the speed . Or is its wave number. Derivation[edit] Here. Room acoustics. Room acoustics describes how sound behaves in an enclosed space. The way that sound behaves in a room can be broken up into roughly four different frequency zones: The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone, sound behaves very much like changes in static air pressure.Above that zone, until the frequency is approximately 11,250(RT60/V)1/2, wavelengths are comparable to the dimensions of the room, and so room resonances dominate.The third region which extends approximately 2 octaves is a transition to the fourth zone.In the fourth zone, sounds behave like rays of light bouncing around the room.
Natural modes[edit] The sound wave has reflections at the walls, floor and ceiling of the room. The incident wave then has interference with the reflected one. This action creates standing waves that generate nodes and high pressure zones.[1] Reverberation of the room[edit] See also[edit] References[edit] Compare[edit] Transducer. Architectural acoustics. Signal processing. Speech. Psychoacoustics. Auditory system. Musical acoustics. Bioacoustics. Underwater acoustics. Vibration. Structural acoustics. Aeroacoustics. Acoustic resonance. Acoustics.