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Reviewer Acoustics (1)

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  PART 1A - INTRODUCTION TO ACOUSTICS SCOPE OF ACOUSTICS 1.   Earth Sciences 2.   Life Sciences 3.   Arts 4.   Engineering ACOUSTICS STUDIES 1.   Acoustical Oceanography 2.   Animal Bioacoustics 3.   Architectural Acoustics 4.   Biomedical Ultrasound/ Bioresponse to Vibration 5.   Engineering Acoustics 6.   Musical Acoustics 7.   Noise 8.   Physical Acoustics 9.   Psychological-Physiological Acoustics 10.   Signal Processing in Acoustics 11.   Speech Communication 12.   Structural Acoustics and Vibration 13.   Underwater Acoustics PSYCHO ACOUSTICS -   Scientific study of sound perception; -   It is the branch of science studying the psychological and physiological responses associated with sound; -   It can be further categorized as a branch of psychophysics -   Includes the mechanics of the human ear and effect of noise on hearing capacities ELECTRO ACOUSTICS -   Deals with the generation and detection of audible sound waves, including sound reproduction -   Concerned with the recording, manipulation and reproduction of audio using electronics SONICS -   Deals with sounds at frequencies too high to be heard by humans -   Deals with the technical application of mechanical waves in basic scientific research, industry, and medicine ENVIRONMENTAL ACOUSTICS -   Deals with the effects of the environment upon audible sound waves -   Further specialized studies  include:    Outdoor Acoustics/ Landscape Acoustics    Indoor Acoustics/ Architectural Acoustics LANDSCAPE ACOUSTICS -   Environmental noise and soundscapes -   Concerned with noise and vibration caused by traffic, aircraft, industrial equipment and recreational activities -   Research work now also has a focus on the positive use of sound in urban environments ARCHITECTURAL ACOUSTICS -   The science of sound phenomena in buildings, dealing with the production, transmission, and absorption of sound -   The science of sound and vibration which refers to stress fluctuations and waves in a material medium -   The art and science of designing a room or building which insures both comfort and communication, and provides special features as the purpose and use of the structure requires SUBDISCIPLINES OF ARCHITECTURAL ACOUSTICS  -   SPACE ACOUSTICS : designing a space that enhances sound for optimum acoustical conditions -   NOISE CONTROL : eliminating, or at least reducing noise and vibration transmission SOUND -   Any vibratory motion of bodies, the transmission of these vibrations in a solid or liquid media, and the sensation produced on the human auditory mechanism -   An alteration in pressure projected and propagated in an elastic material -   Form of energy which continues to subsist until filtered through a material turning into heat by friction PROPERTIES OF SOUND -   Sound must always have a source, a path, and a receiver -   SPEED : sound travels at 344m/s at normal room temperature (68 degF)    Sound travels about 1 million times slower than light    Sound travels faster in denser media -   INTENSITY : the rate at which sound energy is being transmitted, measured at any point in the medium; sound power per unit area    Intensity diminishes inversely as the square of the distance of the source  -   SOUND PRESSURE : the fluctuation in the atmospheric pressure caused by the vibration of air particles due to a sound wave -   LOUDNESS : the subjective attribute of an auditory sensation -   TRESHOLD OF AUDIBILITY : minimum intensity that is capable of evoking an auditory sensation in the ear -   TRESHOLD OF PAIN : minimum sound pressure level which stimulates the ear to a point which is painful -   PERCEIVED SOUND    Dominant frequencies affect sound perception    Sound meters that interpret human hearing use an “A” weighted scale      dB becomes dBA -   WAVELENGTH : the distance a sound wave travels during each complete cycle of vibration -   AMPLITUDE : the strength or power of a wave signal. The “height”  of a wave when viewed as a graph -   FREQUENCY : the number of displacements or oscillations that a particle undergoes in 1 second    The unit of frequency is HERTZ , numerically equal to cycles per second    The audio frequency range is 20-20,000HZ    Standard frequencies, selected arbitrarily, represent significance in environmental acoustics    RELATIONSHIP BETWEEN FREQUENCY AND WAVELENGTH?    Sound within the frequency range of 20-10,000HZ produces wavelengths which extends from 17M to 34mm -   PITCH : the attribute of an auditory system which enables us to pinpoint sounds on a scale extending from high to low frequencies -   TONE : a steady periodic sound; a sensation having pitch -   TIMBRE : combination of the number, prominence, pitch, and intensity of the overtones -   SOUND DIRECTIONALITY : sound sources radiate sound waves in all directions PART 1B  –   CHARACTERISTICS OF SOUND SOUND REFLECTION -   Sound reflected off a surface, usually one which is hard, rigid, and/or flat -   LAW OF REFLECTIVITY : Angle of incidence equals the angle of reflection    Sound waves in contact with concave and convex surfaces will reflect off these surfaces in different directions SOUND ABSORPTION -   Sound waves absorbed into a material upon contact; change of energy into some other form -   SOUND ABSORPTION COEFFICIENT : Fraction of energy of the incident sound absorbed by the surface; rates the efficiency of sound absorption of a material at a specified frequency -   NOISE REDUCTION COEFFICIENT (NRC) : arithmetic average of sound absorption coefficients at the frequencies 250, 500, 1000, and 2000 Hz, expressed to the nearest multiple of 0.05 -   SURFACE ABSORPTION : product of the area of a surface and its absorption coefficient/ noise reduction coefficient -   METHODS OF ABSORPTION    Frictional    Flexural    Resonating SOUND DIFFUSION -   Occurs when sound waves are dispersed equally in a room -   Uniform distribution of sound in a room creates a HOMOGENOUS FIELD  -   There are several methods to create sound diffusion :    Providing several surface irregularities    Providing an alternating application of reflective and absorptive finishes    Providing a random application of reflective and absorptive finishes    Providing diffusers SOUND DIFFRACTION -   An acoustical phenomenon which causes sound waves to be bent or scattered around such obstacles as corners, columns, walls, and beams -   A phenomenon where sound passes through an opening resulting in a change of propagation, as influenced by the ratio of  the sound wavelength to the size of the opening SOUND REFRACTION -   Change of sound wave direction as it moves from one material to another of different density SOUND TRANSMISSION -   Sound which penetrates through a surface -   NOISE REDUCTION (NR) : the level of attenuation provided by the building construction between two rooms -   TRANSMISSION LOSS (TL) : the level of sound isolation provided by a construction, independent of the effects of wall size or room absorption -   SOUND TRANSMISSION CLASS (STC) : laboratory measured rating of how well a building partition attenuates airborne sound STC What can be heard 25 Normal speech can be distinctly understood through wall 30 Loud speech can be understood fairly well, normal speech heard but not understood 35 Loud speech audible but not intelligible 40 Onset of “privacy”  45 Loud speech not audible; 90% of statistical population not annoyed 50 Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed 60+ Superior soundproofing; most sounds inaudible REVERBERATION -   The prolongation of sound as a result of successive reflections in an enclosed space after the source of sound is turned off -   REVERBERATION TIME (RT) : time for the sound pressure in a room to decrease 60db after the sound is stopped -   OPTIMUM RT : the most favourable reverberation time for a particular space and volume ACOUSTICAL DEFECTS -   Flaws and imperfections in the acoustical environment 1.   Echo 2.   Long delayed reflection 3.   Sound shadow 4.   Sound concentration ECHO AND SOUND DELAY: -   ECHO : Sound waves which have been reflected to a listener with sufficient magnitude and time delay so as to be perceived separately from those communicated directly from source to listener    TIME DELAY FOR SPEECH : 1/25 th  of a second    TIME DELAY FOR MUSIC : 1/10 th  of a second -   LONG DELAYED REFLECTION : Similar to echo except that the time delay between direct and reflected sound is somewhat less FLUTTER ECHO -   A rapid succession of noticeable small echoes observed when a short burst of sound is produced between parallel sound reflective surfaces SOUND CONCENTRATION -   Also referred to as SOUND FOCI . Sound reflections from concave surfaces concentrating on a single area -   Areas of sound concentration are referred to as HOT SPOTS  -   The intensity of sound at hot spots are always at the expense of DEAD SPOTS   COUPLED SPACES -   Two rooms adjacent to each other by openings, with at least one space being highly reverberant DISTORTION -   An undesirable change in the quality of musical sound due to the uneven and excessive sound absorption of the boundary surfaces at different frequencies -   ROOM RESONANCE : also called Coloration. Occurs when certain sounds within a narrow band of frequencies tend to sound louder than other frequencies SOUND SHADOW -   Occurs when an area does not receive an adequate amount of direct sound    Noticeable under a balcony    Also noticeable in seating areas outside the maximum 140 degree area from the sound source WHISPERING GALLERY -   Also referred to as Creep. High frequency sounds creeping along large concave  surfaces such as a hemispherical dome, elliptical or ellipsoidal enclosure PART 2A  –   ACOUSTICAL MATERIALS ACOUSTICAL MATERIALS -   Commercial and custom designed building materials and surface treatment which contribute significantly to acoustical control, through absorption, or to noise reduction METHODS OF ABSORPTION 1.   Frictional 2.   Flexural 3.   Resonating GENERAL CLASSIFICATION OF ACOUSTICAL MATERIALS -   Porous Materials -   Panel or Membrane Absorbers -   Cavity (Helmholtz) Resonators POROUS MATERIALS -   The basic acoustical characteristic of these materials is a cellular network of interlocking pores -   Within these pores, incident sound energy is converted into heat -   Examples of these include acoustical boards, diffusers, soft plasters, mineral wool, and acoustical blankets, among others -   CHARACTERISTICS : Their sound absorption is more efficient at high, rather than low frequencies. However, their acoustical efficiency improves in the low frequency range with:    Increased thickness    Distance to their solid backing -   CATEGORIES OF COMMERCIAL MATERIALS :    PRE-FABRICATED ACOUSTICAL UNITS : Various types of perforated, fissured, or textured materials constitute typical units in this group Commonly used materials include : 1.   ACOUSTICAL BOARDS    –  also referred to as acoustic tiles. Porous boards made of various types of sound absorbing material, usually used for ceilings These can be installed in several ways: a.   Cement to a solid backing b.   Nailed or screwed to wood or aluminium framing c.   Laid in a ceiling suspension system Advantages: a.   Trade catalogues contain detailed specification b.   Easy installation and maintenance c.   Flexible absorption Disadvantages: a.   Difficult to conceal joints between units b.   Paint redecoration harmful to absorption c.   Soft structure subject to damage 2.   ACOUSTICAL HANGERS  –  a system of fiber-board panels that are wrapped with insulation and are hung freely using wire or rope  3.   GEOCOUSTIC TILES AND PANELS  –  special prefabricated units for random application on walls and ceilings  4.   DIFFUSERS  –  absorptive material that have irregular surfaces, or angled or curved fonts, so when mounted on a wall or the ceiling they stop parallel wall interference and prevent standing waves      PLASTERS AND SPRAYS : Absorbent paint applied in a semi-plastic consistency. There are two types:   1.   TYPE 1 : Composed of a cementitious material such as gypsum, cement, or lime 2.   TYPE 2 : Fibrous materials combined with a binder agent, sprayed on with an air gun or blower Advantages: 1.   May be used for irregular surfaces   2.   Aesthetic and decorative qualities  Disadvantages: 1.   Difficult application 2.   Difficult maintenance    ACOUSTICAL BLANKETS AND FOAM BOARDS :  ACOUSTIC BLANKETS  –   also referred to as “isolation blankets”. Materials manufactured from rock wool, glass fibers, hair felt, etc ACOUSTIC FOAM BOARDS  –  High density blankets with covering; also act as diffusers    CARPETS AND FABRIC : Carpets do not have high absorptive qualities, unless

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Aug 16, 2019
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