AED122: Sound Characteristics PDF

COLLEGE OF ARCHITECTURAL ENGINEERING & DESIGN AED122: Lesson: Sound Characteristics Dilshan R. Ossen, Ph.D Associate Professor Introduction Room acoustics is concerned with the control of sound within an enclosed space The...

COLLEGE OF ARCHITECTURAL ENGINEERING & DESIGN AED122: Lesson: Sound Characteristics Dilshan R. Ossen, Ph.D Associate Professor Introduction Room acoustics is concerned with the control of sound within an enclosed space The main task is to provide the best conditions for the production and the reception of sound; e.g. speech and music Acoustic requirements for a particular room depend upon the nature and the purpose of the space. E.g. good acoustics in a gymnasium will not be the same as good acoustics in a theatre Acoustic Principles Sound Interaction with Media How sound works? Sound Reflection Sound is reflected in the same way as light. In order for a wall to reflect sound, it needs to be dense & smooth. If one or more of these characteristics is missing, then there will be other phenomena occurring to the wave. Reflection near the source of sound can be useful than at a distance from the source  Absorbed 20% Reflected Sound 70% Incident Sound 100% Specula Reflection Angle of incidence is equal to the angle of reflection For specula reflection to occur the surface needs to be “smooth”, flat [ rface e re and irregularities are much smaller than the a eleng h of o nd], dense & large Diffuse Reflection If the wavelength of sound is larger than the surface irregularities a diffuse reflection occurs In diffuse reflection, incident sound is reflected equally in all directions or Types of Reflectors Sound is Dispersed Sound is Focused Sound is Reflected SS SS SS Flat Surface Convex Surface Concave Surface Convex Reflector Flat Reflector Concave Reflector Best Good Poor Sound Absorption As a sound wave impacts onto a surface, some of the energy is reflected back, some is transmitted through the material, and some is lost within the material itself [absorb and convert to heat energy] and preventing sound from bouncing back. Absorbed 20% Reflected Sound 70% Incident Sound 100% Absorbing materials are generally porous, loose and breathable so that sound waves can pass through and be muted. Materials like; Stone wool & Mineral Fiber [Image source: A. M. Jaramillo and C. Steel , Architectural acoustics , Routledge e-book] So nd ab o ion coefficien [ ] Sound absorption coefficient is used to evaluate the sound absorption efficiency of materials. It is the ratio of absorbed energy to incident energy. Higher values of the absorption coefficient [ ] accompany materials that are; (a) more porous, (b) less smooth, (c) of less weight, (d) thicker (provided the thicker material is porous), (e) of less mass (where more of the energy passes through or is translated to mechanical energy) Materials with absorption coefficients greater than 0.50 are generally considered sound‐absorbent materials, and materials with absorption coefficients less than 0.20 are generally considered sound‐reflective materials. The harder, heavier, and smoother a material is, the poorer it will be at absorbing sound. Material NRC Performance Marble 0 Almost completely reflective Gypsum board 0.05 Very poor absorption Window 0.15 Low absorption Thick residential 0.50 Good absorption carpet Mineral fiber 0.90 Almost complete absorption ceiling tiles [Image ce: E mann, Michael , A chi ec al Ac ic Ill a ed. J hn Wile & S n , Inc., H b ken, Ne Je e.] TYPES OF SOUND ABSORBING MATERIALS Porous Absorbers: Porous absorbers, as the name suggests, consist of porous materials. The sound waves penetrate the surface of the material and flow into the cellular or fibrous structure, where they are dampened and converted into heat (the amount of heat is minimal less than one millionth of a watt). Porous absorbers are most effective when dealing with mid- range frequencies. Typical examples are; different textiles, carpets, curtains, open- cell foams, mineral wool insulation, glass fiber, etc. [Image Source: https://www.soundproofcow.com/product-category/sound-absorption-materials/] Membrane (panel) Absorbers Membrane absorbers are non-porous and non-rigid materials that are most effective in the bass frequency range [60Hz-250Hz] but not with higher frequencies [3000Hz]. Membrane absorbers are solid in appearance For example, common building elements like panel windows, doors, walls, floors all function as panel absorbers, as well as tables, chairs, closets and other furniture.  Resonance Absorbers: Resonance absorbers consist of plates include perforated materials (for example perforated plasterboard or metal) and materials that have holes or other openings These holes act as the bottleneck that traps the sound and locks it into the space located in-behind the material. Perforated Plasterboard or Metal [Image Source: https://www.spec-net.com.au/press/0711/son_060711/Perforated-Plasterboard-Acoustic- Panels-Sontext-Camberwell-VIC-3124] Three of these materials are different from the other three in some way. Circle them and say why they are different? Carpet Glass Gypsum board Window Mineral Wool Upholstered Seats They are different because,. Name three ways in which sound can be reflected and say why they are different?