-ِAcoustics -Arch 353 -Dr. Abdelrahman Elbakheit -References الصوتيات, د. سعود صادق حسن.1

Basics of Room Acoustics -We know that sound from a point source moves in concentric circles in the outdoors almost without obstruction. -In the indoors, it will be received my many surrounding surfaces that will change it’s level, intensity, loudness either to the better ( have more louder sound or more clearer) or to the worse ( have more echoes and less clarity). -To better understand the behaviour of sound in interiors of buildings we can learn what are the responses of sound waves in buildings. -For simplicity we can assume that sound moves in straight rays, perpendicular to the wave front, therefore we can apply the principles of light on sound waves. Sound Reflection: -The quality of Sound Reflection depends greatly on it’s wave length and the dimensions of the receiving surface. -If the receiving surface dimensions are greater than sound wavelength then the reflection follows light reflection principles. -That is angle of incidence is equal to angle of reflection. -The angle of incidence, angle of reflection and the normal on the point of reflection are all on the same plane.

Sound Reflection: -If the receiving surface shorter dimensions are equal to sound wavelength then sound will diffuse in many directions, therefore with low frequencies and high wave length care should be taken to this point. -Accordingly if we have a Concave surface reflects light in a focused manner, whereas convex surfaces reflects light in a scattered manner.

Sound Absorption: - It is the property of some materials and masses to absorb sound. Or it is the reduction of sound level as a result of contacting/passing on a surface. Absorption Co-efficient: - It is that portion of incident sound that the receiving surface hasn’t reflected. -It depends on sound frequencies, type of materials receiving sound and their construction. -Sound absorption is different from sound Isolation, many sound absorbing surfaces have poor sound isolation properties. -Sound absorbing materials can reduce noise levels by 10 db. (they are relatively expensive). Fibrous Absorbers: -Fibrous materials are good for general sound absorption, they can be made from metallic fibres and tissues.

Fibrous Absorbers: -Fibrous materials are good for general sound absorption, they can be made from metallic fibres and tissues. -Their ability for absorption depends on material thickness and sound frequency. -With more thickness there would be more absorption especially in low frequencies. -Thickness can be increased by increasing the gap between absorbing surface and the fixing Wall/ceiling. -With high frequencies absorption the properties of the surface finish and composition of the materials are the most important factors.

Properties of membrane Absorbers.

Membrane Absorbers: -These are panels or sheets that vibrate according to sound and some of the sound transfer with the material and the rest turns into heat. -The main difference between Fibrous and Membrane surfaces is that the later is better in absorbing low Frequencies. Sound Echo's and Sound death: -In any room it is important to finish it surfaces with the right absorbing materials. -Too little absorption can lead to more sound Echoes. -Too high absorption can lead to sound Death. -In both cases there would not be optimum clarity for sound in the space.

- Total Absorption: -Is the total summation for all the absorption occurring in a room. -Calculated by multiplying the surface area of the absorbing surface with it’s absorption co-efficient. Then making the total summation for this. -Audience and furniture absorption is estimated per person/unit. -Reverberation: -Is sound multiple reflections (i.e., thousands) in a single second. -Affects sound clarity and should be reduced to a minimum. -Reverberation Time: -Time needed for sound to be reduced by 6o dbs. -Depends on Room total Absorption and Room Volume. Sabine Formula: Actual Reverberation time (T) in Seconds = 0.16V / A V= Room Volume. A= Total Absorption in m 2

-Optimum Reverberation Time: -The required reverberation time can be determined by hall dimensions and use. -The optimum reverberation time can be arrived at from subjective experiments to a given space. -Stephens and Bates, have developed a formula for optimum reverberation time. it’s an approximation that is best suited to frequencies of 500Hz. T = Optimum reverberation time in Seconds. V =Volume of hall in m 3. R = 4, for Speech halls = 5, for Orchestra Music = 6, for Coral Music. It is recommended to add 40% to the resulting T from this equation in low frequencies.