Sound Absorption - Coursework Example

Question b For example and reference: *Excluding the dBC and noise errors. While the intermediate values may be limited in nature, but the respective NR values have been assumed for the sake of representing the dB values each for the corresponding NR range (. The following is a list of correction values for A-weighting. The correction value is described in 「JIS C 1509」. Add the corresponding value to the Z-weighting value. Question 3: Question # 5 It is evident from the solution presented in an excel sheet that Reverberation Time (RT) of sound in the room decreases after placing different objects in the rooms. The resultsof the RT are also shown below for better comparison. 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz 0.9 0.8 0.9 0.7 0.7 0.6 Average reverberation time of sound in empty classroom 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz 0.59 0.52 0.46 0.48 0.44 0.49 Average reverberation time of sound after placed different objects in the classroom. The main reason for this difference is that every material has some tendency to absorb sound which decreases it’s time to damp resulting in lower reverberation time. The difference between the values of time is inversely proportion to the frequency of time because the sound of higher frequency does not take much time to damp even in the place with the less absorption surfaces. The ability of any object, material or surface to absorb sound or light depends on a factor called as absorption coefficient. Every object has its specific absorption coefficient that varies with the frequency of sound, area and volume density of the surface or object. Following is the diagram that is showing the concept of absorption coefficient. The graphical representation of the concept of absorption coefficient Sound Absorption Coefficient: It is a common phenomenon that when sound interacts with any kind of surface, it usually or most of the times lose some quantity of its amplitude unless it is an ideal situation. The analogy of a mechanical object experiencing friction when moving along a surface is a good example and in the similar manner the sound also loses some strength when coming in contact with a surface(Miyasaki, 1999, p.23). The constant or coefficient which indicates the amount or quantity of absorption of the sound wave that occurs is known as the Sound Absorption Coefficient. Usually the absorption coefficient is represented by the term “a” while the effective absorbing area is represented by the product “as”. The effective absorbing area can be thought of as a factor that helps us in calculating the reverberation time in some large kind of places like halls or auditoriums (Bilova, 2012, p45). The absorption coefficient depends on the frequency of the sound wave and the overall interaction of the two mediums. We can generally use a table representing the various values of absorption coefficients of different surfaces in order to calculate the reverberation times with the allocated Sabine Formula( Leighton, Brown, 2002, p.36). Sabine Formula: The Sabine Formula is a way to reasonably work out the reverberation times in the comparatively large places like the auditoriums and other similar places. If one needs the more accurate approximation for the respectively selected enclosures, than the average overall absorption coefficient is calculated as: (Peters, 2013, p.89) Nature of surface Sound Absorption Coefficients at Frequency (Hz) 125 250 500 1000 2000 4000 Acoustical tile, rigid mount 0.2 0.4 0.7 0.8 0.6 0.4 Acoustic tile, suspended 0.5 0.7 0.6 0.7 0.7 0.5 Acoustical plaster 0.1 0.2 0.5 0.6 0.7 0.7 Ordinary plaster, on lath 0.2 0.15 0.1 0.05 0.04 0.05 Gypsum wallboard, 1/2" on studs 0.3 0.1 0.05 0.04 0.07 0.1 Plywood sheet, 1/4" on studs 0.6 0.3 0.1 0.1 0.1 0.1 Concrete block, unpainted 0.4 0.4 0.3 0.3 0.4 0.3 Concrete block, painted 0.1 0.05 0.06 0.07 0.1 0.1 Concrete, poured 0.01 0.01 0.02 0.02 0.02 0.03 Brick 0.03 0.03 0.03 0.04 0.05 0.07 Vinyl tile on concrete 0.02 0.03 0.03 0.03 0.03 0.02 Heavy carpet on concrete 0.02 0.06 0.15 0.4 0.6 0.6 Heavy carpet on felt backing 0.1 0.3 0.4 0.5 0.6 0.7 Platform floor, wooden 0.4 0.3 0.2 0.2 0.15 0.1 Ordinary window glass 0.3 0.2 0.2 0.1 0.07 0.04 Heavy plate glass 0.2 0.06 0.04 0.03 0.02 0.02 Draperies, medium velour 0.07 0.3 0.5 0.7 0.7 0.6 Upholstered seating, unoccupied 0.2 0.4 0.6 0.7 0.6 0.6 Upholstered seating, occupied 0.4 0.6 0.8 0.9 0.9 0.9 Wood seating, unoccupied 0.02 0.03 0.03 0.06 0.06 0.05 Wooden pews, occupied 0.4 0.4 0.7 0.7 0.8 0.7 Hence, using the Sabine formula and the above listed standard values we were able to compile the table of solutions for the question which is attached with this file as an excel sheet. Effect of distributing the panels evenly throughout the room versus locating the panels in one large area As the common observation speaks, distributing the panels evenly while accommodating more space of the same area leads to the fact that the sound waves will not encounter a large surface area of obstruction which will lead to more absorption and the resultant waves that reach the other side will be of much lower strength. Similar will be the effect on the reverberation times accordingly. Question 6: According to the calculated results the kitchen extract fan is creating more sound than the kitchen supply fan. People in the nearest house in the distance of 25 meters from the plant will experience the sound pressure of 83.3 dB due to kitchen extract fan and 77.7 DBA due to the kitchen supply fan. Frequency 63Hz 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz dB(A) Kitchen Extract 84.0 85.0 83.0 80.0 78.0 76.0 72.0 83.3 Kitchen Supply 76.0 78.0 75.0 75.0 73.0 70.0 68.0 77.7 Sound Pressure Level  The human ear is capable of can hear a wide range of sound pressures. Due to this phenomenon, the respective sound pressure level (In short known as the LP) is also conveniently measured in decibels (or dB) on a dedicated logarithmic scale which specifically goes on to compress the values in a well manageable range( Eagle, 2010, p.3-4). In relation, the direct pressure is generally measured in Pascal (or Pa). LP is apparently calculated as around 10 times the overall logarithmic values of the square of whole ratio of the pressure fluctuations at any accountable instant (below and above the atmospheric pressure) to the net allocated reference pressure:  Lp = 10 × log10(P/Pref)2  Whereas P is the sound pressure at any accountable instant, in units of Pascals, and Pref is the specific reference pressure level in the similar conditions which is also defined as the most ideal and quietest noise a healthy young person can hear which is close to the value of 20 µPa (Corthals, Paul, 1997, p. 98).  Example: When a certain equipment has a sound pressure of around 2 Pa, the specific sound pressure level comes out to be: Lp = 20 x  log10 (2/0.00002) = 20 x log10(100,000) = 20 × 5 = 100 dB  Sound Power Level Sound power level (Lw) is a concept that is very similar  to the wattagero overall power of a bulb. In fact, the law also is measured in the same SI unit watts (W). Different from Lp, Lw generally does not entirely depend on the overall distance from the source of noise under question. The power level of the sound is calculated efficiently using this equation as follows:  Lw = 10 × log10(W/Wref)  Where W is the acoustic power in watts and Wref is the reference acoustic power, 10‐12 (Jonassoh, 2007, p. 526). Example: The power level of sound that is associated with a conventional face‐to‐face conversation that may generally have a  Sound power of 0.00001 W, is specifically calculated as follows:   Lw = 10 × log10(0.00001/10‐12) = 70 dB  Combination and Averaging of Sound Levels Decibels are usually measured using a logarithmic scale(Wooten, 1992, p332). This clearly indicates that the decibels can simply not be added arithmetically.  For example: If there are two sources of noise that are both producing 90 dB each right next to  one another, then the overall combined sound level of noise will be around 93 dB, as opposed to the conventional 180 dB if adding arithmetically was allowed or logical in this case. The following equation is useful for calculating the overall sum of pressure levels of sound, the intensity levels of sound, or the sound power levels:  Total L = 10 × log10 (The logarithmic sum of all the respective levels of noise at various frequencies) Also, the following table on the next page can be useful for reference: Octave Band Center Frequency (Hz) Example Lp A-Scale Correction Factor (dB) Corrected Values (dB) 31.5 94 -39 55 63 95 -26 69 125 92 -16 76 250 95 -9 86 500 97 -3 94 1,000 97 0 97 2,000 102 +1 103 4,000 97 +1 98 8,000 92 -1 91 When we follow the above mentioned procedure we come to know that the sound pressure of 83.3 dBAisdue to the kitchen extract fan and 77.7 dBAisdue to the kitchen supply fan. References. Bilova, Monika (2012) Insitumeasurementofsoundabsorptioncoefficient. SGEM2012 12th International Multidisciplinary Scientific GeoConference Print. HTM. (2010) BS 8233 Hospital acoustics HTM BB93, 2ndedn. Palgrave MacMillan. Corthals, Paul.(1997)Running Speech level of Sound Pressure. Folia PhoniatricaEtLogopaedica Folia PhoniatrLogop 170-81. Print. Eargle, John M.(2010)Sound Pressure and DB Lp (Sound Pressure Level).Electroacoustical Reference Data 2-3. Print. Jonasson, Hans G. (2007) How to determine power level and pressure of emission in sound. Handbook of Noise and Vibration Control 526-33. Print. Leighton, T. G., and N. R. Brown (2002)Technique for measuring the rate of coefficient of sound absorption. Southampton, England: U of Southampton, Institute of Sound and Vibration Research. Print. Miyasaki(1999)Acoustics. How to determine the coefficient of absorption of sound. 3rd Edn. Print. Peters, R. J. (2013) Acoustics and Noise Control. 3rd ed. Hoboken: Taylor and Francis. Print. Smith, B. J. (1970)Acoustics. 2nd Ed. New York: American Elsevier Pub., Print. Wootten, John.(1992) Instrumentation for Space-Averaging Sound Pressure Levels. J. Acoust. Soc. Am. ASoA Journal 337. Print. Read More