Sound Absorption PDF

Sound Absorption Sound Absorption Principles of Absorptive and Reflective Surfaces Sound Absorption Principles of Absorptive and Reflective Surfaces Sound Absorption Principles of Absorptive and Reflective Surfaces  Sound Absorption Absorption Coefficient  The absorption coefficient (α)...

Sound Absorption Sound Absorption Principles of Absorptive and Reflective Surfaces Sound Absorption Principles of Absorptive and Reflective Surfaces Sound Absorption Principles of Absorptive and Reflective Surfaces  Sound Absorption Absorption Coefficient  The absorption coefficient (α) is a number between zero and one, which used to describe the sound absorbing quality of a surface and to quantify the proportion of incident sound energy that does not return to the room in the form of a reflection.  The higher the value, the more sound is absorbed (turned to heat within the material) or transmitted (passed through the material) and the less is reflected; the lower the value, the more sound is reflected and the less is absorbed or transmitted.  An open window: has an absorption coefficient of 1.00 because no sound energy incident on that surface returns to the room.  A theoretical perfect reflector: has an absorption coefficient of 0.00 because all incident sound returns to the room by way of a reflection off the surface.  Sound Absorption Absorption Coefficient  Sound Absorption Absorption Coefficient  Marble, with an absorption coefficient of 0.01, reflects 99% of the sound energy impinging upon it—only 1% is absorbed or transmitted.  Conversely, a suspended ceiling tile, with an absorption coefficient of 0.80, reflects 20% of the sound—80% is absorbed or transmitted.  In reality, all materials have varying absorption coefficients across the frequency spectrum, which we group together and describe with octave‐ band values. So a ceiling tile may have an absorption coefficient of 0.80 at 1,000 Hz, and an absorption coefficient of 0.32 at 125 Hz. Thus, ceiling tiles is claimed to remove 80% of the incident sound, which is an oversimplification.  Many porous materials, absorbent at middle frequencies (speech frequencies), are more sound reflective at lower frequencies. Many panelized assemblies, such as gypsum board over stick construction, are more sound absorbent in low frequencies and sound reflective at speech frequencies.  Sound Absorption Absorption Coefficient 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)Mounted over an airspace.  Sound Absorption Absorption Coefficient  Higher absorption coefficient α values are characterized by a fiber orientation that constructs multitudes of tiny interconnecting air pockets.  Materials with lower absorption coefficient values are smooth, dense, flush‐mounted, and massive.  Materials with absorption coefficients greater than 0.50 are generally considered sound‐absorbent materials,  Materials with absorption coefficients less than 0.20 are generally considered sound‐reflective materials.  We typically don’t perceive an absorption coefficient change of less than 0.10, and we judge a change of greater than 0.40 to be considerable.  Sound Absorption Absorption Coefficient  Sound Absorption Absorption Coefficient Types of Sound Absorbers A) Porous material B) Fibrous material, C) Membrane, D) Panel, and E) Resonant Material F) Composite combinations of these materials  Porous absorbers (and a subset of porous absorbers, fibrous absorbers), collectively termed “fuzz”, include glass fiber, mineral fiber, fiber board, acoustical ceiling tile, cotton, and open celled foams.  Sound Absorption Absorption Coefficient Types of Sound Absorbers  Their absorption coefficients generally rise with frequency, yet they are the most broadband of the absorber types and are therefore by far the most commonly specified to deaden a room. A) Porous Material A) Porous Material A) Porous Material A) Porous Material A) Porous Material ) Porous Material with Perforated Screen Panel ) Porous Material: Microperforated Absorber B) Panel Absorbers B) Panel Absorbers B) Panel Absorbers B) Panel Absorbers  At low frequencies, porous absorbers translate acoustic energy to heat;  at higher frequencies, sound energy is damped because of the friction encountered when incident sound weaves through the interconnected pores of the absorber.  Still more sound energy is lost as sound changes direction within the absorber, and through a complex process called acoustic impedance mismatch— which occurs when sound moves between two media (air and the absorber) that differ in their acoustic densities.  Absorption effectiveness is a function of thickness, fiber orientation, density, and porosity. Closed‐cell insulating foams, whose pores are not interconnected, fail to perform as effective porous absorbers.  To check if a porous material might make a good absorber, blow C) Volume Resonators C) Volume Resonators C) Volume Resonators C) Volume Resonators  Panel and resonant absorbers are more narrow‐band in their absorption character than porous absorbers, and are thus used primarily in specialized applications.  Because of their particular absorption spectrum, designers employ these systems for controlling sounds that are narrow‐band, are low‐ frequency, and have frequency content easily predicted beforehand.  This might include the thud of a basketball dribble in a gymnasium, the groan of a pump, or the pure‐tone hum of an electrical transformer. These two types of absorbers, panel and resonant, may be tuned to peak their effectiveness at the frequency of the unwanted sound by adjusting the absorber’s mass, stiffness, or geometry. C) Volume Resonators  Because panel and resonant absorption spectrum characteristics complement those of porous absorbers, which are less effective at low frequencies, panel absorbers or resonant absorbers may be used in conjunction with porous absorbers in rooms like recording studios to flatten the absorption frequency spectrum. The two types of absorbers together are more broadband than either one is alone. Room Constant Room Constant The total absorption in a room, the “room constant,” measured in a unit calle sabins, is not only the result of the absorption coefficient of the surfaces, but also of the total surface area. More‐absorbent surfaces attenuate sound energy through loss to friction, but so do more surfaces of the same absorption profile. To calculate the total absorption in sabins, Where A is the total absorption in the room, termed the “room constant” and measured in a unit called sabins, α1 is the absorption coefficient of the first surface, α2 is the absorption coefficient of the second surface, α3 is the absorption coefficient of the third surface, and so on s1 is the area of the first surface, s2 is the area of the second surface, s3 is the area of the third surface, and so on Room Constant So to calculate the total room absorption at 1,000 Hz of a small office with 100 square feet of wood floor (α1 = 0.06) and 500 square feet of gypsum board (α2 = 0.04), multiply each absorption coefficient by its corresponding surface area, and sum them up. The total sound absorption in the office measures 26 sabins. If we replace 100 square feet of gypsum board in the office with 100 square feet of a porous absorber (α3 = 0.90), the total absorption climbs more than fourfold to 112 sabins. If we then add more surfaces by breaking up the office with 100 additional square feet of partial‐ height gypsum board partitions (α2 = 0.04), we’ve added an additional 4 sabins for a total of 116.For reference, a small sound‐reflective room may have a room constant on the order of 25 sabins, and a large, sound‐absorbent room may have a room constant on the order Room Average Absorption  To find an average absorption in a room ( ), it is not enough to arithmetically average the absorption coefficients of all the materials.  Suppose you occupied a large all‐marble room, with a 1,000‐Hz absorption coefficient of 0.01. Then you dropped a small fleck of (α = 0.80) shredded fiberboard acoustical ceiling tile to the floor. By doing so, you obviously didn’t move the average absorption coefficient of the room to the average of 0.01 and 0.80, or about 0.40.  There is far more marble than fiberboard, so the average absorption coefficient for the whole room must be closer to that of the marble. We therefore area‐weight the average absorption to reflect the surface area of the marble relative to that of the fiberboard. Room Average Absorption Whereis the area‐weighted average absorption coefficient, “alpha‐bar”. α1 is the absorption coefficient of the first surface, α2 is the absorption coefficient of the second surface, α3 is the absorption coefficient of the third surface, and so on. s1 is the area of the first surface, s2 is the area of the second surface, s 3 is the area of the third surface, and so on. stotal is the total area of all surfaces in the room. Room Average Absorption So to calculate the average absorption at 1,000 Hz of that same small office with 100 square feet of wood floor (α1 = 0.06) and 500 square feet of gypsum board (α2 = 0.04), multiply each absorption coefficient by its corresponding surface area, sum them up, and divide the sum by the total surface area in the room. The area‐weighted average sound absorption coefficient in the office measures 0.043. Because there is more gypsum board (α2 = 0.04) than wood (α1 = 0.06), the area‐weighted average is closer to that of gypsum board than to that of wood. Room Average Absorption If we replace 100 square feet of gypsum board in the office with 100 square feet of a porous absorber (α3 = 0.90), theclimbs from 0.043 to 0.186, about four times the value. For reference, a sound‐absorbent room, such as a recording studio, may have an average absorption coefficient of 0.70, and a racquetball court may have an average absorption coefficient of 0.02. Room Average Absorption  As designers add absorption to a room, it approaches a free‐field condition (no surfaces to reflect off), reverberance is lowered, and sound energy is removed from the space. Room Average Absorption  We use sound‐absorbing materials to quiet a noisy space, reduce reverberance for speech intelligibility (a classroom), or apply sound‐ absorbing materials to a surface that might otherwise create an acoustic defect (an echo from a distant surface).  We use sound reflecting surfaces when we want to increase the reverberance in a space (concert hall), or we specify sound‐reflecting surfaces to provide beneficial sound reflections that might bolster loudness (surfaces of a lecture room near the lecturer).  Some styles of music (romantic classical) require rooms with more sound reflections, and others (club music) require rooms with more sound absorption.  This might necessitate a room with variable acoustics. Absorbent velour banners or curtains can retract or deploy to change the acoustic quality of the room, or panels may slide or rotate to hide a sound‐ reflective surface and simultaneously expose a sound‐absorbing surface, or they may reveal a sound‐reflecting surface to cover a sound absorbing one. Room Average Absorption Room Average Absorption Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC)  The absorption coefficients of common building materials and tested building products can be easily obtained by searching online or perusing published tables, like the ones that follow.  Though the 63‐Hz octave‐band data is often omitted because it’s difficult to reliably test for, tables generally offer absorption coefficients at each of the relevant octave bands from 125 Hz to 4,000 Hz.  There are times, however, when for quick comparison of one absorber to another, expedience demands a single number that summarizes performance across several octave bands.  Encompassing speech frequencies, that single‐number rating is called the noise reduction coefficient (NRC).  This value can be found by averaging the sound absorption coefficients in the four octave bands 250 Hz through 2,000 Hz, then rounding off to the nearest 0.05. Noise Reduction Coefficient (NRC) Where NRC is the noise reduction coefficient, a single‐number average for mid‐frequency absorption coefficients associated with a building’s surface. A higher number describes a more absorbent surface. α 250 is the absorption coefficient of the surface at 250 Hz, α 500 is the absorption coefficient of the surface at 500 Hz, and so on. Noise Reduction Coefficient (NRC) To calculate the noise reduction coefficient (NRC) of heavy carpet on a pad, survey the absorption coefficient at the four relevant octave bands: The average of the four speech frequencies, 250 Hz through 2,000 Hz, is 0.37, which rounded off to the nearest 0.05 outputs an NRC of 0.35. Noise Reduction Coefficient (NRC)  Simplifying and summarizing the absorption coefficients across the frequency spectrum into a single number is both useful and convenient, but comes at the expense of valuable information only accessible at octave‐band resolution.  In the carpet example, we see that with an NRC of 0.35, heavy carpet is neither particularly sound absorptive nor particularly sound reflective. Lost in that summarized value is the sound‐ reflective nature of the surface at 125 Hz (α125 = 0.08).  It should be noted that, contrary to its reputation, carpet is not an effective sound absorber. The thinner, padless carpet used in commercial applications is even more sound reflective, with an NRC of 0.10 and an α125 of 0.02. Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC) Noise Reduction Coefficient (NRC)