The Numbers used in Absorption Co-Efficient tables

Hi people... Just trying to get something straight in my head. I'm doing a University course that involves Acoustics as one of the modules and there is something i'm a little confused about. We have been taught that the absorption co-efficient of a surface can range from 0 to 1 with 1 being totally absorbant and 0 being totally reflective. We have also been taught that there is no such thin as a 0 or a 1 as nothing is 100% absorbant and nothing is 100% reflective. I have been looking at some absorption co-efficients of acoustic panels and materials and have noticed that some of them go over 1... i.e. some go up to 1.30. I have seen this on Bob Golds co-efficient charts and also in an article i read by Ethan Winer... Obviously 2 experts in the field who know what they are talking about. Is there different systems for working out co-efficients or have i been taught information that is incorrect. If anyone can help me out with this i would really appreciate it as i am currently writing my dissertation and need to understand it. Thanks very much John

agreed, there is nothing which does not absorb, nor something that absorbs everything. however, the coefficient is just that, a number of relative absorption at a given frequency for a given mounting for a given sample and size. due to sound entering at an incidence angle causing additional length of travel through the material (an edge effect uses this same principle) the material can exceed the absorption versus a 0 degree angle. so in theory, theory is just like reality, except it isn't... :twisted:

Thanks Glenn.... but... errrrrmmmmm. What does that mean? Why do people put numbers higher than 1 in there co-efficient charts when its impossible to get 100% absorption. Let alone even more than that!!! I know using maths to figure out the RT60 is never going to be totally accurate and it is only an estimated projection, but using numbers that are above 1 just mean your projection is going to be even more innacurate than it would be anyway!!! John

maybe call it "attenuation coefficient" rather than absorption coefficient. then consider reciprocal length. it will be much easier to grasp what is happening. on the practical side, understanding the # of Sabin per unit of the material make computations much easier and you will find that there is no material which has more than 100% absorption since a Sabin by its definition is 100% absorption for 1 m2 material e.g. "the open window". here's a link to Sabine's book which explains his methodology and give you a true appreciation for the level of effort he undertook to get to the simple formula we take for granted today. http://vlp.mpiwg-berlin.mpg.de/library/data/lit39364?

Why do people put numbers higher than 1 in there co-efficient charts when its impossible to get 100% absorption.

The way I understand it, the issue is not that you are "really" getting over-unity absorption, but rather the method of measuring it. One way of measuring the coefficient is to place a sample of the material at one end of a large tube, so that it completely covers the end wall, put a speaker at the other end of the tube, and a movable microphone in the tube that you can slide around from one end to the other. For example, if the tube has a diameter of one meter, then you cut out a circle of material that is also one meter in diameter and place it up tight against the end wall of the tube. If the tube is 17 meters long, and you can move the mic from right in front of the speaker all the way up to right next to the material, then you can conduct full wave measurements of that sample over the entire audible spectrum. By sending a fixed frequency tone to the speaker and moving the mic around inside the tube, you can measure the "standing wave ratio" ("SWR" - roughly meaning the ratio of peaks to nulls), which tells you how much of the standing wave the absorption is removing. If you had an SWR of exactly 1.0, that would mean that the material was a perfect absorber, effectively equivalent to the "open window" that Glenn mentioned. In that case, the coefficient of absorption would also be exactly 1.0, which is of course impossible. (The "open window" here would be the entire end cap being removed from the tube, and even then it would not give a reading of 1, since there would be impedance mismatches at the end of the tube, so some small fraction of the energy would still be reflected back). Anyway, that's the theoretical way to measure absorption. Based on the SWR, you can calculate how much of the wave the material is "absorbing". However, that isn't the way that acoustic labs measure the coefficients of acoustic materials. Rather, they place a standardized sample of the material on the floor of a large reverb chamber (much lager than the sample), and measure the results with a mic hanging over the sample. By "standardized sample", they mean one whose surface area is carefully measured and known, and they compare the results for the frontal surface area o that sample against the theoretical "open window" of the same surface area. So for example, they measure the results for a one-square-meter sample of product "X", and compare that against the theoretical 1 square meter "open window". So far so good! Sounds logical, right? Except for one small point: The sample actually has MORE than one square meter of surface area, since it also has thickness. It's not the thickness per se that matters, but thickness also implies edges: It doesn't just have a front: it also has sides. Edges and sides imply more surface area, and also diffraction. And in the case of shaped acoustic foams, with wedges sticking out or wavy surfaces, things are even more complex. So in reality, they aren't just measuring the absorption of the frontal area, they are also measuring the absorption from the sides and other portions of the sample, yet they are still comparing it against the same "one-square-meter theoretical open window". Obviously, some materials absorb really well on the front and the sides and the protuberances as well, so they are absorbing over an area that is GREATER than the apparent one square meter frontal area. Hence the coefficient can be greater than one. The other issue is that with the "sample at the end of a tube" method, you are ONLY measuring normally incident waves (waves that hit the material at an angle of exactly 90°). But in the reverb chamber, you also have waves bouncing around at all kinds of angles, not just 90°, so that also skews the results to a certain extent. So why measure like this? Why not do all measurements in SWR tubes? Because an SWR tube isn't a realistic representation of a room. A reverb chamber is. You want to get a measurement that reasonably reflects the way a real piece of the material will react in a real studio, living room, home theater, etc., and the acoustic lab test in a reverb chamber does that much better than the tube does. As far as I know, that's the reason for the apparently impossible over-unity absorption of some materials. It isn't really over-unity at all: it's just a side effect of the way the measurements are done. So you really need to take a look at how thick the material is when you compare coefficients, to get an idea about how good they really are at absorbing. For example, a material that is 1 inch thick and has a coefficient of 0.85 (for example) at a certain frequency would most likely be a MUCH better absorber than a different material that is 10 inches thick but has a coefficient of 0.9 at the same frequency. Confusing, isn't it? Ain't acoustics wonderfully logical, and intuitive? :) (There may be other factors that also skew the readings, but those are the ones that I've heard of.) - Stuart -

vinyljunkie wrote:

We have been taught that the absorption co-efficient of a surface can range from 0 to 1 with 1 being totally absorbant and 0 being totally reflective. We have also been taught that there is no such thin as a 0 or a 1 as nothing is 100% absorbant and nothing is 100% reflective.

You're already getting good explanations, but I got your PM so I'll add a bit anyway. :D One reason absorption can seem to exceed 100 percent is because the edges of the absorber panels are often exposed to the room. With a 4-inch thick 2x4 foot panel those edges add 50 percent to the surface area! Because these questions come up so often, I wrote this article for Sound & Vibration magazine: Test Methods for Acoustic Treatment Products --Ethan

John, You have some good advice here, I would only add the following. ASTM does not necessarily attribute this to the thickness of the specimen, the following is taken directly from ASTM C423

5.3 Diffraction effects usually cause the apparent area of a specimen to be greater than its geometrical area, thereby increasing the coefficients measured according to this test method. When the test specimen is highly absorptive, these values may exceed unity.

The above includes (after the word "effects") a reference to footnote 5 - which reads (in whole)

5 Chrisler, V., “Dependence of Sound Absorption Upon the Area and Distribution of the Absorbent Material,” Journal of Research, National Bureau of Standards, Vol 13, 1934, p. 169: Northwood, T. D., Grisaru, M. T., and Medcof, M. A., “Absorption of Sound by a Strip of Absorptive Material in a Diffuse Sound Field,” Journal of the Acoustical Society of America, Vol 31, 1959, p. 595: and Northwood, T. D., “Absorption of Diffuse Sound by a Strip or Rectangular Patch of Absorptive Material,” Journal of the Acoustical Society of America, Vol 35, 1963, p. 1173.

You could reference that for further information in this regard. The face of a panel tends to absorb - the edges of a panel tend to diffract, diffraction is the tendency for sound to bend around obstacles in its path. Picture taking a rigid panel and placing it widthwise in a fast running stream. There would be a tremendous energy exerted against the face of the panel. Now take that same panel and rotate it 90 degrees, so that the force of the stream was pushing only the thinner edge. There would be very little pressure - the water just would easily just push past that edge. Sound-waves work in much the same manner as water in this case: Sound-waves that strike an object have 3 (distinct) possibilities as to how they might react, these are dependent on the makeup of the material they are encountering coupled with the angle they are striking the object in question, along with the frequency in question, including it's amplitude (temperature and humidity also come into play to a lesser degree): The 1st being to push through the object (and in the process be absorbed to whatever degree,) The 2nd being to reflect (deflect), at which point they will leave the object at an angle which is incident to the angle at which they struck the object. The same way a pool ball travels off a bank (assuming you do not strike it using English, or with enough force that other factors will enter into the picture.) The 3rd is that waves can simply bend around it - diffract. For example, a certain amount of energy might be absorbed, while the remainder can diffract. However, when you take finish your measurements, you sum the difference of the sound energy in the room for the room in an empty state - and the room with treatments. The difference is then (again in accordance with ASTM standards) strictly divided by the total area of the face of the panels that were tested. Now - to the question of why would one include a number that exceeds unity in a test result: Any laboratory testing - regardless of the nature of the lab or tests being conducted, has only one goal in mind - which is to gather (hopefully meaningful) data, and report those findings, along with all of the (relevant) conditions existing at the time of the test in question. Any lab that would choose to omit, or alter, data that was gathered , could lose their certification, or, at the very least, their credibility. ASTM requires reporting of the test results to be in accordance with the following:

12. Report 12.1 The report shall include the following: 12.1.1 A statement, if true in every respect, that the test was conducted according to this test method. If not true in every respect, the exceptions shall be noted. 12.1.2 A description of the test specimen, its size, mounting, weight, and any other details that may be necessary to identify another sample of the same material or kind of object. When sound absorption coefficients are reported, the area used to calculate them shall be reported. Mountings that are defined in Practices E 795 may be described by citing the applicable type designation. 12.1.3 When the specimen is an extended plane surface or an office screen, the results to be reported are the absorption coefficients at the eighteen measuring frequencies rounded to the nearest multiple of 0.01, together with the sound absorption average (SAA) and the noise reduction coefficient (NRC).

Note the requirement to round to 2 decimal points. The do not allow for the test results to be adjusted to unity. I hope this (along with the rest you have been offered) was a help. Rod Gervais Director of Education GIK Acoustics http://www.gikacoustics.com (USA) http://www.gikacoustics.co.uk (Europe) Tel.(US)[removed] Tel.(UK)+44(0)20.7558.8976

Stuart, Glenn, Ethan & Rod Thank you very much guys, this has been a massive help. Much appreciated. John