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Sabins And Sound Absorption Coefficients

Dennis Foley March 26, 2014 21 Comments
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There is much confusion in the acoustical sales literature when it comes to sound absorption and the terms used to describe the process and testing methodology used in sound absorption calculations. Two terms that are widely used and confused are Sabins and sound absorption coefficients. Lets examine both terms for definition and application in our world as it relates to sound and the sound absorption management of different frequencies.

Here’s my video explanation and below is a more detailed written discussion.

The Relationship Between Sabins And Sound Absorption Coefficients

Sound absorption is defined, as the incident sound that strikes a material that is not reflected back. It is the ratio of absorbed energy to incident energy. An open window is an excellent absorber since the sounds passing through the open window are not reflected back. This process of having the window open makes for a poor sound barrier. A painted concrete block is a good sound barrier but will reflect back about 97% if the incident sound striking it.

Sound Is Vibration

When a sound wave strikes an acoustical material the sound wave causes the fibers or particle makeup of the absorbing material to vibrate. This vibration causes tiny amounts of heat due to the friction and thus sound absorption is accomplished by way of energy to heat conversion. The more fibrous a material is the better the absorption; conversely denser materials are less absorptive. The sound absorbing characteristics of acoustical materials vary significantly with frequency. In general low frequency sounds are very difficult to absorb because of their long wavelength.

Absorber Thickness

For the vast majority of conventional acoustical materials, the material thickness has the greatest impact on the material’s sound absorbing qualities. While the inherent composition of the acoustical material determines the material’s acoustical performance, other factors can be brought to bear to improve or influence the acoustical performance. Incorporating an air space behind an acoustical ceiling or wall panel often serves to improve low frequency performance. Designing cabinets that have air space and material fillers in that air space will go a long way to improve overall cabinet absorption rates and levels.

This video explains the diaphragmatic absorption process in greater detail.

ACDA-12 Performance Absorption Chart

Reverberation Vs. Impedance Tube

There are two methods of measuring the amounts of sound absorbed by various materials. One is the reverberation method of W. C. Sabine, in which a specimen of the material to be tested is mounted on the walls of a reverberation chamber and the coefficient of absorption is deduced from the effect which the presence of the specimen has on the rate of decay of sound in the chamber, The other consists in placing the specimen at the end of a pipe down which sound-waves are made to pass. Measurement microphones are placed at both ends of the tube. The reflected and incident waves interfere, and the coefficient of absorption is calculated from observations made on the interference pattern within the pipe. Here is link for impedance tube testing:

http://sine.ni.com/cs/app/doc/p/id/cs-654

Sabins

A Sabin is actually a scientific term for a unit of measurement of sound absorption. It is the basic unit of measurement that has been formulated and calculated by Wallace Sabin over a hundred years ago. Riverbank Labs, now Alion Research is the lab that was created by Wallace Sabin for testing the amount of absorption a material has and is then assigned a value in Sabins. It is calculated by using one square foot and assigning a maximum value of 1.00 if the material tested has 100 % absorption of that particular frequency. If you are using the metric system, you would use one square meter as your reference size and it would yield a value of 1.00 also if 100 % absorption occurred at any chosen test frequency.

Sabins Measured

In summary, a Sabin is a unit of measure and any material tested will produce so many Sabins per square foot or per square meter depending on your standard of reference. If a square foot of any given material had a Sabin count of 30, you would know that it is equal to 30 sq.ft. of 100 % absorption at that frequency of that test sample. Lets examine our own Riverbank Test Data to look at these values.

Our ACDA-12 units shows at 40 Hz. that our test sample size produced 44.59 Sabins. At 50 Hz., our test sample produced 77.87 Sabins. Following the definition of Sabins where a perfect absorber at 1 sq.ft. receives a value of 1.00, we have almost 45 sq.ft. of 100 % absorption and at 50 Hz. we have almost 78 sq. ft. Our sample size was 72 sq. ft., so we can safely say that 72 sq.ft. of our ACDA-12 units absorbed 100 % of all the 50 cycle energy that was introduced. Now, lets look at what absorption coefficient means. Here is the full test data report on the ACDA-12 performed by Riverbank Acoustical Labs.

ACDA-10 Performance Absorption Graph

Sound Absorption Coefficient

The sound absorption coefficient is the ratio of reflected energy that strikes our sample size to the amount of energy absorbed by our sample size. It is expressed in terms of 1.00, which is equal to 100 % sound absorption at that tested frequency and sample size. Back to our Riverbank Test Data, at 40 Hz. we had an absorption coefficient of .62. What does that mean?

It means that 62% of the energy at 40 Hz. striking the sample size was absorbed and 38% was reflected back. At 50 Hz., we had an absorption coefficient of 1.08. Obviously, our number can not be greater than 1 so the numbers to the right of the decimal point are due to testing bias. We can use 100 % as our number for absorption calculation and not be discrediting the test data. At 50 Hz., our sample size absorbed all the 50 cycle energy thrown at it during the test.

I hope this explanation helps. If you have any questions please don’t hesitate to call on me as I am always available to help. Feel free to leave comments below and I will get back to you. Be sure to like or tweet this out to other audio friends. If you want to learn more about this subject please sign up for our free room acoustic treatment videos and 150+ page e book which provide step by step instructions. Get instant access by signing up now.

Thanks and speak soon,
Dennis

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others. Connect with me on Google+

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21 thoughts on “Sabins And Sound Absorption Coefficients

  1. Dear Mr. Foley,

    I have the sabine coefficient of absorptions with the gradual increase of frequency so how do I can use in BEM modeling? In the BEM model software accepts only damping factor, reflection coefficient, and admittance. what is the exact relationship between sabine absorption coefficient and damping factor?

    • Hi Venkat, I am unsure about the relationship between the two variables. Using definition, there seems to be a correlation between the two variables, but I do not know how to program that.

  2. Dear mr. foley,
    i wanna design a concert hall.. about for 1500 people .. what will be the sound treatment. how will be control it..thank you…?

  3. Dear Mr Foley,
    I was just wondering what the reason is for the following question; why does the thickness (of say a foam) material increase the amount of sound absorbed. Is there a formula that exists to model this relationship ??

    • Hi Teodora, Depth or thickness is related to absorption. The greater the depth, the lower frequency of resonance that can be achieved. Frequencies above the resonant are absorbed, those below are not. Open celled foam composition has come a long way since its inception. Depth increases the lower end resonant frequencies to between 100 – 125 Hz. with open celled foam. Is there a way to model this performance. Yes, there are ways with a little research on the net. Each methodology is specific to the medium you have chosen for absorption. A 2″ piece of open celled foam will absorb at different rates and levels than a 2″ thick drape.

  4. Dear Mr.Foley,

    I see a lot of American manufacturers publishing test reports with NRC ratings up to 1,55 and actually being very proud of it.I do understand that you can have some small deviations due to testing bias,but do you have any idea how this could be explained.I also barely see any serious reporting with testing conditions and the actual testing method,including a picture of how they tested the material(with or without air gap..)
    In Europe acoustic engineers or consultants wouldn’t even take those reports seriously,but I really try to understand if they use other (accepted?)measuring methods?
    Thanks and keep up the good work.

    • Hi Daniel, I see this on numerous occasions and like you, am at a loss on how to explain it. I understand that some measurements may exceed 100 % due to testing variations and error rates but this 1.55 number is baffling to me.

  5. Dear sir ,
    How we can determine the sample size and cut off frequency for reverberation chamber. Is there any relationship equation to determine both of them ?

  6. Dear Mr.Foley,
    If you used micro-perforated materials as sound absorbents before, what is the optimum perforation ration to get the highest absorption coefficient?.
    Is there any correlation equation between normal incidence (impedance tube) and random incidence (reverberation chamber) sound absorption coefficient?
    Thank You

    • H, You must match the technology to the frequency issues you are experiencing in your room. I do not have any experience with micro-perforated materials. Sample size in impedance tube testing is related to reverberation chamber but it is frequency dependent.

  7. Hi I need to design a auditorium , whose surface areas of various parts are known( like ceiling area , stage area , seat area , walls area etc) , I am confused to chose for what frequency should I calculate absorption – 125 Hz or 250Hz or 500 Hz etc. coz there are different absorption coefficients for different frequencies for same material

  8. Dennis:

    On other websites, I have seen absorption coefficients quoted as being greater than 1.00. How can this be? Above, you admit it can happen and say it occurs through “testing bias”. What is testing bias and even if it affected the measurement of an absorption coefficient, why would anyone quote a coefficient greater than 1.00?

  9. Thank you for the video explaining the difference between sabins and sound absorption coefficient.
    If the sound absorption coefficient of a sample is 0.3 at 250 Hz, and the sample size was 12 sq.m, can I apply this same sound absorption for 1 sq.m also? Does the sound absorption vary with the area of the material used?

      • Thank you very much Mr. Dennis, I am into central air conditioning. In air-conditioning, the air from the noisy roof top unit goes through a few metres of acoustically lined duct for some noise attenuation before the air enters into occupied space. If the level of absorption is constant across the sample, would you know on what basis do the duct designers decide that they require x meters of liner is required to bring the noise down from 70 dB to lesser.

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