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.
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.
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:
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.
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.
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,