In a previous blog,Acoustical Control: Understanding Insertion Loss Part 1, we talked about the concept of insertion loss. As a quick recap, insertion loss quantifies the reduction in sound that is caused by adding insulation and jacketing/lagging to a pipe or duct system. We’ll delve deeper into the practical applications of insertion loss in this article as we discuss how to use insertion loss to design piping insulation systems with an emphasis on sound control.
Typically, the optimal way to approach system design is to have a complete understanding of what we want to accomplish prior to beginning the actual design. This can help remove much of the preliminary confusion that surrounds the intricate relationships between sound absorption coefficients, sound transmission loss, and insertion loss.
As safety is frequently a top priority for most manufacturing facilities, we can generally assume that protecting worker hearing by maintaining sound levels below 85 dBA is going to be one of the target goals. To help facilitate this most manufacturers that create industrial equipment also provide acoustical data regarding the amount of noise produced by the equipment they manufacture. System designers then use this information to quantify how much noise will be produced by the equipment as well as the amount of noise that will be produced by the environment as a whole. This enables them to design an insulation and jacketing/lagging system that can provide the appropriate amount of acoustical control to maintain acceptable ambient noise levels.
In some applications, there are design guides that provide prescriptive approaches to ensure that the system design is both accurate and efficient. One such guide that provides information for designing acoustical insulation systems is ISO 15665, “Acoustic Insulation for Pipes, Valves and Flanges.”1 The guide provides both performance and prescriptive requirements for acoustical pipe applications.
The performance requirements per ISO 15665 mandate that the insulation and jacketing/lagging system be tested as a complete system. This means that the test results apply specifically to the system configuration that is being tested. Should the pipe size, insulation material, insulation thickness, or jacketing/lagging change, a new test must be conducted to determine the performance characteristics of the new system configuration. Fortunately, the prescriptive requirements provided by ISO 15665 are the result of several tests of similar configurations to gather a complete picture of the system performance.
The ISO 15665 prescriptive system-requirements establish 3 different sound control ratings (Class A, B and C). These classes are broken out into three sub-sets pertaining to large, medium, and small diameter pipes, and they describe the acoustical performance characteristics of each class. The ISO standard has the characteristics of each class listed in Table 1: Minimum Insertion Loss Required by Each Class.2
In order to use this information, it is critical to understand precisely what you want to achieve with your sound control system. For example, if we need to reduce the noise level by 15 dB at 1000 Hz for a 300 mm pipe, then we would need to use a “B2” type design (as determined by Table 1 in the ISO standard).2 Knowing this information, we can then use the prescriptive requirements established by ISO 15665 to determine the necessary design requirements to meet this specification. In the ISO standard, this information can be found in Table 5: Insulation Constructions Meeting Classes of Acoustic Insulation, and it describes the insulation thickness and stiffness and the weight of the jacketing that are required to design a system that meets the ISO 15665 standard.3
In our example, we would use the information in Table 5 that is designated for Class B designs to determine that the system would require 100 mm (4”) of porous insulation and jacketing/lagging that has a thickness of at least 0.8 mm (.32”) of the same weight per square foot in order to achieve the acoustical performance specifications that we established from Table 1 and meet intended requirements of ISO 156653.
The class differentiation is a function of thickness and jacketing weight, where increased insulation thickness improves performance when going from a Class A to a Class B, while the change in jacketing weight improves performance in the transition from Class B to Class C.3
When a system is designed using a prescriptive approach, like the one outlined in the ISO standard, the anticipated performance can be ensured by meeting the insulation material requirements and the correct weight of jacketing or lagging set forth by the ISO standard. Since prescriptive requirements are the result of several tests of similar configurations, the performance can be summarized by insulation thickness, density and jacketing/lagging weight. In order to use the prescriptive data correctly, the insulation must have the following characteristics1:
- The insulation layer must be porous (like fiber glass or mineral wool, not calcium silicate or perlite)
- Density must be within 80kg/m3 (5.0 lbs/ft3) to 120 kg/m3 (7.5 lbs/ft3)
- Mechanical stiffness must be equal to or less than what is shown above in Table 2
- Airflow resistivity (the impediment of air as it travels through the insulation layer) should range from 25,000 Ns/m4 (25k mks Rayl/m) to 75,000 Ns/m4 (75k mks Rayl/m)
Bear in mind that not only the thickness of the porous insulation, but also the stiffness of the insulation must be taken into consideration when designing for acoustical control. When these conditions are met, and the system is designed as prescribed, the system should perform as outlined in Table 1: Minimum Insertion Loss Required by Each Class.
Acoustical control can be a little confusing, but using resources like the ISO 15665 guide can help present a clearer picture of how to achieve the desired acoustical performance. Additionally, Johns Manville Industrial Insulation Group has dedicated technical specification specialists who can help your design team ensure your specifications are on target. Click here to contact us.
- *ISO 15665:2003-Acoustic Insulation for Pipes, Valves, and Flanges
- Table 1: ISO 15665:2003 Minimum Insertion Loss Required by Each Class
- Table 5: Insulation Constructions Meeting Classes of Acoustic Insulation