It is common belief that a series of elbows near an air handling unit (AHU) can accomplish the same insertion loss as a manufactured duct sound attenuator. It’s true that elbows are less expensive than sound attenuators, but what acoustical performance is really achieved with this “low cost” substitution?
Let’s compare the two designs. Using the tables presented in the 2007 ASHRAE Handbook – HVAC Applications, for any type of elbow (round, rectangular, acoustically lined or not), the width of an elbow (in the plane that it turns) must equal or exceed 30 inches to get ANY insertion loss at 63 Hz. A 30 inch wide elbow provides only 1 dB of insertion loss (IL) at 125 Hz . These two frequencies are typically of concern when reducing sound from AHUs.
At 63 Hz, an elbow must be at least 60 inches wide to break into the IL domain of sound attenuators at 4 dB. This width dimension is a minimum of 30 inches for 125 Hz.
Ultimately, the cap on elbow IL tops out as follows in the table. To obtain this amount of IL at 125 Hz, an elbow must be at least 60 inches wide.
|Lined square elbow with turning vanes (duct must be lined before and after the elbow)||
|Unlined square elbow with turning vanes||
|Lined square elbow without turning vanes||
|Unlined square elbow without turning vanes||
|Lined round elbows||
|Unlined round elbows||
Sound attenuators typically have roughly 4 to 10 dB of IL at 63 Hz and 8 to 20 dB at 125 Hz. Some designs provide substantially higher IL than this.
It can quickly be seen that for smaller elbows, say 30 inches and under, IL equivalence between elbows and sound attenuators is difficult or impossible to achieve. At the larger duct sizes, the system would have to be carefully designed to obtain equivalent low frequency IL.
What about higher frequencies? Sound attenuators typically provide 20-40 dB of IL in the mid frequency bands and 10-15 at the highest tested frequencies. If this is where you need IL (vane axial and mixed flow fans come to mind), a sound attenuator is far superior to a series of elbows.
Furthermore, to avoid aerodynamically induced noise, the airflow velocity must be slow enough and the elbows must be spaced far enough apart. Extra sheet metal is needed in both cases, driving up the cost of the “cheap” elbow configuration. Additionally, the attenuator will accomplish the required IL in a much shorter distance…an attractive proposition for HVAC systems tight on space or that enter acoustically sensitive areas quickly. Duct lagging, heavy gauge sheet metal and long lengths of duct lining might help, but all have their cost implications.
That “expensive” sound attenuator might not look so costly after all.