Newbie muffler question

[jstipins]

So suppose I want to use some insulated duct between the blower and the filters, to help reduce noise.

I understand (using BP's spreadsheet, for example) that the curvature of that duct can affect the static pressure loss of the whole system. But what effect does the curvature have on noise abatement?

For example: Is there a difference in noise reduction between a straight-line 8-foot muffler, and a coiled 8-foot muffler?

Thanks,
-JS3

 

[McRabbet]

JS3:

There will be a slight increase in Static Pressure (SP) loss with a curve segment of duct versus a straight run, as long as there are not significant bends. The key is to build a smooth transition from the 5-1/2" x 7" opening of the CV-1800 blower housing to 8" insulated flexible HVAC duct. The normal CV 90* MDF elbow that is supplied to direct the exhaust air from the blower to the filter stack is replaced with a straight transition piece (called an "evolution" or "square-to-round") and it is connected to a section of HVAC duct.

Several years ago, I found a good web link to layout a custom evolution, which I made to make the transition. Using the link and the associated spreadsheet, I made an evolution using 24 gauge galvanized sheet metal that measures 7" x 9" at the rectangular end (to fit over the outside of the blower housing with small 3/4" x 3/4" x 5-1/2" long oak blocks outside the blower mouth to take screws to hold it in place) to 8" diameter over a 13" length. I pop riveted the seam together and pop riveted the round end to an 8" metal starter collar (sheet metal and starter collar both available at your Lowe's or Home Depot or other home center). The starter collar crimp "fingers" fit outside the round end of the square-to-round and are sealed off with metal HVAC tape. The other end of the starter collar has a crimped end and it fits inside the flexible HVAC duct. You can use another starter collar to make the connection to your filters.

Support the flexible duct along its route from your storage closet to keep it as straight as possible and you are in business. The shorter this duct run, the less SP loss, but it will not be as effective in attenuating the sound.

Hope this helps.

 

[jstipins]

Thank you McRabbet, that's very helpful information. I do understand that curvature and rough transitions will both increase the SP loss of the system.

But my question was whether the curvature of the muffler has an effect on its noise reduction. Do you know if a coiled muffler gives more or less sound attenuation than a straight muffler of the same length?

Thanks,
-Janis

 

[McRabbet]

That one I cannot answer without lots more info. I doubt there will be much difference, but it depends on the style of the muffler, its length and diameter, and the degree of curved path. I would suggest that a straight muffler is going to have less SP "drag" than a coiled one -- I recall a great example of a shop-built muffler in the ClearVue Photo Gallery that one of the submitters made a straight muffler using an outer tube that was about 12" diameter and 1/4" mesh hardware cloth formed into a tube with acoustic foam loosely wrapped around the mesh. You'll find the details in a bunch of great pictures in Louie's Custom Built Godzilla Cyclone in the ClearVue Photo Gallery here.

 

[woody12]

i'd like to use insulated hvac flex, coming out of blower with a sraight transition, them to a filter stack. but cant find any info on the minimum length of the flex (for use as a muffler)