What do you think of this approach compared to having acoustic foam on all inside surfaces, as I haven't seen any others where the ducting continues through the box.
Youtube is filled with hilariously incorrect studio builds.
First off, silencer boxes work off of a few principles that his completely ignore. Here are some points:
- low end is attenuated through the gross impedance changes achieved by immediate cross sectional changes. The larger the change, the better. His design uses a conical transition which allows for an impedance match and eliminates this very important characteristic of a good silencer. Look at the muffler on your car, do you see it gradually getting larger? Nope.
- Next, he used insulated, not lined flex duct. If he had used the correct stuff (lined), here are some lab results:
Lined Flexible Duct Insertion Loss Chart.png
Flex duct is famous for introducing gross amounts of static pressure and ASHRAE recommends no longer than 14' in a system. So, if this guy actually uses the correct number of silencers in his studio, he is guaranteed to go over that. To achieve any gross impedance change, he'd need very large flex duct, and as you can see in the chart, the bigger that duct, the less insertion loss is achieved.
- Proper duct liner should be used in silencer boxes. I highly recommend Owens Corning QuietR Rotary. This guy used fibreglass insulation that is way too light to be good for anything acoustically speaking. The fact that he didn't know to use the correct insulation alone shows that he doesn't really know what he's doing.
- His boxes are made out of material that doesn't have enough surface density. The surface density of this box must match his walls.
- He is using flex duct to penetrate the walls. That right there will trash his isolation as the sound will blow right through it. The penetrating material (we call it sleeves), must match the surface density of the wall/box.
- The sharp corners in lined ducts that are greater than 60 degrees are great at attenuating higher frequencies. This guy used round duct. On the forum, we suggest rectangular. Here's why:
Insertion Loss of Square Elbows without Turning Vanes.png
Insertion Loss of Unlined and Lined Square Elbows with Turning Vanes.png
Insertion Loss of Round Elbows.png
Lastly, watching the end of the video after the guy hoisted up his silencer box (obviously very heavy) with a pulley that was attached to a friggin box handle!, I noticed that his outer leaf has no sheathing whatsoever. In other words, this guy is a joke.
Greg
Oh yeah! Some of them built dangerously, and a few probably illegal, too.