I saw this on the following YouTube video:
I saw a video on YouTube that "proved" that the Earth is flat, and another that "proved" that the queen of England is actually a reptile alien invader!
I would never, ever build a studio based on the videos on YouTube about building studios! Most of those are plain silly, many more show gross ignorance of acoustics and wishful-thinking, even more are pure myth and "my girl-friend's cousin's step-father's great-uncle's best friend's daughter used to date the brother of a guy who worked at a studio, and he said....". Some of those videos show dangerously unsafe practices. Some are just plain illegal. A very, very few of them show good, solid, sound advice. The one you saw is not in the latter category:)
the ducts help to prevent the box from developing a musty smell (which is what happened to boxes he made earlier without ducting).
Which indicates that he didn't know how to make the "earlier" one right either! Then he didn't learn his lesson from that, and instead of doing research on how to do it right, he came up with yet another hair-brained scheme...
If he would have built his first one correctly, building a properly sealed massive box from the right materials and using proper duct-liner for the interior, instead of products that were never made to be used inside air ducts, then he would not have had the "musty" problem.
I wonder what are the other pros and cons are of using ducts inside silencer boxes?
The pro's are that it is cheap. The cons are that it won't work, and will make things worse for the HVAC system, not better. That's the brief summary... now for the details:
Are they likely to be less soundproof at certain frequencies?
Yes. Mostly in the range 20 Hz to 20 kHz...
More seriously, there's no useful mass in that box at all, so it is NOT going to isolate, and in fact will probably resonate. You can clearly see that it is built from very thin plywood panels attached to a light frame, which seems to be MDF. So it's basically a resonant box, with membranes stretched over a frame. In reality, the box needs to have roughly the same surface density as the wall or ceiling through which it penetrates, since it is an extension of that wall or ceiling.... Which brings us to the second point of that box in the video: It isn't even on a wall penetration!
It just has a duct going in, and another duct going out, and one of those ducts then goes through the wall.... Sigh! Unbelievable. What's the point of having a silencer box that is supposed to replace the missing mass in the wall, but then not do that, and run a very low mass duct though the wall instead? Pure ignorance. The guy doesn't have a clue of what he's even TRYING to do...
Then there's the issue of impedance mismatch: there isn't any on this "design"! The cross sectional area of the air path remains roughly the same throughout, so he's missing a key benefit of having a large silencer box: the impedance mismatch at the entry and exit, where there's a sudden very large change in cross-sectional area. When that happens, low-frequency sound is partially reflected back up the way it came, due to the difference in acoustic impedance. That larger the change in cross-section, the more pronounced this effect is. Minimum useful is a change of twice or half the area, better is four or five times the area (or 1/4, 1/5). In this case there's no change at all, thus no benefit.
I imagine that higher frequencies might bounce through the tube a bit
Highs tend to act in ray-like fashion, moving more or less in straight lines... and if you look at that second picture you posted, you can see that an imaginary straight line going up the center of the input duct is going to hit the curve at the far end of the "bend"m then carry on straight through, where there's almost no insulation outside of the duct, then continue on through the thin, resonant panel that is supposed to be the wall of the "silencer" box.... So at best, there's only slight attenuation of the highs.
but lower frequencies would pass through the walls of the duct and into the box.
Right! And then continue onwards, through the thin resonant panel that is supposed to be the wall....
Might the curve of the ducts result in improved airflow,
Think of it this way: Does traffic on a road move faster and more smoothly down a long straight stretch of road, or does it move faster and more smoothly through a pair of tight hair-pin bends, back-to-back?
There's your answer. Any time you have a bend in an HVAC duct, you are basically making it harder for air to get through, because it has to change direction to do that. It take energy to make the air change it's route, and something is supplying that energy: the fan. So if the air is forced to go around many tight corners, the fan has to work harder to make that happen, because it feels more "resistance" in the air flow. Simple illustration: blow through a drinking straw, then pinch the straw a bit... it's harder to blow through like that. You have increased the resistance to the flow of air. Technically, this is called "static pressure". One goal of HVAC design is to keep the static pressure low, so the air can move through easily and the fan isnt overloaded. The more bends and corners you have, the higher the static pressure. The more you pinch down the air flow by narrowing the path, the more you increase static pressure. Etc.
So no, those curves do not improve airflow: they make it WORSE! In a normal silencer box, there's actually a
larger cross sectional area for the air flow around the corners, because the diagonal distance from the tip of the baffle into the opposite corner, is larger than the straight-across section just before and after the corner.
or would a box of similar size without ducts provide better airflow because there would be more free space for air to move (hence less static pressure)?
Exactly!
Yup.
Might the extra insulation be beneficial for soundproofing?
Nope! It does nothing useful, because the entire box does nothing useful.
So, to summarize:
1) There's no mass in this box, thus no isolation.
2) The box is resonant, thus probably amplifying at some frequencies.
3) There's no changes in cross section, thus no impedance mismatch.
4) High static pressure.
5) There's no massive "sleeve" that penetrates through the leaf it is attempting to isolate, thus no isolation.
Those are the issues that I noticed at first glance.... There's probably other issues too...
I think you can safely ignore all videos from this guy...
- Stuart -