Dok wrote: ↑Sun Jul 23, 2023 6:56 am
øøøøøøø wrote: ↑Sat Jul 22, 2023 5:55 pm
Dok wrote: ↑Fri Jul 21, 2023 7:42 pm
real broadband absorption requires mass
(Not really true, unless you’re exclusively employing the old brute-force “add more soft stuff” method—apologies for the pedantry!)
Can you please clarify? My understanding of how broadband acoustic panels work is generally that the thicker they are, the lower the frequencies they'll absorb (to a point). And it's why bass traps tend to be super thick so they can deal with longer wavelengths. Perhaps I should have said density instead of mass? I definitely don't want to contribute any more to the decades of internet acoustical misinformation.
I won't speak for Brad, but I think I get his point.
1) You're talking about velocity-based absorption. The moving air particles ram into a matrix of fibers that vibrate upon impact and convert kinetic energy into tiny amounts of waste heat, leaving the air molecules with less kinetic energy and therefore lower amplitude sound waves.
There is an entirely difference class of absorbers, the pressure-based ones. Examples include Helmholtz resonators. You can still make "broadband" pressure-based treatments by combining various styles of pressure-based traps that are each effective in different frequency ranges instead of one that is tuned to a very narrow band (like a Helmholtz). These treatments don't rely on mass so much as the size and location of holes in a thin, rigid board, for example. It is actually often the act of
removing mass, like cutting out holes or slits, carving out air cavities, etc, that provides the effective absorption. Introducing a lightweight, limp membrane into the structure is another great way to make a pressure-based absorber, and in that case, adding more mass would destroy its effectiveness by making it too heavy or rigid to serve its function.
2) Even with velocity-based absorbers, it is not the act of adding mass itself that increases the width of the effective frequency band. From a physics perspective, you making several big assumptions by stating it that way, including: A) That the physical structure of the absorptive material is homogeneous, B) That you are adding mass by increasing the depth of the material relative to the incident soundwave rather than length or width (a 36"x12"x1" thick piece of mineral wool has a broader range of absorption than a 72"x"72"x0.5" one of equal density, though the second is more massive), and several more assumptions that become increasingly pedantic to the point that I don't feel like laying them out because I'll just end up paraphrasing an entire chapter or two out of an Acoustics textbook.
I took a graduate-level course on acoustics and studio design so I love getting into the nitty-gritty details of this shit, and even Brad was apologetic about how pedantic this could get, because we aren't talking about figuring out how to design a room, we are talking about how to employ a bunch of free rigid felt boards, which are obviously going to be used as simple velocity-based broadband absorbers.
The artist formerly known as mbene085.
