I was looking to "trap" frequencies at 80hz. To do that, I'd either need some non-porous material or an air gap of 3.5 ft. My room is small, so I can really only afford an air gap of 2 ft.
That being said, I'm looking for an easy to work with, non-porous material that I can use for a bass trap. I know fiberglass is the standard, but that's a bit too difficult to handle in my case. I'd prefer using rockwool, but I think that's a porous material and wouldn't be heavy enough for either a superchunk or an air-gapped material at 2ft.
Hey, TeaGuy. I'm happy you're interested in solving your acoustic issue. Even so, please allow me to help you understand a few things.
There are three main types of absorbers you'll use in a typical studio environment: fluffy insulation, rigid insulation, and open-cell foam. While I'm not sure exactly what you mean by 'non-porous', you are probably better off thinking in terms of these three options. While closed-cell foam exists, it's not an acoustic absorber.
Regarding 80Hz, I would forget about targeting a specific frequency unless you've deployed extensive broadband treatment. Based on what you said, I'm assuming you haven't done this yet - forgive me if I'm wrong. In all cases I've seen, no frequency-specific targeting has been required once this has been done.
The last thing I'll say is that rockwool is definitely an option for super chunks or gapping. For whatever reason, maybe you're in a room with 7' and 14' dimensions and have a very specific need to tame 80Hz, the best thing you can do is use either a 3.5'-thick fluffy absorber or a 1.75' fluffy absorber with a 1x gap. You don't want to make a 6" rigid panel and gap it 3'. While that will target 80Hz, it isn't a best practice.
I stand corrected. 'Porous' is indeed a legit way to describe this type of treatment. To me, it seems to apply more to foam but, again, I neglected to remember it applies to both foam and fibrous materials.
There are a number of differences among those materials. Let me not indicate that I know all of them. As far as I'm concerned, it comes down to budget, floor space, availability, and your targets.
The easiest, most expensive, option is foam but it might not be the most effective for a given task. This is because you can make fluffy as thick as you can manage and target all the bass you want. Rigid seems like a great option since most people don't need extremely thick installations and it takes up less room. The disadvantage with rigid is that it only works optimally up to 8", again limiting your bass targeting - not that you'll probably exceed its limitations given enough coverage.
11' of absorption would target frequencies below 16Hz. Unless you're building an anechoic chamber, you don't need this kind of installation. For critical listening spaces, you just need to target down to about 63Hz and, even then, you don't need immediate decay.
If 80Hz has a 14' wavelength then to totally kill it, you use a 3.5' absorber since it functions from the quarter wavelength. I'm not a physicist but, in practice, I know this is how it works. Again, though you want to actually achieve nominal decay targets down to about 63Hz, you don't need absorbers to that quarter wavelength since thinner absorbers still work over that range. The key is to have enough absorbers in your room since sheer coverage seems to trump a solitary mega absorber with limited other dimensions.
Long story short, I have 20 4" rigid panels and 9 4'h 31.5" wide (diagonal 24" fluffy) super chunks in a roughly 2500ft3 room and I achieve 20dB decay within 150ms above 50Hz with an SPL within +-10dB at a listening position with speakers about 3' away plus a subwoofer.
There's no way for me to give you a comprehensive education with one post. The best thing I can tell you is to follow the fundamentals - contained in the stickies and elsewhere, take measurements, then improve iteratively. If you have a significant project, like building a studio, you're better off hiring a consultant.
I realize this might sound rude - it's just my nature, apparently. In any case, know that the way I think of this whole 'acoustics' thing is 'see problem, solve it'. It should be clear that I don't know all the details about this but I don't know exactly what your problem is. I don't know your room size or its intended application. All I can tell you is that basics + targeted problem solving is the way I like to deal with it. If you want to know more about the math and physics side of things then I recommend the Acoustic Absorbers and Diffusers book.
You're not being rude at all! But I think we're getting a little misconnected. Let me try being more clear about what I'm strugling with.
The forumla on that website I linked you to and Ethan's formula both state a quarter wavelength rule for an "optimal" gap for porous material dependent on the hz being targeted. Wave Length in Feet = 1130 / Frequency. As far as my understanding goes, it would be 1130/80 = 14.125. Then you take a quarter of 14.125 and that gets you 3.53. The optimal gap for 80hz with a porous material would be 3.53 feet.
I only have 2.5 feet to work with, so that's not quite going to work for me. The only other option would be to 1) build a "superchunk" bass trap or 2) find a material that can absorb low frequencies without taking up as much space.
In regards to option 1, as far as I know, if I wanted to target 80hz with only rockwool and no air gap, I'd need 11 feet of the stuff to do so. Clearly not an option. If this assumption is incorrect, then do you know how much of the rockwool I'd need to properly target around 200hz and below in a superchunk format?
In regards to option 2, the question is sort of self explanatory. But is there a material or someting I could build that handles low frequencies that's less than 2.5 feet?
Indeed, the quarter wavelength is the ideal place to put an absorber. That doesn't mean there needs to be a quarter-wavelength gap, mind you. All it means is the 1/4 and 3/4 parts of a wave are it's phases with maximum velocity and minimum pressure - perfect for velocity/porous/frictional absorbers. I included an image in this video and blog post I made contrasting this type with resonant absorbers.
Unless I'm gravely mistaken, you can forget all about this 11ft idea - I don't believe it has any valid basis. I don't remember the exact fraction but something like 1/64th the wavelength-thick absorber will have some effect. When you look at absorption coefficient charts, you'll see that even 2" rigid panels are rated down to 125Hz. If everyone needed 11'-thick treatment, I don't think we'd be using these materials or very few people would ever attempt to treat their room because it'd be hopeless.
Again, when you follow the basics and create a RFZ, install some dedicated bass treatment like fluffy or rigid super chunks or even angled panels, fine tune your modal response with additional surface coverage, and lastly use diffusion should you want it, you get closer and closer to a room which is less of an acoustic nuisance and more of a sonic supporter. The last thing I can say about it is that you're best to follow the plan, take notes, then make targeted adjustments. Maybe I'm wrong but I feel like you're looking for chinks in the theoretical armor without ever having donned the practical suit.
Treat your room using proven wisdom first, find fallacies later.
Let's look at this not as absolute an cutoff frequency, but as a gradual roll-off. So for a 3.5 ft thick absorber you can absorb or cancel %100 of 80Hz ... (and here's the kicker) "ONLY in the space that the 3.5 ft thick absorber exists".
So yeah, if you line your entire room with 3.5ft of absorption, you'll theoretically get absolute absorption down to 80Hz. But if you don't line the entire room, you likely wont get the same performance.
Now, on the other hand, it's important to understand that an absorber less than 3.5ft thick WILL absorb SOME 80Hz AND even lower frequencies... just not as much as the 3.5 ft thick absorber. And this is the point: An absorber whose 100% absorption is at 500Hz will still absorb SOMETHING at 80Hz...just not 100%... but which may be enough for what you need.
Bottom line: All broadband porous absorbers are "broadband". In general the thicker they are, the more effective at lower frequencies BUT even thinner porous absorbers will have some effect on lower frequencies, just less than the thicker ones.
Another important idea is the coverage in the room. In general, it's better to have more % coverage with less thickness than less % coverage with greater thickness.
Ethan has stated all these details in his posts, web pages and writings. As Hexspa says "Treat your room using proven wisdom first, find fallacies later."
I can't even begin to find fallacies because I don't really know what I'm doing. I'm trying to treat my room using proven wisdom, but I'm having trouble understanding "what" that proven wisdom is.
Numerous blogs are saying that I can't just tack up 3.5 inch absorption panels to absorb bass frequencies, that they will only absorb mid-high range frequencies and not deal with the bass at all. I can take that 3.5 inch absorption panel and make it thicker, let's say, 2 feet thick, but will making it thicker absorb bass frequencies or will it only more aggressively target the mid-high range frequencies without touching the bass frequencies all all?
At the end of the day, my question is simply this. In a standard bedroom, what material and what length of it do I need to put on my wall to absorb bass frequencies?
Did you read the "sticky" at the top of this forum called "Please read this first"? I think your answers are there. Please read all the links. If you have questions after reading that, we'll try to help you out. BTW, what blogs are you reading?
I already told you what I have in my room and the results I'm getting. All you have to do is decide whether to use rigid or fluffy - or, heaven forbid, foam - and start installing it. From there, take measurements and decide whether you need to move it, or adjust the quantity.
That's really all there is to it.
I agree with what rock seems to be implying, though. 'Follow two paths, go nowhere.' You're better off committing to a single method - preferably by a single author - and moving forward with a bit of trust.
When I first started, Ethan convinced me so I followed his lead. He wasn't wrong and my trust was validated. Ethan is, as far as I'm concerned, the OG who brought acoustics from the highbrow down to the unwashed masses.
I advocate for simple, actionable advice and you can find that within a few clicks from here.
You don't necessarily want it 'as far off the wall as doable'. What you want is to select a thickness with which you can manage a significant percentage of wall coverage and use about a 1x gap. In other words, more thinner panels is way better than a few thick ones spaced 2' off the wall.