So much good info in this thread! I could talk for days about diffusers and absorbers.
Here is a great visual example I like to use to help people wrap their heads around what's going on and how to solve it. I did a Google image search for "salt sound experiment" - lets see if I can add it to this thread.
![](https://i.ibb.co/D9BzS9j/Salt-Sound.jpg)
I posted videos of this here a few years ago.
So this is a steel plate that is physically connected to a driver of some sort, a tone run through it, and salt or sand poured on it. So the steel plate can represent your listening room, and the salt patterns are a physical representation of the sound waves in your room crashing into each other and causing peaks and nulls. Each frequency draws different patterns because each frequency has different lengths. ie a 40hz bass note is much longer than a 200hz bass note, so they crash into each other (after bouncing off walls, floor, ceiling) in different locations...which is represented by the salt pooling up in different locations at different frequencies. This is super apparent with bass because the waves are so big that you can stand in one spot and it's like the center of a tornado, step 3 feet away and your sinuses are rattling!
This happens at ALL FREQUENCIES! At higher frequencies this is called comb filtering because when you look at at a measurement graph of your room, your otherwise flat speakers have these notches at regular intervals.
![](https://i.ibb.co/FBx5vm2/Comb-Filtering.jpg)
So how does this visual representation help us understand how to address the problem? Well, look at those salt patterns, and imagine your listening position in one of those black (dead) spots. You're picturing a tiny little chair on that steel plate now right? So what happens if you physically move your chair? Hot damn, you can be in that salty goodness! I mean, you can be in a bass happy zone. Or, alternatively you can move your speakers, which moves the salt pattern, till the bass happy zone now covers your little chair! But...how far do I need to move my speakers? Can I just toe them in? Do I move Forward, or backward, or wider? (shrug) Way too many variables for any guidance. There are some room tuning programs that pro sound engineers use that can map out a room in 3d and help, but that's all very specialized stuff. There are also some really basic 2d calculators that can help, but IMHO without good measurement gear and LOTS of time, your best bet is to use the measurement devices on the sides of your head!
But, but, but Mr Raven Sir...I have adjusted everything as best as I can, and the bass is way better...but I can clearly spot a few bass notes that are just dead, dead, dead!
Yes, unless your listening room is in an empty field (no boundaries to cause reflections and crashes) or your living room is the size of a basketball court (waves are weakened by the time they reflect and crash due to distance), well then us mere mortals who have typical listening rooms will just have nulls and peaks!
That's where the room treatment comes in! Diffusers help break up those reflections and time delay the reflected sound to reduce comb filtering...but diffusers for bass frequencies would need to be a MINIMUM of 12" to even get to 150hz or so. They would really need to be a few feet deep to really get down to the good stuff, and that's just not possible in our living spaces. So what we do is add large amounts of absorption to sort of fake having that basketball court sized listening room. We're trying to suck up those big bass notes as they are crashing into the walls so the bass waves are weakened by the time they crash in your listening position, causing less of a peak or null and making the bass more even.
Unfortunately this too can take up a lot of space in your room. You just can't cheat physics - it's all about trade-offs.
A couple notes about adding fiberglass and other absorbing materials:
Many people say it's not good to have exposed fiberglass that you could be breathing in particles every listening session. I don't think this is as bad as people say (unless your rocking out and punching the fiberglass to disturb it).
Too much absorption can kill the liveliness of your room because it's sucking up all frequencies. That's part of why Randy's bag of fiberglass worked so well; the plastic wrapping reflected most frequencies, while the bass notes pass through the plastic and still get absorbed by the fiberglass.
If you are concerned about having fiberglass or mineral wool in your listening room, they do make acoustic insulation made from recycled denim. Super safe, and works pretty well...just a bit expensive compared to the pink stuff or mineral wool. I'm very partial to mineral wool myself - there is a reason more expensive housing uses it in the walls between rooms...because it soaks up sound so well for its thickness!