The only 2 numbers we need to know
For all intents and purposes all kinds of porous insulation material absorb sound in the same way. The movement of air particles (called sound velocity) is reduced by friction with the tiny fibers in the material. This converts the energy of movement into heat.
Depending on how densely the fibers are packed the air particle movement is affected more or less. And the denser it is packed, the heavier the material is.
So a) the weight of the material
determines how good it is at absorbing sound.
Of course b) its depth
will also affect how air particle motion is affected. If the sound travels through enough of a very lightly packed material it will also eventually be stopped completely.
But we don’t necessarily want to waist tons of physical space with a light material if we can achieve the same result with a denser material that is thinner.
Although we don’t want to take that to extremes either. Eventually the material will be so dense that it starts reflecting sound. That would kind of defeat the purpose.
The thing is: Theory says that the deeper an absorber is, the lower a frequency it will absorb. It’s called the quarter wavelength effect and it definitely holds true. We do want to absorb as much bass as possible.
So where is the sweet spot?
What is the ideal ratio of density to depth that will give us the best absorption for the least amount of material?
I’ve spent a lot of time testing this. Some designs were shallow, some ended up being huge! Some were more successful, others not so much.
Here are two designs I tried:
The one on the left was my version of a modular “super chunk”. It worked, but man was it tedious to build. And cumbersome to move! That thing was HEAVY!
The one on the right didn’t work so well. It was simply too thin. It never achieved any useful bass absorption.
Finally I ended up with a simple, straight forward solution.
It’s kind of beautiful actually.
The best bang for the buck absorption is achieved with material 16cm (6″) deep, using insulation material of 40kg/m3 (2.5lb/cft).
And it doesn’t matter what that material is made of! Mineral wool, fibre glass, hemp, recycled jeans. They all work as long as they’re the right weight.
This is the case both for corner traps (bass traps) AND first reflection traps (panels). We can use the same design for everything!
The sheets of insulation material should be around 60cm wide. Especially for the corner traps where straddling the corner will allow absorption down into the low bass frequencies.
Looking at the data
This is the bass response in a room that I treated using only 16cm deep, 60cm wide absorbers:
The standing waves are pretty much eliminated down to 35Hz. Also note the scaling of the y-axis. The frequency response has no more ripple than about +-2.5dB. And that with a smoothing of 1/48th Oct.
This proves that you do not need complicated traps to get controlled bass.
Now, using THE SAME panels at the early reflection points, here is the effect on the impulse response of the left speaker:
Each peak in this graph represents a reflection arriving back at the microphone following the direct sound from the speaker (represented by the peak at 0 seconds).
As a very general rule of thumb, we want to suppress any strong peaks within the first 50 milliseconds.
These early reflections are largely responsible for messing with the stereo image and timbre of what we are hearing. Suppressing them hugely improves the clarity and detail of the stereo sound stage and the body of the instruments.
As a side note: The peak circled in blue arrives just 2ms after the direct sound in both measurements. This is the reflection off of the table. It’s the main reason even a great sounding room will still have a messed up frequency response. It’s also the main reason that you see some mastering engineers work with no table at all!
All this was done using a single absorber design based on 16cm (6″) deep material with a weight of 40kg/m3 (2.5lb/cft).
The beauty is that the reverb time ends up being even all the way down to the bass. A slight bump remains because the room is still uncontrolled below 35Hz. And yes, the overall reverb time is short, but the room does not feel dark or over damped. This only happens when the reverb time is cut down in the high frequencies, but left unchecked in the lows.
That’s another reason to avoid thin absorber panels. They only reduce reverb time in the high and mid frequencies, making the room sound unbalanced and very difficult to work with.
So you see, it can be done. It does not need to be complicated. It’s just a matter of getting the fundamentals right.
To get you started
Here’s list of suitable insulation materials I’ve found in various countries.
If your country doesn’t appear in the list have a look at Knauf which operates in most countries across the globe.
A few things to note:
If you are concerned about your health, you can relax, most insulation material nowadays does not seem to pose a serious health threat
. But to be safe I recommend you give it a few days to vent before processing and that you wrap it in a thin plastic foil to avoid fibre flight. Wear protection when handling as the fibers do physically irritate the skin and it’s still probably better not to inhale them.
If you are wondering where foam fits into all of this: It does what it’s supposed to do, just at a much higher price. It’s also sold in panels that are way too thin and so only effective in the high frequencies. As I mentioned earlier, this is something you really want to avoid.