Perhaps Creatology foam would be better than felt. Also, Tyll makes a good point that felt, over time, may absorb moisture, or dry up.
Can we talk more about foam.
There is two kind of foam. Open cell and closed cell. If you're going to use foam to change the acoustic make sure you use open cell foam.
I think the foam you talking about is not open.
Closed cell foam don't "absorb" sound as open do. More like change the tuning. Closed cell is like small mini chambers that isolate the sound.
Open cell foam pass through the air and sound and absorb the unwanted sound and resonance.
But foam does only absorb higher frequencies from mids and highs. Depending on it density.
In all materials, a thick density absorb lower frequencies. Thinner density material like foam and thin felt absorb the higher frequencies.
A thick and hard felt does absorb a wider range of frequencies then foam because it have a light surface and heavy thick bottom.
Heavy materials like bitumen and wood aborb low frequency. When changing acoustic in rooms, hard packed glassfiber wool and even wood is used for "bass traps".
When using light material like foam and felt, it will absorb the upper mids and highs and give a very bassy sound. So if you want more bass use this.
You can combine a heavier material like bitumen with foam on top to get a really wide range of absorbsion.
Cork have lite thicker density then foam and felt but is still airy. Probably don't absorb the lowest frequency and the top highs. There are many types of cork that act differently.
Please tell what results all of you have got by using foam
And is felt good for to have so near exposed and sensitive parts?
Have anyone mod their headphones with bitumen or another similar material?