Why does acoustic foam control reverb but not stop sound from leaving or entering a room?

Question

Acoustic foam, e.g. made of polyether or polyester, with a thickness of 2–3 inch, is often used to control reverb and enhance acoustics. It works through absorption of sound, and the foam’s absorption coefficient is often given at 0.5 to 1.0 for frequencies that correspond to human speech.

Now, any sound wave will experience either reflection or absorption. The latter is what acoustic foam does to the sound of speech and other sounds, to a large degree. Then, perhaps a little unintuitively, absorption can be further divided into dissipation and transmission.

If there is, mostly, either dissipation or transmission for speech, given that the foam has a high absorption coefficient for those frequencies, the remaining question is which of the two contributes the most to the effect.

But since foam is known to reduce reverb and reflections, most of the effect cannot be transmission, right? Otherwise the sound would go through the foam, be reflected from the wall behind it, pass through the foam again and enter the room as a reflection. But that’s not happening with foam panels. So it must actually be dissipation to a high degree, which “transforms” the sound into heat, right?

However, if acoustic foam turns most of the sound into heat, how is it then not also preventing sound from leaving the room, and outside sound from entering the room (perhaps with a small gap between wall and foam)? Various sources say that such soundproofing is not what acoustic foam can do:

I explain that acoustic foam does not block sound about 5 times a day to prospective customers wanting to know how to soundproof. […] I would love to have foam that could stop sound from going through walls. I would sell a ton of it. But physics is physics. […] Even covering the wall 100% with 2” thick foam is not going to, to the extent of the person’s expectations, stop that sound from traveling right through the wall. Acoustical foam is porous and does not block sound simply because it is porous which allows sound to pass through. [1]

We have been fielding calls from people looking to solve sound problems for years. These callers often explain that they want sound that is being made within the room to stay in the room or they want to keep sound out of their space. […] This misconception is incredibly common […] Foam doesn’t stop a sound, it absorbs or reduces echo within the room. [2]

There’s no way around it: the laws of physics dictate that to block transmission, you need materials that are mass-loaded, dense and resilient. [3]

I have come across many people who really believe that foam is great for soundproofing spaces such as home recording studios and bedrooms. […] Foam does not work effectively for soundproofing as it has insubstantial mass to BLOCK sound whereas it is highly capable of ABSORBING sound. That is why ‘acoustic foam’ is for real and ‘soundproof foam’ is a myth. [4]

The kinetic energy of the sound wave moves into the air of the bass traps, absorption panels, or monitor isolation pads, which then rubs against the fiberglass threads. And when this happens, the energy is then transferred into the fibers as heat! It's then dissipated back out to the atmosphere of your room over time. [5]

Whether it be soundproofing against noisy neighbours, or building your own home studio we are always asked the same question regarding soundproofing foam. "Is is good for Soundproofing?" The short answer is No. Unfortunately egg box type foam does not stop sound transferring through your wall from your neighbour or from leaving your room. All it will do is absorb some of the sound within your room and stop it echoing and amplifying. It will not block sound from neighbours or escaping out of your room. [6]

[1] https://acousticalsolutions.com/how-to-soundproof-acoustic-foam-does-not-block-sound/

[2] https://www.acousticalsurfaces.com/blog/soundproofing/soundproofing-vs-sound-absorbing

[3] https://sound-zero.com/acoustic-panels-for-noise-transmission-reduction/

[4] https://soundproofcentral.com/soundproof-foam/

[5] https://ledgernote.com/blog/q-and-a/acoustic-treatment-where-does-the-absorbed-sound-go/

[6] https://www.soundproofingstore.co.uk/does-soundproofing-foam-work

Answer 1

As Tetsujin have stated in their comments (an can be seen at the references you provide), in order to prevent transmission you need mass. Foam does not have enough mass to do that and so it cannot stop sound passing through. Not only that, but guess what... It does allow sound to pass through and this is part of the mechanism that provides the dissipation you mention. In the simplest foam structures there's some "dead ends" where the sound is "trapped", thus part of the energy passes through the material of the foam and part of it is reflected at these "dead ends". In addition to that, with the process of passing through the foam, sound dissipates as heat (mostly due to viscocity).

This means that part of the incident sound will go through the foam and reach the back wall (or any other material placed behind the foam panel). Of course part of it will be reflected and what fraction of the energy is reflected or passes through depends on the (total) impedance that the sound "sees" at the interface of the foam (which in turn depends on the placement and other materials' impedances behind the foam panel). Behind the foam, sound will be reflected and move back into the foam going through the same process again, dissipating yet more energy as heat. This is a quite efficient mechanism just because sound is reflected at the back wall and has to go through a place that "absorbs" its energy (turning it into heat, mostly).

Now, imagine a "wall of foam" with a loudspeaker on one side and yourself on the other side. What do you think would happen if someone turned on the loudspeaker? You would be able to hear the sound quite well. Of course this is because foam lets sound through it and now that there's no back wall to be reflected it will just move on to reach your ears.

You have to keep in mind that the control of reverberation is not something that happens with only a couple of reflections. If this was the case you would just cover one wall of a room with foam and just from that you would experience but a handful of reflections with all sound reaching that wall "dying away" immediately. Control of reverberation is a more complex topic and it is mostly treated in a statistical manner (unless you are in the low frequency regime where you are trying to counter specific wave phenomena). There's much information on both reverberation (control) and room acoustics as well as absorption and absorber technology in Room Acoustics by Heinrich Kuttruff (room acoustics and reverberation control) and Acoustic Absorbers and Diffusers by Cox and D'Antonio (absorbers technology as the title states). Please keep in mind that these are mere suggestions based on personal preference and past experience and no there's no intend on promoting specific references.