[paul.fr]

and what properties make them good at soundproofing?

[Soul Surfer]

Two factors make good soundproofing, one is mass so the air pressure waves cant move it around and the second one is "soggyness" so that when the sound wves get into the material they are absorbed.  a combination of layers of heavy but accoustically dead material like lead and a lighter but highly absorbant material like maybe sawdust would probably be the most effective.

[another_someone]

While checking some of the information regarding skin pigmentation for another question:

http://en.wikipedia.org/wiki/Melanin#Melanin_deficiency_in_genetic_disorders_and_disease_states

It may be that melanin, the best sound absorbing material known

http://en.wikipedia.org/wiki/Melanin#Physical_properties_and_technological_applications

melanin is the best sound-absorbing material known due to strong electron-phonon coupling.

[lyner]

Audible sound has a huge range of frequencies .
Soundproofing can be achieved
1. By absorbing sound  energy or
2, By reflecting it.

High frequencies are best dealt with with a 'lossy' substance. This will absorb the sound as it passes through it (and transfer it to thermal energy). To absorb the sound, the material must distort and the internal friction dissipates the sound energy as it moves.   I should imagine the statement about melanin is a bit 'sweeping' and may apply to some frequencies, only.
To insulate against low frequencies, it is  hard to get a substance that will absorb the energy well.- it just vibrates the whole thing and lets the sound through.  You can make structures - like low frequency resonators  with damping mechanisms inside These will help to absorb lower frequencies.
But, for really low frequency sounds, you just have to reflect them.  The material has to be as massive as possible, so that it won't vibrate in the first place.  It's normal to use as much mass as you can afford - really thick concrete walls -and this will reflect the low frequency sound. 
A really good - quality 'free field' room  or sound studio has very thick walls.  'Egg boxes' are just not good enough.

[paul.fr]

A really good - quality 'free field' room  or sound studio has very thick walls.  'Egg boxes' are just not good enough.

Now you tell me! What i am supposed to do with all these box's now?

[lyner]

I guess you could buy some chickens!

[paulthree]

doh! eggboxes are a common myth, i fell for it myself in my naieve younger days - they may disperse a little bit of the higher frequencies but that's about it. If it's a room with a flatter response you require for recording & producing music then a quick and cheap solution is to simply break up large flat reflective surfaces with furnishings, pictures, shelves, sofas, rugs etc - even the odd carefully placed square of acoutsic foam tile can work wonders (that's partly where the eggbox thing comes from as they have a similar surface structure). If it's actual sound proofing, ie: reducing escaping sound levels, then more serious construction is required, using floating walls and floors filled with rockwool for example.

[lyner]

Plus very massive walls!
Once your loudspeakers have produced a good 'thumpy' level of low frequency sound inside your room it will tend to get through everything but a good heavy concrete box (walls, floor and ceiling) with massive, double thickness doors.
No wonder the lads in their cars  with 'sound systems' make such a racket going down the road; thin steel just doesn't work.
Also, holes for cables and pipes can let through an incredible amount of sound. They must be sealed with mastic. None of this is cheap.
Paulthree is right about the ideal listening conditions; anything too extreme can sound oppressively dead and makes your ears hurt 'cos it's too quiet.  I remember a similar, weird, feeling when I was a child in a draper's shop - piled high with rolls of fabric.
All you need to do is avoid too much 'colouration' for good listening.
It makes me smile that people are obsessed with flat responses for the Hi Fi stuff they buy and then they take it home and play it in an average room with immense response peaks and troughs - not to mention the traffic outside and the central heating and gas fire - what price signal to noise ratio?

[JimBob]

What? I can't hear you. Speak up, please.

Distance, also.

[another_someone]

Plus very massive walls!
Once your loudspeakers have produced a good 'thumpy' level of low frequency sound inside your room it will tend to get through everything but a good heavy concrete box (walls, floor and ceiling) with massive, double thickness doors.

I suspect the suspending an airtight room on magnetic suspension systems within a vacuum inside an outer room should stop most sound getting out.
 

It makes me smile that people are obsessed with flat responses for the Hi Fi stuff they buy and then they take it home and play it in an average room with immense response peaks and troughs - not to mention the traffic outside and the central heating and gas fire - what price signal to noise ratio?

For some of this you could probably simply use noise cancelling systems.